Geology Articles

Step by Step / Updated 10-06-2022

Living on Earth can be dangerous. Earth is a dynamic planet, geologic processes are always in motion, and while some processes take millions of years (like the formation of a sedimentary rock), others happen very quickly (like a volcanic eruption). All these processes are natural, but they become hazards when they affect human lives and the infrastructure of modern society.

Cheat Sheet / Updated 02-28-2022

Geology is the study of the earth, which begins with the study of the three types of rocks — the building blocks of the earth and its features. The unifying theory of geology is called plate tectonics, which contends that the earth's surface is separated into puzzle-like pieces that move around. Of course, earth hasn't always looked the way it does today, and geologists help tell the story of the earth's evolution by reading the rock layers. In doing so, they have mapped out the geologic timescale, which divides the earth's history into time segments based on major biological changes that occurred in the past 4.5 billion years.

Article / Updated 03-23-2020

The beginning of the Paleozoic is marked by the sudden appearance of a wide variety of animal forms in the geologic record. In fact, the fossils from this period exhibit all the animal body plans that exist even today, 540 million years later. (A body plan is how an organism’s body parts and growth patterns are organized.) This sudden appearance of complex life in the geologic record is called the Cambrian Explosion. The Cambrian Explosion has long been defined by the abundance of creatures preserved in the fossil record at the beginning of the Cambrian period. But it is likely that this sudden appearance of life is a result of the incomplete nature of a history told in rocks. Rather than documenting the first appearance of complex life (which was already present), the Cambrian Explosion documents an important new adaption for life: the building of shells. Toughen up! Developing shells In the Cambrian, creatures living in the sea began to grow shells or exoskeletons. This gave them a tremendous advantage, both during their lifetimes and for being preserved in the geological record. As you can see today, this way of life has lasted for millions of years. External hard parts provide the following benefits: Protection from the sun: During the Paleozoic, immense shallow seas were the primary habitat for life on Earth. Soft-bodied creatures were exposed to the sun’s harmful rays — the same rays you and I avoid with sunscreen and hats. Building an exoskeleton protects the soft tissues and internal organs of a creature from being damaged by the sun. Moisture retention: Large, shallow water environments sometimes experience an occasional absence of water — similar to the beach at low tide. Animals that become stranded when the tide goes out will dry out and die unless they have a shell that retains enough moisture to help them survive until the tide comes back in. Muscular support: Building an exoskeleton provides a framework for muscles to attach themselves to. This function is the same one your internal skeleton provides. Because an exoskeleton provides structure for muscle attachments, it allows an organism to grow larger than it would without such a supporting structure. Protection from predators: Possibly the most important advantage a shell provides is protection from other animals that may attack and devour a soft-bodied creature. While all of these are great reasons to build an exoskeleton, scientists are not certain which advantage started the evolutionary trend toward external hard parts. Evidence in the fossil record of creatures with damaged shells indicates that they were being hunted, attacked, and probably eaten by predators. For some scientists, this fact is enough to conclude that predation was the driving force for the evolution of exoskeletons at the beginning of the Paleozoic. Ruling arthropods of the seafloor: trilobites The first shelly fauna, or animals with exoskeletons, were tiny creatures; their shells were only a few millimeters in size. But it didn’t take long for other animals to follow the shelly trend. The most famous creature of the Paleozoic — possibly its mascot — is the trilobite. Trilobites are arthropods, which today include insects, spiders, and crustaceans such as lobsters. Species of trilobites filled every nook and cranny of the ocean throughout the Paleozoic, but they did not survive the end-Permian extinction. Trilobites had an exoskeleton that is segmented into parts, similar in appearance to roly polys (pill bugs) but with a horseshoe-shaped head segment. They ranged in size from itty bitty (just a few millimeters) to more than 50 centimeters (almost 2 feet) in length, but most of them were around 5 to 10 centimeters (about 3 to 4 inches). Some were blind, others had compound eyes (like some insects today), and certain species could roll up just like a roly poly, presumably for protection. While trilobites continued to cover the seafloor throughout the Paleozoic, they were most diverse during the Cambrian period and begin to be overshadowed in the fossil record by the development of other types of creatures later in the Paleozoic.

