Chapter Two A Living Planet

Space and the Universe If you look at the sky on a clear night, you can see thousands of stars. With a telescope you can see millions. Beyond that exist trillions. All of these stars are part of the universe. The universe is made up of all existing things, including space and Earth.

Space and the Universe Most astronomers believe that the universe has been expanding continuously to an unimaginable size.

The Solar System The Sun and the group of bodies that revolve around it are called the Solar System.

Earth’s Zones Scientist have identified four major zones in Earth’s interior. The planet’s center is like a nuclear furnace, where decaying radioactive elements generate heat. Earth’s center, or core, both temperatures and pressures are very high. The core is divided into inner and outer layers. The inner core is solid. The outer core is mostly dense liquid metal, mainly iron and nickel.

Earth’s Core Beyond the core is the mantle, the zone that has most of Earth’s mass. The upper-most layer is the crust. Although it is up to 25 miles thick, the crust is comparatively thin.

Magma Huge currents carry heat from the core through the mantle to the crust. Liquid rock within the earth is called magma.

Lava When liquid rock spills out into the surface it is called lava. Magma erupts from vents called volcanoes.

The Earth System OBJECTIVES: Describe Earth’s four spheres. Describe how earth’s environment is unique in the solar system.

Key Terms Atmosphere Lithosphere Hydrosphere Biosphere Environment

The Earth’s Four Spheres The geographers divide the earth system into four major parts. Each part is called a sphere because it occupies a shell around the planet. These four spheres are the atmosphere, lithosphere, hydrosphere, and biosphere.

Atmosphere The atmosphere is the envelope of gases that surrounds Earth. It is the least dense and outermost sphere, extending from Earth’s surface to space.

Atmosphere Earth’s gravity holds the atmosphere around the planet. About 78% of Earth’s atmosphere is a gas called nitrogen, and about 21% oxygen. The rest is made up of carbon dioxide ozone, and other gases. These gases and water vapor sustain life on Earth. The atmosphere also protects the planet from the sun’s harmful radiation.

The atmosphere also protects the planet from the Sun’s harmful radiation.

Lithosphere The lithosphere is the solid crust of the planet. This outer crust includes rocks and soil. It forms Earth’s continents, islands, and ocean floor.

Hydrosphere The hydrosphere is all of the Earth’s water. Water covers about 70% of the Earth’s surface.

The hydrosphere includes water in liquid, solid, and gaseous forms The hydrosphere includes water in liquid, solid, and gaseous forms. Liquid water is found in the oceans, lakes, rivers, and underground. Clouds and fog are made up of liquid droplets. Solid water, or ice, is found on both land and sea.

The biosphere is the part of Earth that includes all life forms The biosphere is the part of Earth that includes all life forms. It includes all planets and animals. The biosphere overlaps the other three spheres. It extends from deep ocean floors to high in the atmosphere.

Four Spheres Earth’s four major spheres are all interconnected. Each one affects the other. The hydrosphere supplies people with water. It is also home for plants and animals. The hydrosphere effects the lithosphere when rain breaks up rocks and washes them away. It also constantly interacts with the atmosphere, causing clouds and rain.

Section Two The Hydrosphere

Water is necessary for life, but 97% of the world’s water is too salty for most uses. The process of removing the salt in water is called desalinization. Countries dominated by deserts use this process to get freshwater, but it is very expensive. Less than 3% of the world’s water is drinkable, and most of that is frozen in glaciers and the ice caps.

Hydrologic Cycle The amount of water on Earth stays much the same over time. But its physical state is always changing from gas to liquid to solid. The movement of water through the hydrosphere is called the hydrologic cycle. Solar energy, winds, and gravity drive the cycle. When water is heated by solar energy it may change to vapor. This is called evaporation.

Hydrologic Cycle Most water in the atmosphere has evaporated from the ocean. As water vapor rises, it cools and forms tiny droplets in a process called condensation. These droplets join to form clouds. They then grow to become rain drops heavy enough to fall to Earth.

Precipitation Water falling to Earth (such as rain, snow, or hail) is called precipitation. If it falls on land, it is stored in plants, in a river, a lake, or below the ground. Surface water either flows to the sea in a river or it evaporates again into the atmosphere.

The Hydrologic Cycle

As precipitation falls on continents and islands, it flows down hills and mountains toward the lowlands and coasts. The first and smallest streams from this runoff are called headwaters.

