Geology For Dummies. Alecia M. Spooner
7) with almost no heavy metals. (The astronauts collected surface rocks and also drilled core samples.)This finding suggests that the moon did not form at the same time as the earth or from the same cloud of gas that formed the earth. (I discuss the earth’s formation in Chapter 18.) Rather, the composition of the moon rocks suggests that the moon was created by materials from the surface of the earth, long after the heavy metal core was covered by mantle and crust materials.While this idea is still being hotly debated, scientists have been testing the hypothesis that the materials that formed the moon were removed from the earth by a giant impact. Another planet-type body, possibly as large as Mars, may have crashed into Earth, vaporizing some of its crustal and upper mantle materials, leaving them to orbit the earth until they came together forming the sphere now known as the moon.Wouldn’t there be a giant crater as evidence of such an impact? At the time, yes, but this was 4.5 billion years ago, and since then much about the earth and its surface has dramatically changed. The best evidence scientists have is the composition of the moon rocks. Using isotope studies, geologists continue to test the giant-impact hypothesis of moon formation.
Lithosphere: The uppermost portion of the mantle is attached to the underside of the earth’s crust, and together they make up the lithosphere. This portion of the mantle is approximately 100 kilometers (62 miles) thick, very rigid, and brittle — it breaks and cracks rather than bends or flows. Physically, this part of the mantle is just like the rocks of the crust it is attached to (a brittle solid). But this layer is still mantle because of its mineral composition. In other words, when scientists classify the earth’s layers based on their composition, they separate the upper portion of the mantle from the crust because they are composed of different minerals. But when scientists classify the earth’s layers based on physical properties, both the uppermost part of the mantle and the earth’s crust are part of the brittle lithosphere and different from the flowing solid mantle below.
It’s only skin deep: The earth’s crust
The boundary between mantle rock and crustal rock in the lithosphere is labeled the Moho discontinuity, named after the scientist who discovered it, Andrija Mohorovičić. This boundary, illustrated in Figure 4-4, is where the composition of rocks changes from the more dense mantle rocks to the much lighter crustal rocks, which are composed primarily of silica.
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.