in two types: continental and oceanic. These two types of crust vary in thickness and are composed of slightly different materials:
Continental crust: The crust that composes the continents is pretty thick. At its thinnest sections, continental crust is about 12 miles thick; at its thickest sections (where there are mountains), it is up to about 45 miles thick. The rocks that compose the continental crust are primarily granites (see Chapter 7).
Oceanic crust: This crust, which lies under the earth’s oceans, is thin — only about 5 miles thick. This type of crust is composed of dark, dense silicate rocks such as basalt and gabbro (see Chapter 7). Oceanic crust is relatively young, being created even now from the eruption of molten rock along ridges in the sea floor (see Chapter 10).
DRILLING FOR THE MOHO
In 1909, Andrija Mohorovičić, a Croatian seismologist, noticed that earthquake waves increased their speed as they moved through the lower part of the earth’s rigid lithosphere. He interpreted (correctly) that this meant the lower portion of Earth’s lithosphere is made of a different and slightly denser material than the outer portion. The line where the material in Earth’s lithosphere changes from the crustal rock to the mantle rock is named the Moho line or the Mohorovičić Discontinuity.
For decades scientists have attempted drilling deep into the earth, seeking to reach the Moho line between the mantle rock and crustal rock within the lithosphere. In 2005, a team of scientists with the Integrated Ocean Drilling Program (IODP) came close. The core they drilled near the mid-Atlantic ridge in the Atlantic Ocean reached a depth of 1,416 meters (4,644 feet) into the oceanic crust. But the rocks they recovered appear to be made of crustal rock materials rather than the mantle rocks they were seeking. The researchers concluded that they were close to crossing the Moho discontinuity boundary and plan to attempt drilling a new hole.
Every inch closer to the Moho provides scientists with new information about the composition and formation of the earth’s outermost layer and offers clues to the internal structure of Earth’s lithosphere.
Part 2
Elements, Minerals, and Rocks
IN THIS PART …
Discover the chemistry of elements and compounds.
Learn the basics of minerals.
Take a crash course in rocks.
Chapter 5
It’s Elemental, My Dear: A Very Basic Chemistry of Elements and Compounds
IN THIS CHAPTER
To understand earth processes such as the formation of rocks, it helps to understand some of the basic concepts of chemistry. The science of chemistry explores and describes the properties of substances — gas, liquid, or solid — and explains how and why different substances interact with each other.
In this chapter, I explain that all earth materials are made of atoms, and I show how these atoms interact with one another to create the observable characteristics of rocks and geologic features in the world around you.
The Smallest Matter: Atoms and Atomic Structure
Atoms themselves are composed of smaller, subatomic particles called neutrons, protons, and electrons. Figure 5-1 is a diagram of atomic structure, including the location of each different subatomic particle type. Protons and neutrons are located in the nucleus at the center of the atom. In each atom, the electrons surround the nucleus, organized into orbital shells. The innermost orbital shell of any atom contains no more than two electrons; the second orbital shell contains no more than eight; and each of the outer shells, while chemically stable with eight electrons, can hold more.
FIGURE 5-1: The parts of an atom.
Getting to know the periodic table
The periodic table of elements lists the known elements in order of their atomic number, which is their number of protons. Each square on the periodic table provides you with all the information you need to know about that element and how it will interact with other elements. Figure 5-2 illustrates what the different numbers for each element on the periodic table represent:
FIGURE 5-2: The parts of one square of the periodic table of elements.
Atomic mass number: The atomic mass number of an element is the total number of protons and neutrons in its nucleus.
Atomic number: The atomic number of an element is the number of protons in its nucleus.
Group number: The group number tells you how many electrons in the atom are located in the outermost orbital shell and are, therefore, available to bond it to other atoms. For example, elements in Group I have one electron in the outer electron shell, and Group II elements have two electrons in the outer electron shell. The group number for each element may help you understand why some elements, such as Magnesium (Mg) and Calcium (Ca), which are both in Group II, react in similar ways during rock formation and other geologic processes.
Symbol: The letters on the periodic table are the symbols for each element. These symbols are a shorthand so that
