Chapter 19 Overview

Understandings about the earth come from many branches of science, each with specialized scientists who consider different scales of space and time. Astronomers study the motions of Earth in space, its place in the universe, and how the earth formed and evolved over time. Oceanographers are primarily concerned with the composition and motions of the earth's oceans. Meteorologists are concerned with the composition of the atmosphere and how it changes over time. Geologists are concerned primarily with the rocks, landscapes, and the history of the earth through time. Each of these branches of science has its own subdisciplines. For example, there are geologists who study rocks (petrology), geologists who study earthquakes and the earth's interior (seismology), and geologists who study the early history of life on the earth (paleontology). Together, all the scientists who study the earth are known as earth scientists.

The scope of the earth sciences has been expanded in recent years to include environmental sciences. Environmental sciences are concerned with the environmental conditions on the earth that affect living organisms. Earth science is a natural home for environmental sciences since studies of environmental conditions include the atmosphere and the oceans of the earth, as well as the land surface.

The separation of the earth sciences into independent branches and subdisciplines was traditionally done for convenience. This made it easier to study a large and complex Earth. In the past, scientists in each branch studied their field without considering the earth as an interacting whole. Today, most earth scientists consider changes in the earth as taking place in an overall dynamic system. The parts of the earth's interior, the rocks on the surface, the oceans, the atmosphere, and the environmental conditions are today understood to be parts of a complex, interacting system with a cyclic movement of materials from one part to another.

How can materials cycle through changes from the interior, to the surface, and to the atmosphere and back? As you will see in this and the following chapters, the answer to this question is found in the unique combination of fluids of the earth. No other known planet has Earth's combination of (1) an atmosphere consisting mostly of nitrogen and oxygen, (2) a surface that is mostly covered with liquid water, and (3) an interior that is partly fluid, partly semifluid, and partly solid. Earth's atmosphere is unique both in terms of its composition and in terms of interactions with the liquid water surface (Figure 19.1). These interactions have cycled materials, such as carbon dioxide, from the atmosphere to the land and oceans of the earth. The internal flow of rock materials, on the other hand, produces the large-scale motion of the earth's continents and the associated phenomena of earthquakes and volcanoes. Volcanoes cycle carbon dioxide back into the atmosphere and the movement of land cycles rocks from the earth's interior to the surface and back to the interior again. Altogether, the earth's atmosphere, liquid water, and motion of its landmasses make up a dynamic cycling system that is found only on the planet Earth.

Earth also seems to be unique because there is life on Earth, but apparently not on the other planets. The cycling of atmospheric gases and vapors, waters of the surface, and flowing interior rock materials sustain a wide diversity of life on the earth. There are some two million different species of plants and animals. Yet, there is no evidence of even one species of life existing outside the earth. The existence of life on the earth must be directly related to its unique, dynamic system of interacting fluids.

This and the chapters that follow are about the dynamic nature of planet Earth. This chapter is concerned with earth materials. The remaining chapters will consider the internal structure of the earth, the large-scale motion of the continents of the earth, and changes that occur on the surface, in the atmosphere, and in the oceans.