Article / Updated 03-23-2020

Although the early Paleozoic was ruled by invertebrates, the development of skeletal features had also begun. The evolutionary story of chordates—animals with a nerve chord (which later includes animals with a backbone, or vertebrates)—is missing in the geologic fossil record because there were no hard skeletal parts to preserve. When vertebrate fossils do show up in the fossil record, they are already full-fledged fish with backbones. And due to the presence of an internal skeleton (endoskeleton) and other hard parts (such as teeth and bony scales), the evolution of the fishes is a well-detailed story. Fish evolve body armor, teeth, and . . . legs? The first fish were not much like the fish you see today. They had spinal cords but no jaws and were called ostracoderms (which means “shell skin”) due to the bony plates covering them. Ostracoderms are members of the group called agnathans and are distant relatives of the lamprey and hagfish: two modern fish that do not have jaws or bony skin. Ostracoderms were bottom-feeders, skimming the surface of the seafloor sucking up food while keeping their eyes (located on tops of their heads) peeled for predators. They flourished through the early Paleozoic and lived alongside other fish groups evolving at the same time. The earliest fish with jaws belonged to a group called acanthodians that no longer exist. The evolution of jaws appears to be related to the gill structure of early fish. Scientists think the frontal gill supports made of cartilage or bone may have originally become hinged to allow the gills to open wider, taking in more oxygen and also allowing the intake of more food. This feature proved advantageous to their survival and continued to develop through natural selection, resulting eventually in bony hinged jaws. Scientists are still sorting out the details but think that the acanthodians very likely led to later groups of jawed fish, such as the placoderms, cartilaginous fish, and bony fish. (Keep reading for details on these fish groups.) While the fishes began to evolve in the early Precambrian, they reached their maximum diversity in the Devonian period (about 400 million years ago). For this reason, the Devonian is often called the “Age of the Fishes.” During the 50 million year span of the Devonian period, all the major types of fish are present in the fossil record: ostracoderms, placoderms, cartilaginous fish, and bony fish. Placoderms Placoderm fish flourished in the middle to late Paleozoic but do not have any living ancestors today; by the end of the Devonian period they were extinct. The name placoderm means “plate-skinned” and refers to the heavily armored skin of these fish. They survived in both saltwater and freshwater habitats. Some of them, such as the Dunkleosteus, were frightening predators with razor-sharp teeth and body lengths of 10 to 12 meters (more than 30 feet)! The heavily armored head of a Dunkleosteus is illustrated here. Cartilaginous fish The cartilaginous fish group includes modern sharks and stingrays. These creatures have jaws and teeth, but their internal skeleton is made of cartilage instead of bone. Cartilage is a flexible, sturdy, organic material that is found in many other animals as well; you are most familiar with it because it’s what shapes your ears and nose. Bony fish The bony fish are by far the largest and most diverse fish group. Many of the subgroups of bony fish still exist today, and it appears that amphibians evolved from bony fish. The bony fish are separated into two groups: Ray-finned fish: This is the type of fish you are most familiar with. These fish have fins supported by tiny bones that spread out and, when moved, propel the fish through the water. This group includes modern fish such as trout, bass, and catfish. Lobe-finned fish: This type of fish is much rarer today — and more fascinating evolutionarily because they were the first step toward land-dwelling animals. One type of modern lobe-finned fish, the lungfish, lives in freshwater habitats such as streams or lakes and breathes through gills like other fish. However, when the water dries up, this fish can burrow into the mud and breathe through a lung-type organ until the water returns. Among the extinct lobe-finned fish, the crossopterygians group is thought to have given rise to amphibians, which were the first animals to live outside water. The lobed fins of crossopterygians were muscular enough that scientists think they could have propelled these fish across short distances on land, a precursor to fully developed legs in amphibians. Other similarities between the skeletal and tooth structures of these lobe-finned fish and early amphibians are still being studied as scientists fill in the details of early land-dwelling animals. Venturing onto land: Early amphibians In the middle Paleozoic, while the fish dominated the seas, amphibians evolved. Amphibians are animals that breathe air and can move comfortably outside of water but still spend the first part of their lives (as eggs and larvae) in the water. By the Devonian period, insects and plants had colonized the land, providing food resources for amphibians when they ventured out of the water. The discovery of a fossil called Tiktaalik roseae provided scientists with a missing link between water-dwelling and land-dwelling animals. Tiktaalik roseae is informally called a “fishapod” because it had characteristics of both lobe-finned fish and four-legged animals (called tetrapods). Scientists think that early amphibians developed limbs to help them move around the swampy, shallow water environments of the middle Paleozoic. Amphibians became most diverse and abundant in the late Paleozoic as they spent more time outside the water. It appears that they gave rise to the next major animal group in Earth history: the reptiles. Adapting to life on land: The reptiles Reptiles didn’t begin to dominate the earth until the Mesozoic, but they evolved and established themselves — conquering the land for vertebrates — in the late Paleozoic. In order to live on land, animals had to develop certain characteristics that allowed them to be away from water. Amphibians lived the first stage of their lives in the water and had to return to water to lay their eggs. With the appearance of an amniote egg, reptiles no longer needed water the way amphibians did. An amniote egg is an egg with a yolk sac inside it that provides nutrients to the developing embryo so that when it hatches from the egg, the animal is well past the larval stage and doesn’t need to live in water. Early reptiles included the pelycosaurs, such as the Dimetrodon fossil pictured in this book’s color photo section. These animals had large fins along their backs. Paleontologists think the fins may have helped them control their body temperature. Reptiles are cold blooded, which means they have no internal way of warming themselves up (unlike warm-blooded animals, who regulate their own body temperature). But the pelycosaurs and their later relatives, the therapsids, may have begun to develop methods of body heat regulation that were precursors to the way mammals control their body temperature. Reptiles may have been the first animals to dominate the land, but they were not the first living creatures on land. Long before amphibians ventured out of the water, plants had established themselves on land and were living alongside swarms of insects.

