Appendix B

The Rock Cycle: Product of a Dynamic Planet

B-1

All ecosystems, from oceans to deserts, develop and function on a geologic foundation of rock. However, as we shall see, this geologic foundation is not static but is renewed over very long timescales in a process called the rock cycle. We begin our exploration of the rock cycle with a review of Earth structure.

Earth can be divided into three major layers: core, mantle, and crust (Figure B.1). The crust, which is made up of relatively low-density rocks and is the most superficial of Earth’s layers, forms the continents and ocean floor. Continental crust is 20 to 70 kilometers (12 to 43 miles) thick, whereas the crust making up the ocean floor is only about 8 kilometers (4 miles) thick. Immediately below the crust is the mantle, consisting of rocks of higher density than those that make up the crust and extending to about 2,900 kilometers (1,800 miles) below Earth’s surface. The mantle, which constitutes the largest portion of Earth’s volume, can be divided into upper and lower layers, again differing in density.

The uppermost layer of the upper mantle consists of rigid, relatively brittle rocks, which, along with Earth’s crust, make up the lithosphere. In contrast, most of the upper mantle has a consistency similar to plastic, and a small portion of it is liquid. This softer portion of the upper mantle is called the asthenosphere. The lower mantle is composed of solid rock of higher density than the upper mantle. The core of the planet, which extends from about 2,900 kilometers to 6,370 kilometers (1,800 to 3,955 miles) in depth at the center of the planet, is also two-layered. The core appears to consist of an outer liquid layer of molten iron and nickel and a solid, inner iron–nickel center.

Earth is stratified not only by density but also by temperature and pressure. Temperature and pressure are lowest at Earth’s surface and increase gradually with depth below the surface, until the highest temperatures and pressures are reached at the core. Geologists estimate that the temperature at the solid inner core may be nearly 8,000°C, whereas the temperature of the liquid outer core may be slightly less than 5,000°C. If temperatures increase with depth below the surface of Earth, how can the upper mantle and upper core be liquid while the lower mantle and inner core are solid? Earth scientists explain this apparent contradiction by pointing out that the higher pressures at greater depth keep materials solid at higher temperatures. The physical structure of Earth forms the basis for important Earth processes. One of the most consequential of those processes is plate tectonics.

B-2

The theory of plate tectonics explains the mechanisms responsible for some of Earth’s most dramatic features and processes, including the formation of ocean basins, continents, and mountains, as well as the geographic distributions of earthquakes and volcanic activity. Because it can account for so many geologic phenomena, the theory of plate tectonics unifies geology in a way similar to how the theory of evolution unifies biology.

There is abundant evidence that the surface of Earth is not one continuous layer of crust overlying the mantle. Rather, Earth’s surface is divided into several plates of lower-density lithosphere that “float” on the more dense and plastic asthenosphere (Figure B.2). The plates, which vary in thickness from about 80 to 120 kilometers (49 to 74 miles), consist of a surface layer of crust and a deeper layer of the upper mantle. In general, continental plates are thicker than oceanic plates.

B-3

Geologists propose that convective currents in the asthenosphere drive the movements of the plates. These convection currents form as hotter material deeper within the mantle rises toward the surface of Earth. This material rises because it has a lower density than the cooler material above it. However, as the lower-density material rises and moves across the upper layers of the asthenosphere, it cools and, as it does so, its density increases. Eventually, the density of this moving mass of material within the upper asthenosphere increases enough that it sinks, creating a convection cell, a pattern of circulation caused by differing temperatures in a semi-liquid, liquid, or gas.

B-4

The convection cells within the asthenosphere pull the plates along, putting them in motion. These moving plates are thus literally on a collision course with each other. The fastest ones move only a few centimeters per year, but tremendous forces are involved—forces sufficient to move entire continents and to form and move the floor of entire ocean basins. These same forces raise great mountain ranges, such as the Himalayas and the Rockies, create deep ocean trenches, and produce earthquakes.

Because oceanic plates are of higher density than continental plates when the two plate types collide, the oceanic plate generally moves under the continental plate in a process known as subduction. Collisions between oceanic plates and continental plates form deep trenches in the subduction zone and active volcanoes along the continental margin.

During Earth’s estimated 4.5-billion-year history, the forces of plate tectonics have changed the face of the planet many times. Mountains have been built and worn down by water, wind, and ice. Gigantic continents have formed and broken apart. Submarine volcanic activity has formed oceanic plates that were later driven back into the mantle by subduction and their materials recycled. These various phenomena and processes can be viewed as part of a large-scale process called the rock cycle.

At the center of most Earth processes are rocks, naturally occurring solid materials formed as a mixture of one or more minerals. Igneous rock forms as molten rock cools and solidifies. Sedimentary rock forms either as rock fragments deposited by water, wind, or ice are cemented together and solidify or by chemical precipitation, a process by which a dissolved substance goes from dissolved to solid state in a body of water, such as a pond or sea, and settles to the bottom. Metamorphic rock forms when any type of rock is transformed by exposure to high heat and pressure. During Earth’s history, rocks have been made and remade many times by the rock cycle (Figure B.3).