SURFACE PROCESSES
Weathering
Weathering is the physical disintegration and chemical
decomposition of rock on or near the Earth's surface.
Physical weathering is the breakup of rock into smaller
pieces without any change in its chemical composition.
A common type of physical weathering is frost wedging.
Water enters cracks and crevices in rock material and
freezes during cold weather. Since water expands as it
freezes, it causes the rock surrounding it to fracture into
smaller pieces.
In the far northern latitudes the ground may remain
frozen all year. In this case a permafrost layer exists. If
the topsoil thaws during the summer the remaining
frozen layer can trap water near the surface creating a
wet and spongy layer.
Plants and animals play a smaller role in weathering.
Plant root systems can cause rock to fracture in a
manner similar to frost wedging. Creatures that burrow
like earthworms and ants also bring subsoil to the
surface.
Chemical weathering occurs when there is an actual
change in the chemical composition of the rock. Heat
and moisture play a role in chemical weathering so it is
most prevalent in hot, moist climates.
One of the most common types of chemical weathering
involves limestone which is made of calcite (CaCO3).
Water can combine with carbon dioxide to form a weak
solution of carbonic acid. As this acid comes in contact
with limestone, it reacts chemically to convert insoluble
limestone into water soluble calcium bicarbonate. As
the calcium bicarbonate is carried away, large caverns
can be carved out in underground limestone formations.
If the ceiling of the cavern collapses, a sinkhole appears
on the surface. Sinkholes are fairly common in Florida
and south Georgia since most of the underlying rock is
limestone. Sinkholes north of Macon are rare since the
underlying rock becomes granite or basalt.
Erosion
Erosion is the wearing away of soil and rock by
weathering and mass wasting (downhill movement of
soil and rock) or by running water, glaciers, wind or
waves.
Streams are defined as any flow of water that occurs
between well defined banks. This definition includes
everything from a 1 meter wide brook to a several
kilometer wide river.
The water in a stream comes from runoff(water that
does not soak into the ground) due to rain or melting ice
and snow.
The material transported by a stream is called the
stream load. A stream's load consists of three parts:
dissolved load, suspended load, and bed load.
The dissolved load consists of water soluble minerals
that are in solution and can't be seen. These dissolved
salts add to the salt content of the oceans.
The suspended load is visible as muddy water after a
heavy rain brings soil laden runoff into the stream.
The bed load consists of rocks and particles that are too
big to be suspended. They bounce and roll along the bed
of the stream.
As the water of the stream flows to the sea, the bed is
eroded forming a V shaped valley. The stream cannot
erode the land below its base level. This is usually sea
level.
The flood plain is the area around the stream that will
be occupied by water during heavy rains or spring
thaws. A flood plain may be a 10 year flood plain, a 50
year flood plain or a 100 year flood plain. The number
of years is the time, on average, between floods.
Houses should not be built on flood plains but flood
plains are very fertile land. Houses on flood plains can
be protected somewhat by artificial levees built to keep
the river in its channel.
Rivers form loops and bends called meanders. Erosion
occurs on the outside of a meander and deposition
occurs on the inside of a meander. The net effect is that
the radius of the meander will increase.
A delta results from the deposition of sediment at the
mouth of a river. The Mississippi River deposits 500
million tons of sediment each year into the Gulf of
Mexico. Over the years the Mississippi River Delta has
formed. Delta soil is very fertile and therefore
important for agriculture.
Glaciers
Parts of the Earth are covered with large masses of ice
year round. These large masses of ice are called
glaciers. Greenland and Antarctica are covered with
continental glaciers like the ones that covered North
America during the last Ice Age about 10,000 years ago.
Glaciers form when snow falls faster than it melts. As it
accumulates and becomes deeper, the lower levels are
compressed into solid ice. When enough ice
accumulates, the glacier slowly flows downhill. Icebergs
are large chunks of ice that have broken off of the edges
of the continental glaciers in Greenland and Antarctica.
Cirque glaciers form along mountains in hollow areas
protected from the Sun. If the ice melts, a mountain
lake may be formed.
A valley glacier is formed when snow accumulates in a
valley. The glacier will flow towards the lower, warmer
elevations where it will melt. The flow rate may be from
a few inches to over 30 m per day.
Glaciers can gouge out channels and valleys much like a
stream. The glacier moves much more slowly but can
move very large rocks and amounts of soil.
Rock material transported by ice is called till. The till
may form ridges around and within a glacier called
moraines. Medial moraines are located within the
glacier. Lateral moraines are located alongside the
glacier. The terminal moraine of a glacier marks its
point of greatest advance.
In the U.S. terminal moraines can be found as far south
as Indiana, Ohio, and New York.
Wind
Wind erosion is most evident in deserts where
sandstorms and dust storms move relatively large
amounts of solid matter. A desert is defined as any area
where there is a lack of precipitation producing a very
dry surface with very little vegetation. This lack of
vegetation and moisture makes sand and dust particles
more susceptible to wind erosion.
Waves
Waves along the edge of the ocean can change the
location of sandbars overnight. Waves pounding on the
bases of rocky cliffs can undercut the land above. The
white cliffs of Dover are limestone formations exposed
by this type of weathering.
Mass wasting
Mass wasting is the generic term for the downward
movement of rock, soil, or other debris due to the force
of gravity. Mass wasting may be fast or slow.
Two examples of fast mass wasting are landslides and
mudslides.
