Praxis Elementary Education Exam—The Earth's Layers

In the Science section of the Praxis Elementary Education exam, you'll probably encounter a question or two about the structure of our home planet, Earth.

A cross-section of Earth's interior contains several layers, as shown here.

Deep inside is the inner core, solid and made mostly of iron. Next is the liquid outer core, the source of Earth's magnetic field. The mantle is the largest layer, being roughly 1,800 miles (2,900 kilometers) thick. It contains the lower asthenosphere, which has a fluidity that makes the plates move, and above that the lithosphere (litho- means stone), which is solid. Last is the crust, about 5–30 miles (8–50 kilometers) thick.

The ocean floor has levels, from the depths of an ocean trench (the Mariana Trench is the deepest, at 6.8 miles, almost 11 kilometers), to an abyss, to a continental slope, to the continental shelf where water meets land, or continent.

Earth's atmosphere is made up mostly of nitrogen (78 percent) and oxygen (21 percent). The lowest layer of the atmosphere, the troposphere, extends about 11 miles (17.7 kilometers) and is the source of most of our weather and most of our clouds. The stratosphere extends to about 31 miles (50 kilometers); it contains the ozone layer and, at its lower reaches, a few clouds, planes, and large birds. Next is the mesosphere, extending to about 50 miles (80 kilometers). Most meteors burn up in the mesosphere. The thermosphere extends to about 430 miles (almost 700 kilometers). It is extremely hot and has satellites, little air, and many ions. The exosphere extends to over 6,000 miles (10,000 kilometers) and has very little in it at all.

Practice question

1. Which of the following is NOT true? A. An object falling to the deepest possible part of the ocean would land in the abyss. B. A vulture was seen in the stratosphere. C. The aurora borealis and aurora australis both occur in the thermosphere. D. The tectonic plates of the Earth are in the lithosphere and crust.

Answer and explanation

1. The correct answer is Choice (A). The deepest level of the ocean is the oceanic trench. Choice (B) IS true. A large Rüppell's vulture was found at 11,552 miles, just into the stratosphere. Choice (C) IS true. The aurora borealis and aurora australis (from the north and south polar regions, respectively) occur as a result of charged solar particles (ions) falling through the thermosphere. Choice (D) IS true. The tectonic plates are the hard slabs of Earth's crust (surface) and lithosphere; there are seven or eight large plates and several smaller ones.