Rocks make up the mineral matter of soils (sand, silt, & clay). The products of rock weathering that are near surface of the earth produce the solid mineral particles of the soil. Rocks are any mineral or aggregate of minerals that form an essential part of the earth - or rocks are a combination of minerals.
Minerals are elements in various combinations. Minerals are the building
stones of the earth's crust. They are stony mixtures of one or more of the
ninety-two relatively stable elements that man has found in the earth's surface
and its rocks. A formula for a particular mineral is the same, no matter where
the mineral is found. The quartz sand of Pensacola Beach has the same chemical
formula as does the quartz sand of the Sahara Desert, (1 part Silicon and 2
parts of Oxygen).
Rocks, on the other hand, can be defined as extensive mineral bodies, composed of one or more minerals in varying proportions. The chemical formula for one chunk of rock, may vary widely from the another chunk of rock, even though the two rocks were broken off from the same outcrop. See Minerals for more information on minerals.
Rocks are classified on the basis of their origin: Igneous - from molten rock;
Metamorphic - recrystallized in the
solid state from heat and pressure; Sedimentary
- formed from particles of other rocks or from solution. See the
Rock Cycle diagram.
Igneous Rocks
Igneous rocks are the primary rocks from which all other rocks have been directly or indirectly derived. These rocks have solidified from molten magma and their present structural characteristics are due to variations in the conditions when they solidified. Rocks with large crystals cooled slowly under the earth's surface while small crystalline rocks cooled faster near the surface. Igneous rocks are homogeneous in that any one surface looks like any other surface. Light colored igneous rocks like granite contain more quartz and are considered acidic. Dark colored igneous rocks have an appreciable amount of iron and magnesium compounds and have little or no quartz. They are generally considered basic igneous rock.
Metamorphic
Rocks
Metamorphic rocks may be formed by different processes. Generally, they are
recrystallized in the solid state by pressure and heat and may be influenced by
the introduction of chemically active solutions. Igneous, sedimentary, and
pre-existing metamorphic rocks may become new metamorphic rocks. The change may
be partial and some of the original characteristics retained, or it may be so
complete that new minerals are formed and the texture of the rock is altered.
The metamorphic rock seen here is a weathered gneiss.
Sedimentary Rock -
Limestone bedrock
Sedimentary rocks may be formed from particles of igneous, metamorphic, or other sedimentary rocks. These rocks weather by undergoing disintegration or solution and the resulting particles are transported by wind , water or ice. The new sedimentary rocks are formed by sedimentation or precipitation.
Clastic sedimentary rocks are formed from fragments of pre-existing rock by mechanical weathering; weathering produces gravel, sands, silts which are then cemented together by natural cements such as calcium carbonate, silica, or iron oxide.
In the processes of weathering, some rocks or parts of rocks may be
dissolved. As the solution cools or evaporates, the solid portion is deposited
as a precipitate. Rocks formed in this way are referred to as solution or
chemical sedimentary rocks. The limestone roadcut above is an example of
solution sedimentary rock from SE Minnesota.
Organic Rocks
Rocks formed from the compaction of plant remains (such as coal) are termed
organic rocks.
Rocks into Soil
When rock is close to the surface of the earth, the soil will weather directly from the rock. The resulting soil is termed to be a residual soil, in that the soil horizons form from the rock as it weathers. The four rocks in Minnesota that are close enough to the surface to form a residual soil are: Limestone (see above), St. Peter Sandstone, Basalt, and Granite. These rocks and the residual soil they weather into will be seen in the laboratory and discussed again under parent materials.
Another famous rock in southwest Minnesota is the Sioux Quartzite(See Quartzite). Quartzite is composed of sandstone
that has been metamorphosed. Quartzite is much harder than the parent rock,
sandstone. The quartzite and pipestone were formed when a stream system
deposited layer upon layer of sand and other sediment. The sand was eventually
compressed into sandstone, and the red clay under it into clay stone. Some
sediment was removed by one of the four glaciers which traveled through the
area and scraped the land down to the sandstone. Under the weight of the
glaciers and with extremely high temperatures, the sandstone became quartzite
and the red clay sediment turned into pipestone. The vein of pipestone called
catlinite is sandwiched between two layers of hard quartzite, four to twelve
feet below the earth's surface. Catlinite is the rock that is used by Native
Americans to make pipes and other items. See Pipes Rock &
Layer. This is at Pipestone National Monument.
Go to Catlinite.html for
more information.
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