Download Exogenous Forces and Weathering

Exogenous Forces and Weathering
The motion picture and television industries have treated us to glimpses of many
of the natural wonders of the world. We have scanned the great glaciers and ice fields of
the Artic, gone on safari in deepest Africa, toured the beautiful national parks of the
United States, and flown over the harshest environments known to humanity. Through
the marvels of special-effects camera work, we have experienced earthquakes,
hurricanes, tidal waves, and erupting volcanoes.
Each of these experiences has allowed us to see very different features of the
earth. Thee features have been shaped by natural forces active on our planet. Geologists
tell us that these forces not only shape the surface, but are also in continual conflict with
each other. The forces of weather, waves, rivers, and glaciers are in continual battle with
the rocks of the earth’s crust.
Since the origin of our planet, these forces have been wearing down the features
of the continents. Countering these destructive forces are other forces. These opposing
forces are constructive. They lift up the crust. They build geological formations such as
mountains. And they create new rocks.
The two types of forces together, the tearing down and the building up operate
naturally in a geological cycle. The destructive forces are called exogenous. The
constructive forces are called endogenous. In this lesson, we will explore the exogenous
forces.
Weathering
Weathering = Fragmentation and breaking up
+
= Decay and decomposition
The exogenous forces that act upon the earth are a combination of several
complex natural events. One of these events of major importance is weathering.
Weathering, in turn, involves two processes: fragmentation and decay. That is, rocks
both break up and decompose.
What causes these two processes of weathering? The breaking up occurs by
means of physical weathering. The decomposition occurs by means of chemical
weathering. Rock exposed to these two processes changes size and shape. It is then
transported by gravity, water, wind, and ice.
It is the sun that provides the energy for both chemical and physical weathering.
Solar energy caused the pattern of winds that gives us our climates and weather. The sun
also supplies energy for chemical reactions. Water and gases from the oceans and the air
react with the earth’s rocky surface. The surface is broken up. Material is then transported
from one place to another.
What we have then are two causes of weathering-physical and chemical. The sun
energizes events, both wear down the rocks on the earth’s crust, but only one changes the
physical appearance of the rocks. The other chemically changes rocks into new
substances. Both are exogenous forces.
Summary
Two opposing types of forces operate naturally on earth:
Exogenous
Endogenous
These are destructive.
These are constructive.
One major exogenous force is weathering.
Weathering involves two processes:
a. Physical weathering changes physical features of rocks: fragmentation or
breaking up of the rock.
b. Chemical weathering changes rock into new substances: decomposition or
decaying of the rock.
Rock is exposed to both types of weathering. It changes size and shape. Or it’s altered
chemically into a new substance. Then it is transported to some other place by: gravity,
water, and ice.
Link: http://en.wikipedia.org/wiki/Weathering
Link: http://dsc.discovery.com/news/2008/03/06/grand-canyon-formation.html
Physical Weathering
Physical Weathering is the actual breakup of rocks and minerals into smaller
pieces or fragments. In this process fragmentation occurs through physical force:
In other words, rocks are fragmented by non-chemical or technical methods. They
all mean the same—“physical”, “non-chemical” or “technical”. The final product is the
same—rock fragment. You are already familiar with most of these non-chemical
methods. Rainfall is one of them. It’s less extreme than others. Yet it does act to wear
down rock and transport it somewhere else.
Here are some more obvious physical agents of weathering:
a. Rainfall
b. Running Water: River, Streams, waterfalls
c. Wave action: lakes and oceans
d. Ice
e. Earth movements: earthquakes, landslides
f. Winds
g. Plant growth
These agents do not attack only small rocks. They wear down and destroy larger
formations, such as boulders and mountains.
Chemical Weathering
Minerals in rocks decompose (change) into different Chemical Substances. Chemical
weathering occurs when minerals react with water and atmospheric gases.
Forms of Chemical Weathering
Oxidation- occurs when rock material is exposed to free oxygen. This can happen
in water or air. When the rock and oxygen combine, they from chemical oxides
Hydration-occurs when water is brought into a mineral compound. This is
common to many clay compounds. They swell when they absorb water. Then when they
dry they crumble into a powdery substance
Carbonation-occurs when carbon dioxide combines with water to from carbonic acid (the
acid in soda water). Carbon Dioxide comes from decaying plants and root growth. This
acid dissolves many rocks, such as limestone.