Article / Updated 03-23-2020

It’s easy to get distracted by the abundance and diversity of life that appears and flourishes during the Paleozoic. But life and evolution are influenced by the geologic processes that are always shaping the earth’s environments. The Paleozoic saw periods of intense mountain building, extensive glaciations, widespread shallow seas, and the continued buildup of material onto the continental cratons, building the continents into shapes resembling what you see today. The construction of continents The history of each continent is told in its rocks. Beginning with the ancient cratons that formed during the Precambrian, geologists interpret the geologic history of the continents from the rock sequences and the stories they tell. When the Paleozoic era began, there were six major continents on Earth, none of them as large as the modern continents. These continents moved around under the influence of plate tectonics. Rock layers on the modern continents indicate intense periods of mountain building that occurred during the Paleozoic era as the continents crashed into each other. Each continent grew larger through the accretion of terranes, and mountains were built along mobile belts. Here’s what these processes entail: Enlarging continents through accretion of terranes: Rocky material from one continent can be added to another continent in a process called accretion. The foreign materials (the new rocks) have a different history than the continent they are added to and are called As the accretion of terranes occurs repeatedly through time, the continents grow larger and have different shapes. Building mountains along mobile belts: When two continents collide, crustal material along the edges is forced upward. The result is elevated areas of topography — mountains — in a linear pattern parallel to the edge of the colliding plates. Reading the rocks: Transgressions and regressions Extensive sedimentary rocks formed in the Paleozoic era indicate times when vast, shallow seas covered the continents, depositing sandstone, shale, and limestone. These rocks are informative because sedimentary rocks record important information about the environments in which they are deposited — particularly when the sedimentary rocks are formed by the settling of particles through water. In such cases, the sediment particles are subject to the laws of physics and gravity that still apply today. For example, a larger, heavier particle will settle out of water more quickly than a smaller, lighter particle. When rivers or streams carry sediments from the continent to the sea, the sediments are deposited according to their size as the motion of the water slows down. This means that the largest, heaviest particles are deposited closest to the seashore, while the tiniest particles are carried much farther and deposited in the deep, still waters far from the shore. The result is that the sedimentary rocks formed in the ocean have a distinct pattern to their particle sizes, with sandstone created closer to shore in shallow water and limestone created farther away in deeper water, as illustrated. By understanding how the depth of the water and distance from shore affects the type of rock formed, geologists can look at the rocks and read a story of changing sea levels. When sea levels rise and cover more of the continent, the geologic event is called a marine transgression. The sedimentary rocks being deposited in one spot change from sandstone to shale to limestone as the water gets deeper at that location. This situation is illustrated here. When sea levels drop and expose more of the continent, the geologic event is called a marine regression. As it occurs, the type of rocks forming in one location shift from limestone to shale to sandstone as the water becomes shallower. The rock types in a marine regression are illustrated in this figure. Rock types indicating a marine regression. One aspect of a continent’s history is found in its cratonic sequence, or record of marine transgressions and regressions. North America, for example, has four cratonic sequences dated to the Paleozoic. In each sequence, rocks indicate that the North American craton was covered by transgression of a shallow sea, which then regressed. In some regions, these seas became so shallow that they dried up, leaving rocks called evaporites. Evaporites are minerals that form as water evaporates. In other regions, extensive reefs were built by reef-building invertebrates that later became geologic deposits of limestone. Fossilizing carbon fuels Concern about modern climate change has people talking about reducing the use of fossil or carbon fuels. Both terms refer to coal, oil, and natural gas resources, many of which are found in Carboniferous rock layers created during the late Paleozoic. The abundant plant life in the Carboniferous period left its carbon remains to form geologic deposits in rock layers of coal. In fact, geologists describe these mid- to late- Paleozoic environments as coal swamps. (Some coal swamp deposits were formed during the following period, the Mesozoic.) A rock sequence common in the Carboniferous period, especially the Pennsylvanian period, is called a cyclothem. Cyclothem rock layers indicate a transition between marine and nonmarine environments, similar to what is observed today at a low-lying river delta such as the Mississippi River. These regions are now (and were in the Carboniferous period) full of thick, swampy vegetation. As these plant materials die, become buried by sediments, and accumulate over time, they turn into coal beds. The cyclothems of the Carboniferous are so widespread that geologists are still seeking answers to how they were formed because it seems unreasonable that so much of the earth was covered in swampy, transitional marine environments. Pangaea, the most super of supercontinents By the end of the Paleozoic, all the major continents on Earth came together, forming a single supercontinent. Pangaea is the most super of supercontinents because there is no evidence that ever before — or ever since — all the major landmasses formed a single continent. The remains of Gondwana, the southern landmass of Pangaea, led early geologists to ask the questions that eventually led to the theory of plate tectonics. On the supercontinent of Pangaea in the early Mesozoic is when dinosaurs began to evolve.

