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waves. Tsunamis, or tidal waves, are different. They result

from underwater earthquakes, volcanic eruptions, or

landslides, not wind.

Energy from the Core

Another source of Earth’s energy comes from Earth’s

core. We distinguish four main layers of Earth: the inner

core, the outer core, the rocky mantle, and the crust. The

inner core is a solid mass of iron with a temperature of

about 7,000° F. Most likely, the high temperature is

caused by radioactive decay of uranium and other

radioactive elements. The inner core is approximately

1,500 miles in diameter. The outer core is a mass of

molten iron that surrounds the solid inner core. Electri￾cal currents generated from this area produce the earth’s

magnetic field. The rocky mantle is composed of silicon,

oxygen, magnesium, iron, aluminum, and calcium and is

about 1,750 miles thick. This mantle accounts for most

of the Earth’s mass. When parts of this layer become hot

enough, they turn to slow moving molten rock, or

magma. The Earth’s crust is a layer from four to 25 miles

thick, consisting of sand and rock.

The upper mantle is rigid and is part of the litho￾sphere (together with the crust). The lower mantle flows

slowly, at a rate of a few centimeters per year. The crust

is divided into plates that drift slowly (only a few cen￾timeters each year) on the less rigid mantle. Oceanic

crust is thinner than continental crust.

This motion of the plates is caused by convection

(heat) currents, which carry heat from the hot inner

mantle to the cooler outer mantle. The motion results in

earthquakes and volcanic eruptions. This process is

called plate tectonics.

Tectonics

Evidence suggests that about 200 million years ago, all

continents were a part of one landmass, named Pangaea.

Over the years, the continents slowly separated through

the movement of plates in a process called continental

drift. The movement of the plates is attributed to con￾vection currents in the mantle. The theory of plate tec￾tonics says that there are now twelve large plates that

slowly move on the mantle. According to this theory,

earthquakes and volcanic eruptions occur along the lines

where plates collide. Dramatic changes on Earth’s land￾scape and ocean floor are caused by collision of plates.

These changes include the formation of mountains and

valleys.

Geochemical Cycles

Water, carbon, and nitrogen are recycled in the bios￾phere. A water molecule in the cell of your eye could have

been, at some point, in the ocean, in the atmosphere, in

a leaf of a tree, or in the cell of a bear’s foot. The circula￾tion of elements in the biosphere is called a geochemical

cycle.

Water

Oceans cover 70% of the Earth’s surface and contain

more than 97% of all water on Earth. Sunlight evapo￾rates the water from the oceans, rivers, and lakes.

Living beings need water for both the outside and the

inside of their cells. In fact, vertebrates (you included)

are about 70% water. Plants contain even more water.

Most of the water passes through a plant unaltered.

Plants draw on water from the soil and release it as vapor

through pores in their leaves, through a process called

transpiration.

Our atmosphere can’t hold a lot of water. Evaporated

water condenses to form clouds that produce rain or

snow on to the Earth’s surface. Overall, water moves

from the oceans to the land because more rainfall reaches

the land than is evaporated from the land. (See the figure

on the next page.)

Carbon

Carbon is found in the oceans in the form of bicarbon￾ate ions (HCO3

−), in the atmosphere, in the form of car￾bon dioxide, in living organisms, and in fossil fuels (such

as coal, oil, and natural gas). Plants remove carbon diox￾ide from the atmosphere and convert it to sugars

through photosynthesis. The sugar in plants enters the

food chain, first reaching herbivores, then carnivores,

and finally scavengers and decomposers. All these organ￾isms release carbon dioxide back into the atmosphere

when they breathe. The oceans contain 500 times more

carbon than the atmosphere. Bicarbonate ions (HCO3

–)

settle to the bottoms of oceans and form sedimentary

rocks. Fossil fuels represent the largest reserve of carbon

on Earth. Fossil fuels come from the carbon of organisms

that had lived millions of years ago. Burning fossil fuels

releases energy, which is why these fuels are used to

power human contraptions. When fossil fuels burn, car￾bon dioxide is released into the atmosphere.

Since the Industrial Revolution, people have increased

the concentration of carbon dioxide in the atmosphere

–EARTH AND SPACE SCIENCE–

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