Download Erosion - The Agents of Erosion Are Water, Wind, Ice, and Waves

Erosion - The Agents of Erosion Are Water, Wind, Ice, and Waves
By Matt Rosenberg
The process known as weathering breaks up rocks so that they can be carried away by the process known as
erosion. Water, wind, ice, and waves are the agents of erosion that wear away at the surface of the earth.
Water Erosion
Water is the most important erosional agent and erodes most commonly as running water in streams. However,
water in all its forms is erosional. Raindrops (especially in dry environments) create splash erosion that moves
tiny particles of soil. Water collecting on the surface of the soil collects as it moves towards tiny rivulets and
streams and creates sheet erosion.
In streams, water is a very powerful erosional agent. The faster water moves in streams the larger objects it can
pick up and transport. This is known as critical erosion velocity. Fine sand can be moved by streams flowing as
slowly as three-quarters of a mile per hour.
Streams erode their banks in three different ways: 1) the hydraulic action of the water itself moves the
sediments, 2) water acts to corrode sediments by removing ions and dissolving them, and 3) particles in the
water strike bedrock and erode it.
The water of streams can erode in three different places: 1) lateral erosion erodes the sediment on the sides of
the stream channel, 2) down cutting erodes the stream bed deeper, and 3) headward erosion erodes the channel
upslope.
Wind Erosion
Erosion by wind is known as aeolian (or eolian) erosion (named after Aeolus, the Greek god of winds) and
occurs almost always in deserts. Aeolian erosion of sand in the desert is partially responsible for the formation
of sand dunes. The power of the wind erodes rock and sand.
Ice Erosion
The erosive power of moving ice is actually a bit greater than the power of water but since water is much more
common, it is responsible for a greater amount of erosion on the earth's surface.
Glaciers can perform to erosive functions - they pluck and abrade. Plucking takes place by water entering cracks
under the glacier, freezing, and breaking off pieces of rock that are then transported by the glacier. Abrasion
cuts into the rock under the glacier, scooping rock up like a bulldozer and smoothing and polishing the rock
surface.
Wave Erosion
Waves in oceans and other large bodies of water produce coastal erosion. The power of oceanic waves is
awesome, large storm waves can produce 2000 pounds of pressure per square foot. The pure energy of waves
along with the chemical content of the water is what erodes the rock of the coastline. Erosion of sand is much
easier for the waves and sometimes, there's an annual cycle where sand is removed from a beach during one
season, only to be returned by waves in another.
How Do You Make and Break a Mountain?
How Physical Processes Form Physical Features
By Terry Bain
“Water carries the mountains down to the sea a teaspoon at a time. A day becomes a million
days, and a mountain of rock changes shape.” (from the film "Planet of Man: The Uneventful
Day")
Geographers believe that the Earth’s physical features are created by physical processes--constant, ongoing
actions of nature that change the physical environment. In physical geography, we study the physical features
and the physical processes that create, shape, move, destroy, or recreate them.
One of the best ways to examine these processes is to look at the life cycle of a mountain.
Building a Mountain
A mountain is an elevated landform with a summit and steep sides. According to scientific theory, mountains
are created by a physical process called plate tectonics. The theory of plate tectonics says that the earth’s solid
surface (crust) is broken up into massive pieces, called plates, and each plate is squeezed against other plates.
Plates move slowly but constantly, the result of convection currents or slab pull, and not all at the same speed or
direction. As plates move, so much pressure and stress build at the places where the plates meet (plate
boundaries) that the crust (rock) there starts to bend, fold, or get crumpled. After millions of years, when the
force is great enough, the pressure is released in sudden, brief, violent events as plates slide under, into, by, and
away from each other, breaking rocks or pulling them apart. A mountain starts to build when colliding plates
push up the rock between them. At the rate of just a few millimeters a year, building an entire mountain will
take millions and millions of years. The mountain stops growing when tectonic forces no longer act on it and
crust is no longer being uplifted.
Mountain Breaking
The first step in the process is weathering. Weathering breaks down the mountain’s surface into tiny pieces
called sediment. Over time, the forces of weathering (wind, water, rain, ice, waves, chemicals, gravity, and
organisms) wear down and eventually level the mountain by breaking up or dissolving its rock into smaller and
smaller pieces.