Article / Updated 03-23-2020

More than once in Earth’s long history, geologic events have led to the demise of multiple species. Sometimes whole families of organisms disappeared, putting an end to that particular path of evolution and leaving room for surviving animals to spread into new habitats. Each of these extinctions is well-documented by changes in the fossils preserved in the geologic record. To be extinct means to no longer exist. Technically speaking, at the end of your lifetime you will be extinct, though your species (Homo sapiens sapiens) will not be extinct because many other people will still be living. When scientists talk about extinction, they talk about the extinction of every member of a whole species, or even a genera (group of species) or a family (group of genera). When the term mass extinction is used, it indicates that a very large number of species cease to exist. In geologic time, a mass extinction event may occur over several hundreds of thousands — or even millions — of years. While this seems like a long time relative to a human lifespan, remember that geologically speaking it’s not very long at all. How extinctions are explained Each mass extinction in Earth’s history has been recorded by the sudden absence of fossils of certain organisms in the geological record. These events or periods of extinction affected the entire planet. Scientists think that in each case, some change in the environment resulted in conditions that could no longer support the organisms that had adapted to it. Thus, the organisms died off in great numbers, and some never reappeared. Scientists have not yet determined, unquestionably, what led to each mass extinction, but they have some good ideas, expressed as theories, that are still being tested by modern scientific research. I introduce four such theories in this section. Heads up! Astronomical impacts Earth is only one of many objects moving through the universe. Occasionally, as evidenced by the craters on the moon, flying objects in space may hit one another. When this occurs, it is called an impact event. Scientists have found evidence for impacts on Earth, such as craters resulting from meteorites that have hit Earth’s surface. The span of human history has not recorded any impact large enough to cause a dramatic change in global conditions, but evidence exists that such major events occurred in the past. While it may seem obvious that being struck by a meteor devastates life in the areas surrounding the impact zone, what is not so obvious is the continued after-effects that are experienced all around the globe. The following sequence of events explains how global ecosystems could be negatively affected through an impact event: A large object hits Earth. The impact sends large amounts of rock and other collision debris into the atmosphere, and it starts fires, which add smoke and ash to the atmosphere. The atmosphere is polluted. The particles of ash and rock in the atmosphere do three things: Block sunlight, which plant life depends on Block sun warmth, leading to global cooling Create conditions for acid rain This darkened atmosphere may also be very cold and difficult to breathe in — like a day of heavy smog in modern cities, but a day that lasts for many years. Plant life is affected first. The combination of acid rain, cooler temperatures, and absence of sunlight shuts down the process of photosynthesis and brings plant life to a halt. Herbivores are affected next. Without the plants to support them, herbivorous animal species begin to suffer. The entire ecosystem collapses. As the plants and herbivores disappear, animals that depend on them (carnivores) also suffer. Eventually entire food webs have been affected and begin to collapse. Keep in mind that mass extinction does not occur in one day. The sequence of events following an impact may continue for many hundreds or thousands of years following the impact event itself. The species that can’t adapt to a new way of life will die out. Lava, lava everywhere: Volcanic eruptions and flood basalts Basalt rocks formed by the cooling of lava indicate that at times in Earth’s past, volcanic activity occurred on a massive scale. Entire regions of the continents, called provinces, are covered by layers of basalt rock many miles deep. Regions of the modern continents that are covered in these flood basalts are illustrated in the following figure. Such provinces are not formed by the eruption of lava from a volcanic mountain such as Mount Saint Helens, pictured in this book’s color photo section, but rather from fissures: elongated cracks where the magma below erupts onto the surface without explosive force. Today such fissure eruptions are most common on the flanks of volcanic mountains and in the Hawaiian Island volcanic eruptions. Geologists conclude that the eruption of lava from giant fissures created the flood basalts and would have also altered the global environment. Specifically, such massive eruptions of lava, much larger than the fissures currently erupting on the Hawaiian Islands, would have been accompanied by the release of huge amounts of volcanic gas into the atmosphere. The result would have been rising global temperatures and associated changes in climate patterns due to the added sulfur dioxide and water vapor in the air. Some of these flood basalt events are thought to have lasted for hundreds of thousands of years at a time. While the region affected by the lava itself would be confined to a particular continent, the global effects of changed atmosphere and climate would have reached every part of the earth, both on land and in the oceans. Shifting sea levels During certain periods in Earth’s history, most of life was lived in shallow oceans. A shift in sea level would have had dramatic effects on the environments supporting shallow marine life. Lower sea levels would force shallow sea life into dry, waterless environments. Higher sea levels would leave them in deeper water with less access to sunlight and oxygen found near the ocean’s surface. Such sea-level changes could have been the result of climate changes (the melting or growing of large ice caps, which would change the amount of water in the oceans) or tectonic plate movements. Changing climate Most scientists now consider climate change to be the most important factor in mass extinctions. The earth’s climate changes in response to many different factors, including impacts, tectonic plate movements, and volcanic eruptions. In looking at evidence for mass extinctions in the geologic records, scientists conclude that global-scale changes can most reliably be explained through changes in a global system, such as the climate. Other