Geology For Dummies
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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:

  1. 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.
  2. 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.
  3. 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.
  4. Herbivores are affected next. Without the plants to support them, herbivorous animal species begin to suffer.
  5. 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.

flood basalt rock layers Regions of the modern continents covered in flood basalt rock layers.

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.

extinction rates Extinction rates for five major extinction events.

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.

Chicxulub Crater The location of the Chicxulub crater in the Gulf of Mexico.

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!

About This Article

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Alecia M. Spooner has been teaching at the college level for more than 15 years. She currently teaches at Seattle Central College, where she is Professor of Earth and Environmental Sciences. Alecia teaches earth science courses that are accessible and engaging, while stressing scientific literacy and critical thinking.

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