Download Unit 6: Sedimentary Rocks

Geology
Unit 6: Sedimentary Rocks
Student Information
The minerals formed deep within the earth combine in various ways to form the hard solids
we call rocks. Sedimentary rock is a kind of rock formed when a layer of sediment becomes solid.
This type of rock is formed by the weathering and erosion of existing rocks. Rocks exposed to air and
water slowly wear away. During the process of weathering, small pieces, or particles, break away
from the main rock. Water flowing over the earth's surface picks up sediment, or small pieces of rock,
sand, clay, and other materials. The water flows into streams or rivers. As the flow slows down, some
particles of rock and other materials fall to the bottom of the river and settle out of the water.
Rock particles in flowing water settle out and layers form. The layers of sediment become
covered by other layers.The upper layers press down on the lower layers.The weight of accumulated
particles, along with mineral-laden water, cements everything together. After thousands of years,
layers of sediment become solid and form rocks. Sandstone is an example of a sedimentary rock that
was formed when layers of sediment became solid.
Sedimentary rocks are known for their layered characteristics and often provide clues to the
past in the form of fossils. Sedimentary rocks are classified by the source of their sediments. There are
three classifications of sedimentary rocks.
Chemical sedimentary rocks, such as rock
limestone, halite, and gypsum, are formed
from minerals left behind after water evapor a
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Clastic sedimentary rocks, such as conglomerate, breccias, and sandstone, are formed
when little pieces of broken-up rock pile up
over time and are pressed and cemented toaether.
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Organic sedimentary rocks, such as coal
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All sedimentary rocks are formed by two common processes: compaction and cementation.
Compaction is caused by the weight of the overlying rocks squeezing the sediments together.
Cementation is a chemical process through which water carries and deposits dissolved minerals in
the small spaces between the sediment particles. In a sense, the particles are cemented together.