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In my OWN words this means…GIST,
the summary (15-20 words)
TEXT In the Middle:
Porosity / Water Underground
Imagine that instead of turning on a faucet to get a glass
of water, you pour water from a chunk of solid rock! This
idea may sound crazy, but millions of people get their
water from within rock that is deep underground.
Although you can see some of Earth’s water in streams
and lakes, you cannot see the large amount of water that
flows underground. The water located within the rocks
below the Earth’s surface is called groundwater.
Groundwater not only is an important resource but also
plays an important role in erosion and deposition.
The Location of Groundwater
Surface water seeps underground into the soil and rock.
This underground area is divided into two zones.
Rainwater passes through the upper zone, called the zone
of aeration. Farther down, the water collects in an area
called the zone of saturation. In this zone, the spaces
between the rock particles are filled with water.
These two zones meet at a boundary known as the water
table. The water table rises during wet seasons and falls
during dry seasons. In wet regions, the water table can be
at or just beneath the soil’s surface. In dry regions, such as
deserts, the water table may be hundreds of meters
beneath the ground.
Aquifers
A rock layer that stores groundwater and allows the
flow of groundwater is called an aquifer. An aquifer can
be described by its ability to hold water and its ability to
allow water to pass freely through it.
Porosity
The more open spaces, or pores, between particles in an
aquifer, the more water the aquifer can hold. The
percentage of open space between individual rock
particles in a rock layer is called porosity.
Draw that you visualize while reading the text
Porosity is influenced by the differences in sizes of
the particles in the rock layer. If a rock layer contains
many particles of different sizes, it is likely that small
particles will fill up the different-sized empty spaces
between large particles. Therefore, a rock layer with
particles of different sizes has a low percentage of
open space between particles and has low porosity.
On the other hand, a rock layer containing same-sized
particles has high porosity. This rock layer has high
porosity because smaller particles are not present to
fill the empty space between particles. So, there is
more open space between particles.
Permeability
If the pores of a rock layer are connected,
groundwater can flow through the rock layer. A
rock’s ability to let water pass through is called
permeability. A rock that stops the flow of water is
impermeable.
The larger the particles are, the more permeable
the rock layer is. Because large particles have
less surface area relative to their volume than
small particles do, large particles cause less
friction. Friction is a force that causes moving
objects to slow down. Less friction allows water
to flow more easily through the rock layer.
Aquifer Geology and Geography
The best aquifers usually form in permeable
materials, such as sandstone, limestone, or layers
of sand and gravel. Some aquifers cover large
underground areas and are an important source of
water for cities and agriculture
Watersheds
River systems are divided into regions called
watersheds. A watershed, or drainage basin, is the
area of land that is drained by a water system. The
largest watershed in the United States is the
Mississippi River watershed. The Mississippi River
watershed has hundreds of tributaries that extend
from the Rocky Mountains, in the West, to the
Appalachian Mountains, in the East.
A stream that flows into a lake or into a larger
stream is called a tributary.
The satellite image shows that the Mississippi River
watershed covers more than one-third of the United
States. Other major watersheds in the United States
are the Columbia River, Rio Grande, and Colorado
River watersheds. Watersheds are separated from
each other by an area of higher ground called a
divide.
Millions of people get their water from within rock
that is deep underground. Although you can see some
of Earth’s water in streams and lakes, you cannot see
the large amount of water that flows underground. The
water located within the rocks below the Earth’s surface
is called groundwater. Groundwater not only is an
important resource but also plays an important role in
erosion and deposition.
Springs and Wells
Groundwater movement is determined by the slope of
the water table. Like surface water, groundwater tends
to move down-slope, toward lower elevations. If the
water table reaches the Earth’s surface, water will flow
out from the ground and will form a spring. Springs are
an important source of drinking water. In areas where
the water table is higher than the Earth’s surface, lakes
will form.
Artesian Springs
A sloping layer of permeable rock sandwiched
between two layers of impermeable rock is called an
artesian formation. The permeable rock is an aquifer,
and the top layer of impermeable rock is called a cap
rock, as shown in the Figure. Artesian formations
are the source of water for artesian springs. An
artesian spring is a spring whose water flows from a
crack in the cap rock of the aquifer. Artesian springs
are sometimes found in deserts, where they are often
the only source of water.
Wells
A human-made hole that is deeper than the level of
the water table is called a well. If a well is not deep
enough, as shown in the Figure, it will dry up when
the water table falls below the bottom of the well.
Also, if an area has too many wells, groundwater can
be removed too rapidly. If groundwater is removed
too rapidly, the water table will drop, and all of the
wells will run dry.
Underground Erosion and Deposition
Rivers cause erosion when water removes and
transports rock and soil from its banks. Groundwater
can also cause erosion. However, groundwater causes
erosion by dissolving rock. Some groundwater
contains weak acids, such as carbonic acid, that
dissolve the rock. Also, some types of rock, such as
limestone, dissolve in groundwater more easily than
other types do.
When underground erosion happens, caves can
form. Most of the world’s caves formed over
thousands of years as groundwater dissolved the
limestone of the cave sites.
Cave Formations
Although caves are formed by erosion, they also
show signs of deposition. Water that drips from a
crack in a cave’s ceiling leaves behind deposits of
calcium carbonate. Sharp, icicle-shaped features that
form on cave ceilings are known as stalactites (stuh
LAK tiets). Water that falls to the cave’s floor adds to
cone-shaped features known as stalagmites (stuh
LAG MIETS). If water drips long enough, the
stalactites and stalagmites join to form a dripstone
column.
Sinkholes
When the water table is lower than the level of a
cave, the cave is no longer supported by the water
underneath. The roof of the cave can then collapse,
which leaves a circular depression called a sinkhole.
Surface streams can “disappear” into sinkholes and
then flow through underground caves. Sinkholes
often form lakes in areas where the water table is
high. Central Florida is covered with hundreds of
round sinkhole lakes. Figure 7 shows how the
collapse of an underground cave can affect a
landscape.