What is geology? Geology is the study of Earth—its history, its composition and internal structure, and its surface features.
How do geologists study Earth? Geologists, like other scientists, use the scientific method. They develop and test hypotheses, which are tentative explanations for natural phenomena based on observations and experiments. They share their data and test one another’s hypotheses. A coherent set of hypotheses that have survived repeated challenges constitutes a theory. Hypotheses and theories can be combined into a scientific model that represents a natural system or process. Confidence grows in those hypotheses, theories, and models that withstand repeated tests and are able to predict the results of new observations or experiments.
What is Earth’s shape? Earth’s overall shape is a sphere with an average radius of 6370 km that bulges slightly at the equator and is slightly squashed at the poles due to the planet’s rotation. Its topography varies by about 20 km from the highest point on its surface to the lowest point. Its elevations fall into two main groups: 0 to 1 km above sea level over much of the continents and 4 to 5 km below sea level for much of the ocean basins.
What are Earth’s major layers? Earth’s interior is divided into concentric layers of different compositions separated by sharp, nearly spherical boundaries. The outer layer is the crust, made up mainly of silicate rock, which varies in thickness from about 40 km in the case of continental crust to about 7 km for oceanic crust. Below the crust is the mantle, a thick shell of denser silicate rock that extends to the core-mantle boundary at a depth of about 2890 km. The core, which is composed primarily of iron and nickel, is divided into two layers: a liquid outer core and a solid inner core, separated by a boundary at a depth of 5150 km. Jumps in density between these layers are caused primarily by differences in their chemical composition.
How do we study Earth as a system of interacting components? When we try to understand a complex system such as the Earth system, we find that it is often easier to focus on its subsystems (which we call geosystems). This textbook focuses on three major global geosystems: the climate system, which involves interactions among the atmosphere, hydrosphere, cryosphere, biosphere, and lithosphere; the plate tectonic system, which involves interactions among Earth’s solid components; and the geodynamo, which involves interactions within Earth’s core. The climate system is driven by heat from the Sun, whereas the plate tectonic system and the geodynamo are driven by Earth’s internal heat.
What are the basic elements of plate tectonics? The lithosphere is broken into about a dozen large plates. Driven by convection in the mantle, these plates move over Earth’s surface at rates of a few centimeters per year. Each plate acts as a rigid unit riding on the ductile asthenosphere, which is also in motion. Hot mantle material rises at boundaries where plates form and separate, cooling and becoming more rigid as it moves away. Eventually, most of it sinks back into the mantle at boundaries where plates converge.
What are some major events in Earth’s history? Earth formed 4.56 billion years ago. Rocks as old as 4.3 billion years have survived in Earth’s crust. Liquid water existed on Earth’s surface by 3.8 billion years ago. Rocks about 3.5 billion years old show evidence of a magnetic field, and the earliest evidence of life has been found in rocks of the same age. By 2.7 billion years ago, the oxygen content of the atmosphere was rising because of oxygen production by early organisms, and by 2.5 billion years ago, large continental masses had formed. Animals appeared suddenly about 600 million years ago, diversifying rapidly in a great evolutionary explosion. The subsequent evolution of life was marked by a series of extreme events that killed off many species, allowing new species to evolve. A dramatic example was the impact of a large meteorite 65 million years ago. Our species, Homo sapiens, first appeared about 200,000 years ago.
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