geologic evidence indicates that periods of mass extinction commonly occur during global warming or glaciations, leading scientists to conclude that shifting climate conditions changed global environments so dramatically that many species could not adapt, and perished. End times, at least five times Species go extinct all the time; extinction is part of the natural order of things. Normal rates of extinction through time are part of what scientists call the background extinction rate expected to occur on Earth. The mass extinctions described in this section are periods when the rate of extinction, as indicated in the fossil record, is much more dramatic and extreme than the normal (background) rate. The following figure is a graph that illustrates extinction rates throughout Earth’s history, highlighting the five major extinction events described. Cooling tropical waters The first major mass extinction that scientists know about happened approximately 445 million years ago toward the end of the Ordovician period (in the Paleozoic era). At the time, life was lived in the oceans; no evidence indicates that land plants or animals existed yet. Scientists think the expansive marine environment was affected by a cooling climate and abrupt changes in sea level as extensive glaciers grew over the continents of the South Pole. More than 100 families of marine organisms, primarily those living in tropical regions near the equator, went extinct. This totaled more than 50 percent of the living families of that period. Scientists conclude from the evidence for glaciation that the colder climate at the poles meant conditions in the tropics were also cooler, leaving the warm water–adapted organisms nowhere to go. The amount of water locked up as ice over the South Pole may also have dramatically lowered sea levels all over the planet, reducing the habitat for undersea organisms. Reducing carbon dioxide levels At the end of the Devonian period (also in the Paleozoic era), around 370 million years ago, another extinction event affected marine life. This event seems to have affected reef-building organisms living in shallow marine environments, as well as some groups of early land plants. Only slightly less dramatic than the earlier extinction event, the Late Devonian extinction saw almost 50 percent of the existing families disappear. The fact that organisms in shallow marine waters as well as on land were affected has led scientists to hypothesize that atmospheric conditions, such as changes in carbon dioxide levels, played a large role in this event. The early plants themselves may have altered atmospheric levels of carbon dioxide through photosynthesis. Less carbon dioxide leads to cooler global climate conditions, which may then have affected marine life in warm, shallow sea ecosystems. The Great Dying A mass extinction event that marks the transition between the Paleozoic and Mesozoic eras, about 250 million years ago, is called The Great Dying, the Permian-Triassic event, the Permo-Triassic extinction, or the End-Permian extinction. At this time, more than 96 percent of species in the oceans and 70 percent of the species on land (including some plants) perished. The End-Permian extinction is the only extinction event in Earth’s history to affect insects, resulting in a loss of 33 percent of the insect species of the time. Scientists are not certain what caused the End-Permian extinction. This event appears to have occurred over a few million years, leading scientists to rule out an impact as the primary cause. At the time of this extinction, the supercontinent of Pangaea was forming, which may have changed ocean circulation patterns and temperatures more quickly than species could adapt to. But some scientists argue that by the time of the extinctions, the landmasses had already moved and shouldn’t have further changed the marine environments in any important way. This mass extinction was most severe in the oceans, leading some scientists to conclude that global water conditions must have experienced a disruptive change. One explanation may be that the oceans became anoxic: lacking in dissolved oxygen. Oxygen levels in the ocean are maintained by the circulation of surface waters, which cool near the poles, sink (taking oxygen-rich water into the deep sea), and move back toward the equator. This circulation of water due to changes in temperature and salinity (the amount of salt it contains) is today called the thermohaline ocean conveyor. Scientists propose that toward the end of the Permian period (the last period of the Paleozoic era), climate conditions were so warm all over the planet that ocean circulation was stopped — with the result that no oxygen was brought into the deep sea, which essentially suffocated marine life. On the continents, the Siberian Traps — a massive outflowing of lava from volcanic fissure activity in what is now northern Siberia — likely affected global climate conditions. The release of gases associated with this type of volcanic activity may have been enough to create a global greenhouse, warming temperatures enough to halt the temperature-driven cycling of oxygen in the oceans. Paving the way for dinosaurs At the end of the Triassic period (the first period of the Mesozoic era), about 200 million years ago, approximately 35 percent of the animal families became extinct. While this event is the least dramatic of the five major extinctions, its cause is also still a mystery to scientists. This extinction event was likely spread over a long period of time. The Central Atlantic magmatic province — a region of massive lava flows between the continents of South America, Africa, and Europe as Pangaea split apart — was erupting, and evidence for climate conditions suggests the climate was on a rollercoaster from one extreme to the next. This unsteady climate may have made it difficult for some species to adapt, resulting in their extinction. The end-Triassic extinctions paved the way for dinosaur dominance. As other animal groups died out, dinosaurs expanded to fill the empty niches, eventually covering every environment on Earth. Demolishing dinosaurs: The K/T boundary Possibly the most well-known mass extinction event is the one that ended the reign of the dinosaurs at the end of the Cretaceous period (at the end of the Mesozoic era). In the geologic record, the transition from the Cretaceous period to the Paleogene period is well-marked by the disappearance of dinosaur fossils. Some dinosaurs are the ancestors of birds. These avian dinosaurs are the only ones who survived this extinction event. All the reptilian dinosaurs are found in rock layers below this time period’s geologic layer — not