Download Four main kinds of changes affect the Earth`s surface: (1) weathering

Four main kinds of changes affect the Earth's surface: (1) weathering, (2) erosion, (3)
mass movement, and (4) changes in the Earth's crust.
Weathering is the breaking up of rocks by such agents as water, ice, chemicals, growing
plants, and changing temperature. Soil is an important product of weathering. Soil
consists of bits of weathered rock mixed with living things and the remains. Geologists
speak of two types of weathering: (1) physical weathering, which is also called
mechanical weathering, and (2) chemical weathering.
Physical weathering breaks rock into pieces. One of the main causes of physical
weathering is the formation of ice in cracks within rocks. First, water soaks into the
cracks. Then, if the temperature falls low enough, the water near the rock's surface
freezes and seals in the water that is deep in the cracks. As the rest of the water freezes, it
expands in the cracks and may push hard enough to split the rock. Similarly, tree roots
may grow through cracks in rocks and cause the rocks to split.
Chemical weathering affects the substances that make up rocks and soil. One of the main
causes of chemical weathering is the dissolving action of water. Rain, streams, and
seawater dissolve minerals from rocks, causing the rocks to crumble. For example, water
dissolves the mineral feldspar from granite, leaving grains of quartz, a mineral that forms
sand.
The effects of erosion
Erosion is a combination of weathering and the movement of weathered material from
one place to another. Eroded material generally moves from high places to low places on
the Earth's surface. For example, erosion wears away rock from mountainsides and
carries it down into valleys. Water, glaciers, and wind are three important transporting
agents of erosion.
Erosion by water combines the weathering action of water with water's ability to move
pieces of rock. Rainwater drains from the land into streams that flow downhill. The
moving water cuts into the land as it wears away soil and rock. The faster the stream
flows, the more it wears away the land around it. Bits of rock picked up by the stream
add to the grinding action. Soft rock wears away first, and then hard rock. Sometimes
this action leaves towering masses of hard rock standing alone on a plain. The rock
remains long after the disappearance of the water that wore away soft surrounding rock.
The Grand Canyon of the Colorado River is a spectacular example of water's eroding
power. There, after millions of years, the river has cut 1 mile (1.6 kilometers) deep into
the Earth.
When rivers reach the sea, they leave behind the materials they picked up while flowing
over the land. At the mouths of some rivers, this material forms a triangular-shaped
deposit called a delta. At the mouths of other rivers, the materials are swept into the
ocean. All along the seashore, water gradually changes the shape of the land. Waves and
tides wear away the rocky shore and create sand bars, beaches, cliffs, and headlands.
Water moving underground also changes the land. Spouts of water called geysers shoot
out of the Earth and carry dissolved minerals to the surface. Underground water
dissolves limestone and other rock, and forms caves deep in the ground.
Changes made by mass movement
Mass movement is the slipping of large amounts of rock and soil, as occurs in a landslide
or a mud slide. Most landslides and other forms of rapid mass movement take place
along steep-sided hills and mountains. Slow movement, such as the gradual downhill
creep of soil, takes place unnoticed on gently sloping land. Weathering and erosion help
loosen large chunks of Earth and start them sliding downhill. Earthquakes also
sometimes cause sections of hills and mountains to break off and slide down.
Mass movement may produce a variety of effects. For example, a landslide may fall
across a river, damming the water and causing it to form a lake. The slipping of soil
down the sides of a river valley gradually widens the valley and makes the sides slope
more gently.
Changes in the Earth's crust can be explained by the theory of plate tectonics. According
to this theory, the Earth's crust and upper mantle consist of about 30 rigid plates of
various sizes. A slow, continuous movement of these plates folds and reshapes the
Earth's crust and builds mountains. It also causes earthquakes and volcanic eruptions.
But many movements of the land occur so slowly that they are unseen or hardly noticed.
The Grand Canyon of the Colorado River is a spectacular example of water's eroding power. After
millions of years, the river has cut 1 mile (1.6 kilometers) deep into the Earth.
National Park Service photo
Exploration of the planets by space probes has expanded our understanding of the solar
system. Modern theories about the Earth's origin look at how the Earth fits into the solar
system, the Galaxy, and the universe as a whole. Most scientists agree that the Earth was
probably formed at the same time as the rest of the solar system.