The next step in the process is erosion. Erosion is the carrying away, movement, or removal of weathered rock,
dirt, and other bits of earth from one place to another by wind and water in various forms. One of the more
forceful agents of erosion is running water, which picks up and transports weathered material. This is how
sediment finds its way to a river that moves these weathered materials downstream to new locations.
The next step in the process is deposition. Deposition occurs when sediment carried and transported by a
flowing river gets deposited at other places on the Earth's surface. This typically happens where the current
slows so much that it can no longer move or carry the sediment. As the river approaches an ocean, for example,
it tries to flow downstream, but the ocean pushes it back. At these locations, such as in the mouth of a river,
millions of tons of the weathered mountain drop out and are left behind. Over time more and more sediment
drops out of the river and is deposited at the same place, building up and forming a solid land mass. This new
land mass takes on a triangular, fan shape because the river slows down and veers off course as it approaches
the ocean, splitting into different channels that slice the new landform into sections. The result is a delta, a
triangular landform formed from the sediment that flowed downstream and was deposited at the mouth of a
river or stream where it enters a larger, quieter body of water, like an ocean or lake.
Physical Processes and Mountain Building
Tectonic processes build landforms such as plateaus, volcanoes, valleys, rift valleys, and certain kinds of
islands, as well as mountains. Weathering breaks down landforms, while erosion carries away landforms, and
together they reshape the earth’s surface by creating landforms such as canyons, buttes, mesas, inselbergs,
fjords, hills, lakes, valleys, and sand dunes. Thanks to deposition, what gets worn down gets new life
somewhere else as an alluvial plain, island, beach, or a delta. Tectonic activity, weathering, erosion, and
deposition are actually not steps, but rather ongoing simultaneous forces at work on the Earth’s surface. Even as
a mountain is growing, the physical processes of weathering, erosion and deposition are slowly but relentlessly
breaking and taking its surface and depositing it somewhere else.
Plate Tectonics –
Learn About the History and Principles of Plate Tectonics
By Amanda Briney
Plate tectonics is the scientific theory that attempts to explain the movements of the Earth's lithosphere that have
formed the landscape features we see across the globe today. By definition the word "plate" in geologic terms
means a large slab of solid rock. "Tectonics" is a part of the Greek root for "to build" and together the terms
define how the Earth's surface is built up of moving plates.
The theory of plate tectonics itself says that the Earth's lithosphere is made up individual plates that are broken
down into over a dozen large and small pieces of solid rock. These fragmented plates ride next to each other on
top of the Earth's more fluid lower mantle to create different types of plate boundaries that have shaped the
Earth's landscape over millions of years.
History of Plate Tectonics
Plate tectonics grew out of a theory that was first developed in the early 20th century by the meteorologist
Alfred Wegener. In 1912, Wegener noticed that the coastlines of the east coast of South America and the west
coast of Africa seemed to fit together like a jigsaw puzzle. Further examination of the globe revealed that all of
the Earth's continents fit together somehow and Wegener proposed an idea that all of the continents had at one
time been connected in a single supercontinent called Pangaea. He believed that the continents gradually began
to drift apart around 300 million years ago - this was his theory that became known as continental drift.
The main problem with Wegener's initial theory was that he was unsure of how the continents moved apart
from one another. Throughout his research to find a mechanism for continental drift, Wegener came across
fossil evidence that gave support to his initial theory of Pangaea. In addition he came up with ideas as to how
continental drift worked in the building of the world's mountain ranges. Wegener claimed that the leading edges
of the Earth's continents collided with each other as they moved causing the land to bunch up and form
mountain ranges. He used India moving into the Asian continent to form the Himalayas as an example.
Eventually Wegener came up with an idea that cited the Earth's rotation and its centrifugal force toward the
equator as the mechanism for continental drift. He said that Pangaea started at the South Pole and the Earth's
rotation eventually caused it to break up, sending the continents toward the equator. This idea was rejected by
the scientific community and his theory of continental drift was dismissed as well.
In 1929 Arthur Holmes, a British geologist, introduced a theory of thermal convection to explain the movement
of the Earth's continents. He said that as a substance is heated its density decreases and it rises until it cools
sufficiently to sink again. According to Holmes it was this heating and cooling cycle of the Earth's mantle that
caused the continents to move. This idea gained very little attention at the time.