above it. The geologic layer marking the boundary between the Cretaceous and Paleogene periods is called the K/T boundary. The K stands for the German word for Cretaceous, Kreidezeit; the T is for Tertiary, which is the period between the Cretaceous and the Quaternary (65 to 2.8 million years ago). Modern geologists who no longer recognize the Tertiary period have begun to refer to this transition as the K-Pg boundary (Cretaceous-Paleogene) instead. Many events occurred during this time that may have, together, resulted in the extinction of so many animals. The supercontinent of Pangaea was breaking up, and the Deccan Traps of India were erupting. As I explain earlier in the chapter, tectonic plate movements, as well as massive volcanic activity, can change global atmospheric, climate, and ocean conditions, resulting in animal extinctions. At the K/T (or K-Pg) boundary, however, there is also clear evidence of an impact event. In the Gulf of Mexico, just off the north side of the Yucatan Peninsula, scientists have identified a massive crater. The Chicxulub Crater illustrated in the following figure is the result of a rocky body at least 10 kilometers (6 miles) wide hitting the earth around 65 million years ago. According to the impact theory, such an impact could have created long-lasting darkening of the atmosphere, interfering with plants first, and then rippling up the food chain to devastate the largest creatures: dinosaurs. The strongest line of evidence for this explanation is the amount of iridium found in layers of sediment dating to this time. Iridium is an element that is rare in Earth’s crust but much more common in meteors. Its worldwide presence in layers of clay and sand that must’ve been at the surface of the earth during the K-Pg boundary indicates that somehow, large amounts of iridium were introduced to Earth’s atmosphere. Scientists accept that a large meteor impact is an obvious explanation. Modern extinctions and biodiversity In this section, I describe a significant extinction event in the age of man. After humans evolved and began to spread across the globe, the large mammals of the Cenozoic era began to decline. In this section, I present possible explanations for that decline, and I touch on ideas about how man’s continuing impact on our planet may affect biodiversity. Hunting the megafauna About 14,000 years ago, humans first entered the Americas, probably by way of a land bridge connecting Siberia and Alaska. What they found was a land full of large mammals, or megafauna, such as mammoths, mastodon, bison, horses, ground sloths, and rhinoceros to name a few. Shortly after the arrival of humans, the numbers of large mammal species declined dramatically, leading some scientists to conclude that humans hunted these animals into extinction. The proposal that human hunting resulted in megafauna extinction is called the prehistoric overkill hypothesis. Supporters of this hypothesis claim that animals with no previous exposure to humans do not adapt quickly enough to the predatory skill of humans and their ability to kill large numbers of animals at once. A recent example of an overkill situation is the extinction of the dodo bird from the island of Mauritius in the Indian Ocean. In the year 1600, sailors arrived on the island and started hunting this large flightless bird and its eggs to eat. The dodo, with no natural predators and no previous experience with humans, was completely wiped out by 1681 —only 80 years after it started co-existing with humans. Supporters of the overkill hypothesis also point to similar events in Australia more than 40,000 years ago. When human populations first arrived in Australia, numerous species of large marsupials (kangaroo-like mammals) went extinct. The cause of this extinction — like the North American megafaunal extinctions — is still being debated. Some scientists think it was a direct result of human migration to the continent and overhunting of the animals. Other scientists suggest that human changes to the environment (through the use of fire to clear vegetation) were a more important factor in causing the extinctions. However, other hypotheses have been proposed to explain the disappearance of these large mammals: Asteroid impact: Recently, scientists have presented evidence for a possible impact event that may have led to the extinction of North American megafauna. Researchers are still hotly debating this possibility and looking for evidence (such as where such an impact may have occurred) to support their hypotheses. Climate change: Other scientists claim that climate changes occurring at the same time, such as ice age glaciers melting and global conditions becoming warmer, were significant enough to lead some species into extinction. However, the climate change hypothesis leaves skeptics wondering why large mammal species didn’t migrate to new habitats as the environments changed. And others suggest that the climate didn’t change quickly or dramatically enough to result in extinctions. Reducing biodiversity The human effect on megafauna, or large mammals, is far from over. Today, many of the animals listed as threatened or endangered by humans are the largest existing mammals, including the American bison, Asian elephants, mountain gorillas, various species of whales, and many species of bear. As humans continue to dominate the earth, the pattern of humans moving into new regions and leading species to extinction is ongoing. Human population growth and expansion into new ecosystems threatens many species with extinction. While a background rate of extinction is normal and expected, some ecologists (scientists who study ecosystems) suggest that humans have increased the extinction rate by up to 10,000 times its normal rate. The existence of multiple species is called biological diversity or biodiversity. Scientists realize that biodiversity is very important because ecosystems with high biodiversity are more likely to adapt in response to disturbances (such as wildfires). Richly biodiverse regions like the rainforests of South America are also home to species of plants, insects, and animals that may have important and undiscovered medicinal properties. The most damaging effect humans have on biodiversity is the destruction, fragmentation, and pollution of ecosystems. Many of the most biodiverse regions are also the most fragile and are easily damaged by the building of roads, introduction of non-native species (or farm animals), industrial pollution, and deforestation. If these species become extinct, whether due to human-caused climate change or development and land use, humans may lose something of great value that they didn’t even know they had!