Most scientists believe that the solar system developed from a huge spiraling [spaieral]
nebula (cloud of gas and dust-sized pieces of rock and metal). The sun itself may have
been formed from the central part of this nebula. As the nebula whirled around the sun, it
slowly flattened out. Sections of the cloud began to spin like eddies (whirlpools) in a
stream. Gas and dust collected near the centers of these eddies. The collections of gas and
dust grew by attracting nearby particles of matter. These collections slowly developed
into the spinning planets that now travel around the sun.
There are several Theories on the origin of the solar system
Immanuel Kant, a German philosopher, proposed a nebular theory for the origin of the
solar system in 1755. A French astronomer, Pierre Simon Laplace, refined Kant's
theory in 1796. Laplace suggested that the original nebula was much larger than the
present solar system, and left behind eddies of matter as it became smaller. This theory
assumes that the Earth was first a gas and then a liquid, and finally cooled enough to
have a solid crust.
In 1905, Thomas Chamberlin, an American geologist, and Forest Moulton, an
American astronomer, proposed the planetesimal theory. According to this theory, a
rapidly moving star passed close to the sun but did not collide with it. The gravity of
the passing star pulled long, threadlike "arms" of gas from the sun. Eddies swirled
within the arms of gas. The gas cooled and formed solid particles called planetesimals.
The planetesimals gradually collected in the centers of the eddies, and formed planets.
The planetesimal theory assumes that the Earth was made of solid particles from the
beginning. Meteorites that have fallen to the Earth each day may be evidence that the
Earth is still growing by the gradual collection of solid particles.
Two English scientists, Sir James Jeans and Harold Jeffreys, proposed the tidal or
gaseous theory in 1919. Like the planetesimal theory, this theory begins with arms of
hot gas pulled from the sun by the gravity of a passing star. The gas gathers in eddies
which turn into liquid balls. Each ball slowly cools, and a hard crust forms around it.
The tidal theory assumes that the Earth was first a gas and then a liquid before it
developed a solid crust.
In the 1930's, the English astronomer R. A. Lyttleton proposed the double star theory.
Our galaxy contains many two-star combinations called double stars. Lyttleton
assumed that the sun and a "companion" star once formed a double star. The
companion star exploded into a cloud of gas which was "captured" by the gravity of
the sun. The planets developed from this cloud in much the same way as described in
the tidal theory.
Many scientists support condensation theories that begin with a single exploding star.
These theories were developed during the 1940's and 1950's. They assume that a star
exploded and that most of the exploded material escaped into space. A small part of
the material remained behind to form a nebula that began to rotate and contract. The
sun formed from the central part of this nebula. In orbits at varying distances from the
sun, smaller masses of dust and gases condensed to form the planets.
Erosion by glaciers
Erosion by glaciers has shaped and leveled large areas. The northern Midwestern
plains of the United States were formed hundreds of thousands of years ago when
huge glaciers slid over the land and smoothed it out. Today, glaciers cover all of
Antarctica and most of Greenland. In mountainous areas throughout the world,
glaciers flow among rocky peaks like frozen rivers.
Mountain glaciers form when fallen snow builds up and becomes so thickly packed
that it turns into ice. Many glaciers are more than 1,000 feet (300 meters) thick.
Gravity pulls a glacier downhill. The thick, heavy ice scrapes away any soil and
weathered rock in its path and digs U-shaped valleys in the mountains. It grinds away
rock, sometimes polishing it smooth and at other times leaving deep scratches. Pieces
of rock become frozen inside the ice and add to the grinding action. When the glacier
melts, it drops the rock. Water from the melting ice then spreads out the loose
material.
Erosion by wind
Erosion by wind involves the movement of dust and particles of sand. Wind also
carries ashes from volcanoes great distances before dropping them. During dry
seasons, strong winds pick up large quantities of rock and soil and blow them away. In
deserts and on some beaches, wind-blown sand forms hills called dunes. Some dunes
move little by little because the wind blows sand from one side to the other. Some
dunes cover and destroy forests. Sand particles driven by wind also scrape and wear
away rock surfaces.