Geology is the science that deals with the history and composition of the Earth and its life, especially as recorded in rocks.
In part by studying rocks, scientists have been able to determine that the Earth is made up of three layers. The outermost layer, or crust, comprises roughly one percent of the total Earth’s volume. It varies in thickness from 10 kilometers to as much as 80 kilometers. Beneath the crust lies the mantle, which comprises more than 75 percent of the Earth’s volume. Roughly 3,500 kilometers thick, the mantle contains mostly iron, magnesium, and calcium, and is much hotter and denser than the Earth’s surface because temperature and pressure inside the Earth increase with depth.
At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. The Earth’s core contains two distinct parts: a 2,300-kilometer-thick liquid outer core and a 1,200-kilometer-radius solid inner core.
Structure of the Earth
As noted, the interior of the Earth is quite hot, somewhere between 3,000 and 4,000°C. This heat is generally prevented from escaping thanks to the solid rock in the Earth’s upper mantle and crust. The crust and the rigid upper part of the mantle (the lithosphere) consist of approximately 30 separate pieces called plates. These plates move very slowly upon the more movable mantle beneath (the asthenosphere), and this has caused the continental landmasses to drift slowly apart over the course of hundreds of millions of years.
Along the edges of these plates are fault lines. (Fault lines are simply places where the plates slide relative to each other.) When plates slide relative to each other along fault lines earthquakes can occur. When an earthquake occurs, scientists use the Richter scale or the moment magnitude scale to measure its intensity. The Richter scale is perhaps more familiar to most people. The Richter scale begins at 1, with each larger integer representing a magnitude that is about 10 times greater than the preceding step.
The Earth’s rocks fall into three categories, based upon how they are formed.
| Rock type | How it’s formed | Examples |
|---|---|---|
| Igneous | formed from the hardening of molten rock, or magma, which is called lava when it reaches the surface of the Earth | granite, pumice, basalt, obsidian |
| Sedimentary | formed by the sedimentation, or gradual depositing, of small bits of rock, clay, and other materials. Over time this deposited material becomes cemented together. Most fossils are found in sedimentary rocks. | shale, sandstone, gypsum, dolomite, coal |
| Metamorphic | formed when existing rock material is altered through temperature, pressure, or chemical processes | marble, slate, gneiss, quartzite |
Most of what is known about the history of our planet has been learned by studying the fossil record found in sedimentary rock. By studying rocks, we now know that the Earth is approximately 4.6 billion years old, and that for most of that time, very few fossil traces were left. This is why the period from 4.6 billion years to 570 million years ago is called the Precambrian eon, meaning the period before the fossil record began. It turns out, however, that early geologists who studied the Precambrian eon were unable to recognize early, primitive fossils, and in fact life first appeared on Earth as early as 3.5 billion years ago!
Here’s how a well-prepared test taker would approach a question about geology on Test Day.
| Question | Analysis |
|---|---|
| Studies of the Precambrian eon have shown that life first appeared on Earth approximately | Step 1: The Precambrian eon allowed scientists to narrow down the time frame for life formation. |
| Step 2: The Precambrian eon ranges from 4.6 billion to 570 million years ago. | |
| Step 3: The correct number should be within the range of the Precambrian eon. | |
| (A) 570 million years ago. (B) 3.5 billion years ago. (C) 4.6 billion years ago. (D) 5 billion years ago. |
Step 4: Based on the prediction above, select answer choice (B). |
Now try your hand at the problem below.
Choice (C) is correct. Pumice comes from volcanic rock when it cools very quickly on the surface. The small air bubbles formed in it are visible to the naked eye. The bubbles give it its lightweight feel and increase its usefulness in many everyday products.