Article / Updated 03-23-2020

Here are a few of the most common ways that you use geologic resources every day. This includes in your home, in your neighborhood, and even in your computer! Burning fossil fuels One of the best known and most pervasive ways that humans use geologic resources is through the use of fossil fuels: coal, oil and natural gas, to create energy. The hydrocarbons that are extracted from deep in the earth are the remains of once-living organisms (no, not dinosaurs, most are more likely from algae of the distant past). As ancient organic material is trapped under layers of clay and sand it slowly decays, but without access to oxygen it cannot fully decompose. Instead the hydrocarbons molecules just hang out. Depending on the conditions of burial and the location they form different types of fossil fuel resources that humans can collect. Some organic matter gets crushed over time, turning from a peat bog into bituminous coal that can be mined, such as what is extracted in the Appalachian Mountains of North America. Other organic matter may be trapped in the seafloor sediments and over millions of years form a liquid that can be drilled out as oil, such as that found in the Gulf of Mexico seafloor. Tar sands, such as found in Canada are sandy sediments with sticky hydrocarbon tar filling in all the spaces between grains of sand. And natural gas that is captured by fracking is simply gaseous hydrocarbons hanging out in the tiny spaces between sediments in clay, shale and sandstone rocks. All of these different types of ‘fossil fuel’ release carbon dioxide when burned to provide energy and with recent understanding of how CO2 effects climate change, there has been a push to move away from our global dependence on these resources. Playing with plastics Plastics are man-made products that were engineered out of the leftover molecules from processing fossil fuels like liquid oil and petroleum. Since their invention in the early 1900’s plastics have become the most common material for making almost everything! Unfortunately, because plastics are man-made, they do not recycle back into the ecosystem the way naturally occurring products do, like paper (from wood fiber) or metals. Look around your house and count all the items made of plastic. These count as geologic resources too, because without fossil fuels, there would be no plastics. Gathering gemstones For millennia humans have gathered beautiful minerals, called gemstones, to adorn themselves with as jewelry. Many of the common gemstones, such as those assigned as birth stones to each month of year are simply common minerals with either eye-catching color or intriguing characteristics of hardness. For example, some minerals of quartz have minor amounts of other elements trapped in their crystal structure. When this happens, you get a gemstone! When quartz has manganese trapped within it, it turns purple and we call it an amethyst. When an amethyst is heated its color changes to yellow and we call it a citrine. Similarly, both sapphire and ruby are the mineral corundum, but the sapphires have minor amounts of titanium present and the rubies have minor amounts of chromium present. Other common gemstone minerals include emeralds, aquamarines, and opals. Drinking water Maybe you haven’t thought of water as a geologic resource before, but without rock layers to filter groundwater, the water in your well wouldn’t be as fresh! As water moves across the land surface it also moves underneath the land surface. The movement of water through the tiny pore spaces of permeable rock helps filter and clean the water. By the time it reaches an aquifer where it can be stored and accessed, it may already be clean enough to drink straight from the well. However fresh groundwater is an endangered resource. Rising sea levels threaten to infiltrate fresh water aquifers in coastal regions, which, once contaminated by saltwater, are no longer useful for human consumption. Another danger to the groundwater resources in many parts of the world is fracking for natural gas. While the fracking doesn’t directly contaminate groundwater, fracking does release gasses such as methane that have been trapped in the rocks along with other natural gasses. Once these gasses are released during the fracking process they can’t be contained and kept from moving through the rocks into nearby freshwater aquifers. Creating concrete Maybe you thought the concrete that is used to build sidewalks, buildings, bridges and other structures in your city or town was another type of rock. Or maybe you haven’t thought about it at all. The truth is that concrete is a man-made rock-like material. To make concrete a number of different rocks are actually used. Firstly, you need some crushed rocks and stones and some water. But to glue them together you need cement—or mineral glue. In nature, sedimentary rocks are glued together or cemented with minerals such as calcite or quartz that precipitate between the sediment grains and glue them into one large piece. To make cement humans have mimicked this process by combining limestone, clay and gypsum (another precipitating mineral). Once you have the crushed rocks, water and cement mixture, stir it together and let sit until dry. Now you have concrete! Concrete is the workhorse material for human buildings. It is used in creating the foundation to homes and buildings, as well as shaping the landscape with sidewalks, highways, concrete barriers and other structures. Paving roads Roadways criss-cross almost every landscape now, a sure sign of the Age of Humans or Anthropocene. To build these roads humans depend on a number of different geologic resources to create asphalt, that black stuff that roads are made of. Asphalt is a combination of sticky tar, called bitumen or bituminous coal. The asphalt can be mined where it formed naturally in the rock layers, or refined from other extracted fossil fuels. Combined with gravels, sand and a concrete mix, the asphalt serves as the sticky glue to hold these materials together where they are laid down and compressed to create a roadway. Accessing geothermal heat As our understanding of climate change and the effect of additional carbon in the atmosphere has grown, many regions have looked to alternative sources of energy. One of these sources is geothermal heat. As you dig deeper into the earth you will experience an increase in heat that scientists called the geothermal gradient. The depth at which heat from inside the earth is accessible varies with location. Regions near volcanic activity in particular have access to a great amount of geothermal heat at relatively shallow depth beneath the surface. To access this heat, for example, to heat your home, you need a system of pipes built that extends deep underground below your house. By sending water through theses pipes, the water will be heated in the deep sections simply because the pipes are surrounded by heated rocks. After it’s heated, the water will be circulated back up into your home where it can be used for a heat source instead of using gas, oil or other fossil fuels to generate heat. Fertilizing with phosphate Phosphate is an important mineral to support life and is found in the rocks of Earth’s surface. Natural processes bring phosphate to surface environments through erosion of rocks as they are uplifted on Earth’s surface. Phosphate is also mined and refined into fertilizers for agricultural use. The expanding agricultural business of the last century has ramped up the mining and application of phosphates — the mineral helps plants grow and when applied to crops can increase yields dramatically. However much of the phosphate used is not absorbed by plants and ends up washing into nearby water ways, which can become overloaded with the mineral, leading to negative side effects such as excessive algae blooms. Constructing computers Computers and handheld device technology depend heavily on geologic resources. You can start by examining the plastics in the device shell or casing but the real story is on the inside. Computers are built using a huge variety of elements mined from minerals found all over the world. Here is a few of the elements found in certain parts of your computer or smartphone: Screen display: Displays are commonly made of materials containing lead and tin, as well as quartz and potassium or lead for the glass. The lead for strengthening the glass screens is mined from the mineral galena. Circuit board: The internal workings of a computer or smart device depend on a long list of metals and other elements mined from minerals, including but not limited to: silicon from quartz, copper from chalcopyrite, and aluminum from bauxite. Function: Certain functions of your smart phone depend on the magnets inside fueled by the rare-earth element neodymium. Without this vital resource your phone would not be able to run its vibration motor for notifications. Building with beautiful stone The rocks we find on Earth are beautiful with their wide array of minerals, textures, and colors. And while the foundation of your home may be the man-made concrete, the many attractive accents or outer layer may be adorned with rock selected not for its strength, but simply for its beauty. Using attractive stone for construction and decoration began in ancient times in many societies, and the tradition continues today. Although the front of many homes and building may have natural stone features, or accessories, in recent decades there has been a boom in using polished granites, marbles, quartzites, and other rock types for interior decoration as well. Countertops of cut stone are popular, and if you shop around you will find slabs of interesting and beautiful stone in many different colors to choose from. Similarly, interesting stone is used for flowing tiles and even in showers and bathrooms as tile.

