GED Science Test: Earth’s Layers and Landforms

By Murray Shukyn, Achim K. Krull

Prepare yourself for the GED Science test by delving into the Earth’s layers and landforms. Earth’s layers and landforms are the earthy parts of Earth — everything that’s not water, air, or a living thing. The planet Earth comprises the following three layers:

  • Core: At the center of the Earth is a very hot, dense core thought to be made mainly of a metal alloy (mixture) of nickel and iron. The core contains most of Earth’s mass and is its primary source of internal heat, emitting heat as radioactive materials within the core decompose into more stable elements. The core is subdivided into two layers:

    • Inner core: The inner core is solid because of the pressure exerted by the other layers and the force of gravity, which compacts the atoms so tight that they can’t pass into a liquid state.

    • Outer core: The outer core is liquid because of the intense heat at the center of the Earth. The pressure in the outer core isn’t sufficient to prevent the hot metal from turning to liquid.

  • Mantle: The mantle that surrounds the core is estimated to be approximately 2,000 miles (3,000 kilometers) thick and is made of two layers of rock:

    • Upper mantle: The upper mantle is made of cooler, brittle rock that can break when subjected to stress. The breaking and shifting of this rock is responsible for earthquakes.

    • Lower mantle: Hot, soft rock composes the lower mantle. This rock flows when subjected to stress.

    Activity in the mantle is responsible for creating mountains and producing earthquakes and volcanoes.

  • Crust: The crust is the very thin layer that surrounds the Earth, and it differs depending on its location:

    • Oceanic crust: The crust below the oceans is relatively thin (3 to 4 miles) and is composed mainly of basalt.

    • Continental crust: The crust beneath the continents is approximately 20 to 30 miles thick and is composed primarily of granite.

The crust and the upper mantle form the lithosphere, a layer of brittle rock that floats atop the lower mantle. The lithosphere is broken up into several major and many minor tectonic plates that move, causing many of the geological events that can be observed on Earth’s surface. Plate tectonics is the scientific theory that explains the movements of tectonic plates and the various geological events that occur as a result. Plate tectonics is responsible for the following:

  • Continental movement: The various continents on Earth were thought to be one supercontinent commonly referred to as Pangea. As soft rock oozes up from the lower mantle, it pushes apart the plates on which the continents rest, causing them to “drift” apart over long periods of time.

  • Earthquakes: Moving tectonic plates cause earthquakes as they drift apart, press against each other, or the edge of one plate slides below another.

  • Mountains: As plates push into each other, the rock has nowhere to go but up, creating mountains.

  • Tsunamis: When the edge of one tectonic plate slips beneath the edge of another at the bottom of the ocean, massive amounts of water are displaced, forming a wave that can be very destructive when it washes up on land.

  • Volcanoes: Molten rock flows up between plates to create volcanoes. When the heat and pressure reach a certain point, the volcano erupts, sending rock and ash into the atmosphere and creating lava flows. The heat from a volcano may also melt snow and ice, creating mud flows.

Check out the three types of tectonic plate boundaries:

  • Convergent: When two tectonic plates move toward or against each other, they form a convergent boundary characterized by mountain ranges, ocean trenches, volcanoes, and earthquakes. If the edge of one plate slips beneath the edge of another, the lower plate is referred to as a subducting plate, which is typically responsible for creating ocean trenches. When the edge of the subducting plate reaches a certain depth, it is absorbed back into the lower mantle.

  • Divergent: When two tectonic plates move away from each other, a divergent boundary is formed characterized by frequent earthquakes, lava flows, and geysers. Under all this, a layer of molten rock flows slowly into the gap and hardens to form solid rock.

  • Transform: Two plates sliding past each other form a transform plate boundary. At these boundaries, rocks are pulverized as the plates grind along, creating a linear fault valley or undersea canyon. As the plates alternately jam and jump against each other, earthquakes rattle through a wide boundary zone. In contrast to convergent and divergent boundaries, no magma is formed. Thus, crust is cracked and broken at transform margins, but isn’t created or destroyed.

    Tectonic plate boundaries.

    Tectonic plate boundaries.
  1. What are the three layers of Earth’s geosphere?

    • (A) inner core, middle core, outer core

    • (B) core, mantle, lithosphere

    • (C) core, mantle, crust

    • (D) core, mantle, tectonic plates

  2. Plate tectonics help scientists explain which of the following?

    • (A) continental movement

    • (B) what causes volcanoes and tsunamis

    • (C) how mountains are formed

    • (D) all of the above

  3. Which type of plate boundary is most likely responsible for the movement of continents away from each other?

    • (A) divergent

    • (B) convergent

    • (C) transform

    • (D) subducting

  4. The crust is part of which of the following?

    • (A) the upper mantle

    • (B) the lower mantle

    • (C) the lithosphere

    • (D) the atmosphere

Now check your answers:

  1. The three layers that compose Earth’s geosphere are the core, mantle, and crust, Choice (C).

  2. Plate tectonics help scientists explain continental movement along with the formation of mountains, volcanoes, and tsunamis, Choice (D), all of the above.

  3. A divergent plate boundary, Choice (A), would be most responsible for causing continents to drift apart.

  4. The crust is part of the lithosphere, Choice (C). The other part of the lithosphere is the upper mantle.