During landslides small or large amounts of relatively
dry rock and soil will move rapidly down a hill. In areas
where the slope of the land is rather severe, rocks may
break off and tumble down the side of the hill. This is
called a rockslide.
Mudslides occur when water from precipitation,
melting snow and ice, or volcanic eruptions causes the
soil to become semi fluid and unstable. Mudslides in
California, Italy and other mountainous areas occur
when vegetation is removed from the hillsides.
Construction and fires reduce the amount of vegetation
holding the soil in place. If there is enough rainfall, the
soil becomes unstable and flows down the hillside.
Slow mass wasting is less dramatic but much more
widespread. Creep is the slow particle-by-particle
movement of weathered debris down a slope year after
year. Although the process is too slow to be seen as it
happens, the effects of creep over several years are
easily seen. Think of a fence on a hillside that was
installed perfectly straight. In 10 to 20 years, it will have
moved downhill enough in sections to have bends and
curves.
Groundwater
Every living thing must have water. Most of the water
we use and drink comes from the Earth's groundwater.
Groundwater is the water that is found under the
surface of the Earth in aquifers and saturated soil.
The hydrologic cycle helps to explain how groundwater
is replaced.
During the hydrologic cycle, water evaporates from lakes,
streams, and oceans to form clouds. Precipitation falls on the
surface of the Earth in liquid or solid form. Runoff returns
water to lakes, streams, and oceans and in the case of glacial
flow, ice is eventually returned to the ocean.
The water that soaks into the ground and does not run off
replenishes the groundwater. This groundwater can be tapped
by shallow wells with a depth of 50 feet or less. This water may
not be safe to drink due to surface contamination (hog pen,
chicken pen, septic tank). In most cases drinking water comes
from a deep well that has a depth of 120 feet or more. This
water has gone through many layers of sand and other types of
soil that effectively filter out any harmful organisms.
Most of these wells reach into an aquifer. An aquifer is a body
of permeable rock through which water moves. Water takes
about 5000 years to reach this level unless there is an aquifer
recharge point upstream. In South Georgia there are several
known sinkholes that direct water into the aquifer with less
filtering than usual. One of the best known is north of town
and has led to Valdosta's trihalomethane problem.
Groundwater is able to be stored in soil and rock because there
is empty space between particles. The amount of empty space
is called porosity. Sandstone seems to be the most common
water storage rock.
The ability of the water to move through the soil and rock is
called permeability. Sand and gravel have both high porosity
and high permeability. Clay is less porous and has a lower
permeability. If you decide to install a septic tank and drain
field on your property you must have a percolation test
performed by the health department. If the clay content of the
property is too high or the land is too low, the property may
not pass.
The depth at which the ground is saturated is called the water
table. This is the upper boundary of the zone of saturation.
Above the water table, the soil is said to be in the zone of
aeration since the spaces between particles are filled with air.
Artesian wells are wells in an aquifer that produce water under
pressure due to the fact that the recharge point is at a higher
altitude than the well. Natural springs also can result from this
condition.
When water is extracted from an aquifer, it must be
replenished by natural processes. If this does not happen, the
aquifer can be depleted to the point that very little water can
be drawn from it. The Ogallala aquifer which runs from South
Dakota to Texas is being depleted at 10 times the rate it is
being recharged. Eventually this aquifer will not be usable as a
source of water.
In coastal regions, depletion of groundwater can lead to salt
water contamination of wells. Normally, salt water lies under
the freshwater near the coast. If too much freshwater is
pumped from a well, the salt water rises up to replace it.
Water from wells can contain bacteria, viruses or harmful
chemicals. Chlorination is used to kill harmful organisms.
Drinking water in cities and towns is tested for harmful
chemicals and can be treated if the concentrations are above
acceptable standards.
Water that is hard is not considered a health risk. Hard water
contains calcium, magnesium or iron compounds that make it
difficult (hard) to produce suds with soap. Effects of hard
water can be seen in the ring around the tub, poor cleaning
when using soap or the scale that forms in hot water heaters
and wherever water dries.
The solution is to install a water softening device. It will have
some operating cost and will require some maintenance.
Shoreline and Seafloor Topography.
Oceans cover about 70% of the Earth's surface. The five major
oceans in order of decreasing size are: Pacific, Atlantic, Indian,
Antarctic, and Arctic. The greatest depth is about 11 km in the
Marianas Trench in the western Pacific.
Ocean water is always moving as waves, currents and tides.
Waves are caused by wind disturbing the surface of the ocean.
As a wave moves toward shore, the reduction in depth of the
water causes the wave to increase its height until it falls over as
surf.
Waves sometimes approach the shore at an angle. When this
happens, a long-shore current is set up that travels parallel to
the shore.
There are major currents in the ocean which are caused by the
sun's heating and different densities of sea water due to
evaporation. One such current is the Gulf Stream which flows
from the Gulf of Mexico into the Atlantic, past England and
into the North Atlantic where it sinks.
The tides are caused by the gravitational pull of the Sun and
Moon.
Seafloor Topography
Isolated, submarine, volcanic mountains are called seamounts.
A seamount with a flat top which indicates it may at one time
have been an island is called a guyot.
Large, flat areas of ocean floor are called abyssal plains. They
form near continents from sediment from the erosion of the
continent.
At the edges of the continents are continental shelves and
continental slopes. Continental shelves are relatively shallow
areas that are important to commercial fishing and oil
exploration.
Continental slopes are the transition areas between continental
shelves and the deep ocean basin. They define the edge of the
continental crust.