Article / Updated 03-26-2016

Scientists estimate that the Earth is about 4.5 billion years old, based on radioisotope dating techniques. To understand how this process works, you need to know a little bit about atoms and isotopes. Often, any one atom has several different forms, called isotopes. Atoms are made up of electrons, protons, and neutrons, and the number of electrons and protons determines the type of atom. Hydrogen, for example, has one electron and one proton. Sometimes, it also has a neutron, in which case it is called deuterium. Heavy water refers to water in which each hydrogen atom has a neutron. Some isotopes, like deuterium, are stable; they're perfectly happy with the number of electrons, protons, and neutrons they have. Other isotopes are unstable because the different number of neutrons interacts with the other atomic components in such a way that, over a period of time, the isotope changes into some other atom. When these unstable isotopes change to a different atom, they emit radioactivity. For that reason, they're called radioisotopes. An important property of radioactive isotopes is the half-life — the time it takes for half of the atoms to undergo the transition from one atom to the other. In the first half-life, half the atoms make the transition. In the second half-life, half of the remaining atoms transition, leaving one quarter of the original parent material. In the third half-life, half again transition, and so on. To determine the age of material, researchers compare the ratio of the parent and daughter products that were initially in the sample with the ratio of these products at the current time. By doing so, they can calculate how much time has passed. Numerous radioactive isotopes exist. One system that has been very successful in dating the ages of fossils is potassium-argon dating. Potassium is an extremely common element. Although most potassium isotopes aren't radioactive, one of them is, and one of its decay products is the gas argon. Although potassium is a solid, argon is a gas. When rock is melted (think lava), all the argon in the rock escapes, and when the rock solidifies again, only potassium is left. The melting of the rock and releasing of any argon set the potassium-argon clock to zero. As time passes, argon accumulates in the rock as a result of radioactive potassium decay. When scientists analyze these rocks and compute the ratio of argon to potassium, they can determine how long it's been since the lava cooled. When scientists date rocks from our planet this way, the oldest dates they find are 4.5 billion years. By dating the lava flows above and below a fossil find, scientists can put exact boundaries on the maximum and minimum age of that fossil. With radioactive dating, scientists can now get within a few percentage points of the actual date. They know this because they have been able to accurately date lava flows that happened recently enough for their dates to be known historically, such as the eruption of Mount Vesuvius at Pompeii.

Article / Updated 03-26-2016

Plate tectonics is the unifying theory of geology. This theory explains how crustal plates move around the surface of the earth, and it allows geologists to find explanations for geologic events such as earthquakes and volcanoes, as well as the many other processes that form, transform, and destroy rocks. The crust of the earth is separated into ten major plates and a few smaller ones. These plates interact with each other along their edges as they shift position on the earth's surface. The motion of crustal plates is described as the relative motion between two plates where they touch; this motion fits into one of three categories: Convergent: Where two plates are moving toward one another, they form a convergent plate boundary. Divergent: Where two plates are moving away from each other, they form a divergent plate boundary. Transform: Where two plates are moving alongside one another, they form a transform boundary. Early in the twentieth century, a scientist named Alfred Wegener proposed that the continents had once been attached to one another, forming a single large land mass or supercontinent, and had then drifted apart. While he had some good evidence to support his hypothesis, it wasn't until after World War I that scientists made progress in developing a solid theory of plate movement. The use of submarines in WWI prompted extensive mapping and study of the ocean floor. Through these studies, scientists discovered that the rocks on the floor of the Atlantic Ocean were of different ages — and the ages could be traced from oldest (nearest the continents) to youngest (along a ridge down the middle of the ocean). What they had discovered was that new ocean floor is created along a divergent plate boundary in the middle of the Atlantic Ocean. This finding in the Atlantic Ocean provided new energy to supporters of Wegener's earlier hypothesis and led the way to decades of intensive undersea geological research. Only fairly recently — in the 1960s — did researchers have enough evidence to propose an explanation for how crustal plates move around the earth's surface and interact with one another: the theory of plate tectonics.

Article / Updated 03-26-2016

Geologists classify the rocks of earth's crust in one of three categories — igneous, metamorphic, or sedimentary — based on how the rock was created. Each type of rock has its own unique characteristics: Igneous: Igneous rocks form from the cooling of melted rock (either lava or magma) into solid form. If the cooling occurs underground, the rock is an intrusive, or plutonic, igneous rock. If the cooling occurs on the earth's surface, the rock is an extrusive or volcanic rock. Geologists describe different igneous rocks according to their texture and composition. Metamorphic: Metamorphic rocks form when existing rocks are subjected to intense heat and pressure, usually deep below the earth's surface. These conditions change the original minerals of the rock into new minerals. Geologists classify metamorphic rocks according to how much they have been changed from the original, or parent, rock. Low-grade metamorphic rocks still appear very similar to the parent rock, while high-grade metamorphic rocks have been changed so much that they look very different from the parent rock. Sedimentary: Sedimentary rocks are either detrital or chemical. Detrital rocks are formed by the compaction of separate particles, or sediments, into a rock. The particles are pieces of a different, pre-existing rock that have been weathered and transported by wind, water, ice, or gravity. Chemical sedimentary rocks form from minerals that have been dissolved in water and precipitate out, forming a solid rock. Geologists describe sedimentary rocks according to the size and shape of the particles in them or their mineral composition (in the case of chemical sedimentary rocks). The rocks of earth's crust are constantly being recycled and changed into new forms through geologic processes. This continual transformation of rocks from one type to another is called the rock cycle. Through processes such as weathering, heating, melting, cooling, and compaction, any one rock type can be changed into a different rock type as its chemical composition and physical characteristics are transformed.