Erosion: Another Major Exogenous Force
Weathering is a slow and constant process. It’s important for the breakup of rock masses.
The process that actually sculpts or shapes the crust is erosion. Some erosion may be
slow. Erosion differs from weathering in that it involves the transportation of material.
The agents of erosion that we see the most are:
1.
Running water-deposits most of material
2.
Moving ice (glaciers)-common in only high mountain regions or near the
poles. Are masses of compact snow and ice? Controlled by gravity, which
causes them to flow down hill. Changes hillsides because they scrape it.
3.
Wind-Travels at high speeds, the wind can carry large amounts of rock
fragments. Wind often carries silt and sand. These Particles carried by the
wind also strike against rocks and other features. As they continue to hit a
surface they erode it. If the Particles are sand the process is called
sandblasting.
4.
Gravity
All four of these have something in common: they all move and deposit material from on
place to another. This deposited material is sediment.
The agents of Erosion
Running Water: Of these four agents, it is running water that deposits the majority of the
earth’s sediments: Streams, creeks, and rivers erode and carry sediment into many of our
country’s rivers. This is how the muddy Mississippi got its name. A more extreme
example of erosion by running water is a flash flood. A flash flood is swift and powerful.
Erosion thus occurs very quickly in a flash flood.
Glaciers: The second agent glaciers, is common only in high mountain regions or near the
north and south poles. Glaciers and masses of compact snow and ice move soil. They’re
formed in regions that have heavy snowfalls and snowfields the year round.
Gravity controls glaciers. That is, gravity causes glaciers to flow downhill. As they move
downhill, the glaciers pick up dirt, stones, and boulders. The stones and boulders then
become embedded or frozen into the ice. As the ice continues moving downhill, the
stones and boulders scrape and gouge the land’s surface. This changes the hillside’s
shape.
Wind: The third agent, wind, is very familiar. When it travels at high speeds, the wind
can carry large quantities of rock fragments. Silt and sand are often carried by the wind.
Near most natural beaches you’ll find sand dunes huge piles of sand deposited by the
wind.
The particles carries by the wind also strike against rocks and other features. As they
continue to hit a surface, they erode it. If the particles are grains of sand, for example, the
process of erosion is called sandblasting.
Gravity: Gravity is largely responsible for the erosion of land. When gravity alone pulls
soil and rocks downhill, the movement is called mass wasting. But usually another force
acts with gravity to speed up the movement. For instance, movements of the earth hasten
downhill soil movement to cause rapid landslides.
In 1964 a major earthquake occurred in Alaska. Most of damage that resulted was from
rock and landslides.
This image is in the public domain because it contains materials that originally came from the United States Geological Survey, an
agency of the United States Department of Interior. For more information, see the official USGS copyright policy
Mudflows are another example. They occur in hilly regions after a series of rainstorms.
Mudflows sound harmless, but a mudflow moves down a hillside like a river of mud. It
can carry boulders, trees, or even whole houses.
Summary
Weathering
Breaks up rocks.
Usually slow and constant.
We think of the exogenous forces of weathering and erosion as being destructive. They
break down, wear down, and transport the rocks and minerals of the crust. But these
events do result in one very beneficial product soil. Soil is crucial to us, of course,
because it supports our crops.
Soil is formed very slowly. “Slowly” here means hundreds of thousands of years of
evolution. You can see why farmers get concerned about soil erosion.
We already know that rocks and minerals are found in soil. But what else? We find water
and air, and we have a top layer of humus. This is made of tiny bits of decayed leaves,
twigs, and animal remains. These are sediments, as are the rocks and minerals.
The composition of the humus and rocks and minerals depends on what kinds of minerals
were eroded to form the soil. If a soil is sandy, we know sand is or was situated nearby.
Not all soils are good for growing plants. Harmful materials, such as pollutants, may have
eroded into the soil. The best soil is rich in humus and minerals that support plant growth.
These soils are usually found in the temperate regions of the world.
Summary
Exogenous Forces
Weathering and Erosion
Sediments
Soils
In hundreds or thousands of years, the soil matures:
The composition of the humus and minerals is dependent on the original sediments.
Humus and rocks and minerals are considered sediments. The best soils usually form in
climates that are temperate.