Geology Reference
In-Depth Information
12
I. Introduction to Geology
2.
After you have identified the metamorphic rocks, note infor-
mation regarding the geologic, environmental, and economic
significance of these rocks in the proper column in Table 1
.8.
sand, and gravel, which correspond to the sedimen-
tary rock materials of shale, siltstone, sandstone, and
conglomerate, respectively. Sorting of sedimentary
material into these grain sizes occurs by wind or mov-
ing water. Unsorted sediments are a mixture of many
of these grain sizes and include such materials as those
deposited directly from glaciers (till) and from land-
slides (colluvium). See Table 1.9.
Regolith—Unconsolidated Material
on Bedrock
One of the most important geologic materials with
which the human colony interacts at the surface of
the Earth is regolith. This unconsolidated material
that overlies the bedrock is often loose and readily
excavated. There are four basic processes of forma-
tion: (1) in place by
weathering
of underlying bedrock;
(2) by
mass-wasting
of bedrock and sediments moving
downslope under the force of gravity; (3) by
accumu-
lation oforganics
such as peat; or (4) by
fluvial, glacial, or
eolian transport
of eroded material, which is deposited
as sediment. The general term used by geologists for
all these materials is regolith; however, for the same
unconsolidated materials over bedrock engineers
apply the term soil. Geologists, biologists, and soil sci-
entists generally use the term soil for those weathered
horizons developed at the top of the transported
regolith or directly from weathering of the bedrock. The
biologist's soil is considered to be able to support life.
Most regolith is described primarily on the basis
of grain size (and sorting) in much the same way as are
detrital sedimentary rocks (see Table
1.5).
Grain sizes
(or textures) of unconsolidated materials are clay, silt,
SIGNIFICANCE
Many of the environmental problems
encountered in use of the surface of the Earth occur in
or on the regolith. Failure or movement of this mater-
ial often occurs during hazardous geologic processes
and may result in destruction of buildings and high-
ways. The regolith is where solid and liquid waste dis-
posal and recharge, protection, storage, and extraction
of groundwater can occur. In this material we also find
gravel and sand resources for construction and agri-
cultural soils for crops. And in glacial and other
deposits, including those in rivers, lakes, and bogs, we
are able to decipher Quaternary history, which shows
us how climate, geologic processes, and landscapes
have changed over the last 2 million years. Thus the
regolith provides essential resources for humanity and
provides scientific data that helps us understand what
the future might hold. Where does the regolith fit in
the rock cycle? It is basically unconsolidated sediment
that overlies the rocks.
TABLE 1.9 Common Regolith (Unconsolidated Sedimentary) Materials
Texture (or grain
size—gravel, sand,
silt, clay) (sizes in
Table 1.5)
Sorting
(well sorted,
poorly sorted,
unsorted)
Sediment
Type (detrital,
organic,
residual)*
Possible Origin
(e.g., river, beach,
humans, wetland,
dune, glacier)
Geologic Significance
(good aquifer, mineral
resource, or building
site, etc.)
Sample
Number
* Detrital, loose grains of broken rocks and minerals (e.g., sand, etc.); organic, peat or loose plant matter; residual, chemically weathered residual regolith
with loose silicate fragments, clay minerals, and concentrations of iron oxide and aluminum oxide.
