Chapter Outline

Earth Science

Earth Science is the study of the physical properties of Earth. Earth science seeks to understand how Earth formed and how Earth continues to evolve.

There are many fields of study in Earth science. The most familiar are listed below:

• geology—the study of the composition and structure of Earth itself, including the study of volcanoes and earthquakes
• meteorology—the study of the atmosphere, weather patterns, and long-term climate
• oceanography—the study of Earth's oceans and the effects of sunlight on ocean temperatures and ocean currents
• paleontology—the study of prehistoric animals and plant life through the study of fossils
• geochemistry—the study of the distribution of chemical elements in Earth's crust, mantle, and core
• ecology—the study of organisms in relationship to environmental factors such as light, heat, solar radiation, moisture, and wind
• environmental science—the study of how human beings interact with the environment, both living and nonliving

The Early Earth

The Earth formed about 4.6 billion years ago, most likely from the collapse of a solar nebula—a cloud of interstellar gas and dust.

• The hot interior of the early Earth was due to gravitational pressure and to the heating effects of radioactive elements in the interior. These heating effects continue at the present time.
• Volcanic eruptions on Earth's surface brought carbon monoxide, carbon dioxide, water vapor, and methane out of the crust and into the atmosphere. Carbon, hydrogen, and oxygen were most likely added by impacting meteorites and comets.
• Water vapor from volcanic eruptions and ice from comets were most likely the two main sources of water on early Earth. By 4 billion years ago, Earth had cooled enough so that rainwater would stay on the surface. The early Earth was totally covered by ocean water.
• As the crust developed, heavier elements sank into the interior and lighter elements rose to the surface. The light surface elements eventually cooled enough to harden and form the continents and ocean bottoms. This crust now floats on a hot, liquid mantle below.

The Earth Today

The Structure of Earth

• Earth's surface, or crust, is about 71 percent water and 29 percent land.The land is divided into 7 continents and many islands.
• The mantle, the supporting structure beneath the crust, is about 1,800 miles thick.
• The core is divided into a solid inner core—about 800 miles in radius—and a liquid outer core—about 1,400 miles thick.

Energy in Today's Earth

• Earth's hot interior energy comes from two main sources: gravitational energy from Earth's formation and radioactive elements.
• Earth's main external energy source is the Sun. Electromagnetic radiation, part of which is visible sunlight, flows through the vacuum of space from the Sun to Earth.
• A geochemical cycle is the movement of an element from one chemical storehouse to another.

Plate Tectonics

• Pangea is the name given to the single supercontinent that may have existed 200 million years ago.
• The slow movement of continents over Earth's surface is known as continental drift. Continents continue to move about two inches each year. At this rate, continents could move more than 6,000 miles in 200 million years.
• Plate tectonics is the theory that Earth's is made up of about 12 large plates and several small plates. These crustal plates float on the liquid mantle and slowly move over Earth's surface. At the boundary between crustal plates, several disruptive events occur as plates collide into one another.

Earth's Atmosphere

The atmosphere is the blanket of air that surrounds Earth. Gravity holds the atmosphere next to Earth's surface and keeps it from escaping into space. Earth's atmosphere plays several roles that makes life possible on Earth:

• The atmosphere provides oxygen, nitrogen, and carbon dioxide, the three gases necessary for life.
• The atmosphere protects human beings from harmful ultraviolet sunlight.
• The atmosphere gives us weather, regulating the amount of sunlight that reaches Earth, and controlling the escape of heat energy into space.

The atmosphere is divided into several layers.

• troposphere—layer closest to ground
• stratosphere—just above the troposphere
• mesosphere—just above the stratosphere
• thermosphere—outermost layer of the atmosphere, rising to an altitude of about 200 miles

Earth's Seasons

There are two reasons that Earth has seasons:

• Earth rotates (turns) on its axis, completing a turn each 24 hours. The point where the rotation axis passes through Earth's surface in the Northern Hemisphere is known as the North Pole. The point where the rotation axis passes through Earth's surface in the Southern Hemisphere is the South Pole.
• Earth's rotation axis is tilted with respect to Earth's direction from the Sun. During summer months, the Northern Hemisphere is tilting toward the Sun, receiving more direct sunlight. During winter months, the Northern Hemisphere is tilted away from the Sun, receiving less direct sunlight.

Earth's Weather

Weather occurs because of changes that take place in Earth's atmosphere. Weather is usually thought of as having four characteristics:

• temperature—a measure of the warmth of the air
• humidity—a measure of water vapor in the air
• wind—a measure of air movement
• air pressure (barometric pressure)—a measure of the weight of the atmosphere

The Changing Earth

Weathering is the breaking down of rock into smaller pieces by natural processes:

• physical weathering—the breaking of rocks without the occurrence of any chemical changes in the rocks
• chemical weathering—the softening and crumbling of rocks brought about by chemical changes

Erosion is the movement of rock fragments over the surface of Earth:

• gravity erosion—the falling of rock fragments due to gravity
• wind erosion—the movement of rock fragments due to wind
• water erosion—the movement of rock fragments due to water

A fossil is a trace remain of an organism from a past geological time. There are many types of fossils and much information provided by them:

• the atmospheric conditions at the time of the organism's life
• evidence of continental drift