By the 1960s Holmes' idea began to gain more credibility as scientists increased their understanding of the
ocean floor via mapping, discovered its mid-ocean ridges and learned more about its age. In 1961 and 1962
scientists proposed the process of sea floor spreading caused by mantle convection to explain the movement of
the Earth's continents and plate tectonics.
Principles of Plate Tectonics Today
Scientists today have a better understanding of the make-up of the Earth's tectonic plates, the driving forces of
their movement, and the ways in which they interact with one another. A tectonic plate itself is defined as a
rigid segment of the Earth's lithosphere that moves separately from those surrounding it. There are seven major
plates (North America, South America, Eurasia, Africa, Indo-Australian, Pacific and Antarctica) as well as
many smaller, microplates such as the Juan de Fuca plate near the United States' state of Washington (map of
plates).
There are three main driving forces for the movement of the Earth's tectonic plates. They are mantle convection,
gravity and the Earth's rotation. Mantle convection is the most widely studied method of tectonic plate
movement and it is very similar to the theory developed by Holmes in 1929. There are large convection currents
of molten material in the Earth's upper mantle. As these currents transmit energy to the Earth's asthenosphere
(the fluid portion of the Earth's lower mantle below the lithosphere) new lithospheric material is pushed up
toward the Earth's crust. Evidence of this is shown at mid-ocean ridges where younger land is pushed up
through the ridge, causing the older land to move out and away from the ridge, thus moving the tectonic plates.
Gravity is a secondary driving force for the movement of the Earth's tectonic plates. At mid-ocean ridges the
elevation is higher than the surrounding ocean floor. As the convection currents within the Earth cause new
lithospheric material to rise and spread away from the ridge, gravity causes the older material to sink toward the
ocean floor and aid in the movement of the plates. The Earth's rotation is the final mechanism for the movement
of the Earth's plates but it is minor in comparison to mantle convection and gravity.
As the Earth's tectonic plates move they interact in a number of different ways and they form different types of
plate boundaries. Divergent boundaries are where the plates move away from each other and new crust is
created. Mid-ocean ridges are an example of divergent boundaries. Convergent boundaries are where the plates
collide with one another causing the subduction of one plate beneath the other. Transform boundaries are the
final type of plate boundary and at these locations the no new crust is created and none is destroyed. Instead the
plates slide horizontally past one another. No matter the type of boundary though, the movement of the Earth's
tectonic plates is essential in the formation of the various landscape features we see across the globe today.
The Four Spheres of the Earth
Atmosphere, Biosphere, Hydrosphere, Lithosphere
By Matt Rosenberg
The area near the surface of the earth can be divided up into four inter-connected "geo-spheres:" the lithosphere,
hydrosphere, biosphere, and atmosphere. Scientists can classify life and material on or near the surface of the
earth to be in any of these four spheres.
The names of the four spheres are derived from the Greek words for stone (litho), air (atmo), water (hydro), and
life (bio).
Lithosphere
The lithosphere is the solid, rocky crust covering entire planet. This crust is inorganic and is composed of
minerals. It covers the entire surface of the earth from the top of Mount Everest to the bottom of the Mariana
Trench.
Hydrosphere
The hydrosphere is composed of all of the water on or near the earth. This includes the oceans, rivers, lakes, and
even the moisture in the air. Ninety-seven percent of the earth's water is in the oceans. The remaining three
percent is fresh water; three-quarters of the fresh water is solid and exists in ice sheets
Biosphere
The biosphere is composed of all living organisms. Plants, animals, and one-celled organisms are all part of the
biosphere. Most of the planet's life is found from three meters below the ground to thirty meters above it and in
the top 200 meters of the oceans and seas.
Atmosphere
The atmosphere is the body of air which surrounds our planet. Most of our atmosphere is located close to the
earth's surface where it is most dense. The air of our planet is 79% nitrogen and just under 21% oxygen; the
small amount remaining is composed of carbon dioxide and other gasses.
All four spheres can be and often are present in a single location. For example, a piece of soil will of course
have mineral material from the lithosphere. Additionally, there will be elements of the hydrosphere present as
moisture within the soil, the biosphere as insects and plants, and even the atmosphere as pockets of air between
soil pieces.