As these headwaters join, they form larger streams, and farther downstream they eventually form rivers. Any smaller stream or river that flows into a larger river is called a tributary. In the U. S. the Arkansas, Missouri, and Ohio Rivers are major tributaries of the Mississippi River. Tributary

Continental Shelves The continental surface extends under the shallow ocean water around the continents. These areas are called continental shelves. At the edges of the continental shelves, the seafloor drops steeply down to the abyssal plain.

Plate Tectonics The theory of plate tectonics explains how forces within the planet create landforms. The tectonics theory views Earth’s crust as divided into more than a dozen rigid, slow moving plates. The plates can be compared to the cracked shell of a hard boiled egg.

Pangaea Scientist believe that about 200 million years ago all of the modern continent were part of one supercontinent called Pangaea.

The plates slowly move across the upper mantle, usually less than an inch per year. This process is called continental drift. Along the plate boundaries, the crust is subject to stresses that lead to melting, bending, and breaking.

Volcanoes often form long rows and signal that a plate boundary is nearby. Earthquakes take place when tectonic forces cause masses of rock inside the crust to break.

Plate Movement Three types of movement at plate boundaries are possible. First, the plates can move apart, or spread. The second plate movement occurs when plates collide. The third way plates move is laterally when they slip past each other.

Divergent Plate Movement Long ago the Earth’s history the crust sorted itself into layers of different kinds of rocks. The lower layer, made of heavier rock, is found on the ocean floor. Lying on top is a patchy layer of lighter rock. This layer makes up the continents.

Earthquakes are also common near plate boundaries.

Subduction Zones When two plates on the ocean floor collide, one slides underneath the other. This plate boundary is called a subduction zone, and the deep valley marking the plate collision is called a trench. The plate sliding downward generates heat as it grinds against the plate above it. This heat may produce a row of volcanoes which rise enough to become islands.

Convergent Plate Movement Convergent plate boundaries are locations where lithospheric plates are moving towards one another. The plate collisions that occur in these areas can produce earthquakes, volcanic activity, and crustal deformation.

Transform Plate Movement Transform plate boundaries are locations where two plates slide past one another. The fracture zone that forms a transform plate boundary is known as a transform fault. Most transform faults are found in the ocean basin and connect offsets in the mid-ocean ridges.

Plates Moving Laterally When plates move laterally past each other, long fractures develop along the edges of both plates. The pressure along these boundaries is seldom uniform. While squeezing produces low mountains, a little spreading generates broad valleys. Earthquakes can be frequent in these areas. The San Andreas Fault is and example of this.

Rift Valleys A few spreading plate boundaries lie under continents. In these places, the crust stretches until it breaks, forming rift valleys. The biggest rift valleys are in eastern Africa.

Rift Valley This is an example of a rift valley in Kenya. Note the recessive line due to crust separation. Would this be a suitable location build permanent structures?

Weathering and Erosion Rocks break and decay over time in a process called weathering. Weathering is usually slow and difficult to detect. However, even the hardest rock will eventually wear down.

Chemical Weathering Chemical processes cause some weathering. Substances in the air and water react with the rocks, creating acids and slowly dissolving the rock.

Weathering Weathering is also caused by physical processes that break rocks into smaller pieces. In high mountains repeated freezing and thawing of water inside a cracked rock can cause it to break even more. The roots of trees can pry rocks apart. Weathering breaks rocks into smaller particles of gravel, sand, and mud called sediment.

Erosion Along with weathering, the other process changing landforms on earth’s surface is erosion. Erosion is the movement of surface material from one location to another.

Erosion Water, wind, and ice can cause erosion. What types of erosion are at work?

Water, Waves, and Wind Water is the most important force of erosion. Rainfall can cause rapid erosion where few plants protect the ground. Water erosion can begin as tiny channels on hillsides. If erosion is severe, a channel may grow into a gully. Running water can even carve deep canyons, such as the Grand Canyon.

Water Erosion Wave action is another powerful force of erosion. During a storm, waves can tear away tons of beach sand within a few hours. Waves can slowly change shorelines over many years.

Wind Erosion Wind is another force that causes erosion. Plants protect most land surfaces from wind action. However, in dry lands, on beaches, and in places where people or animals have destroyed the vegetation, wind can cause significant erosion.