USGS Frequently Asked Questions
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Question:
What are sedimentary rocks?
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Answer: Sedimentary rocks are formed from
pre-existing rocks or pieces of once-living organisms.
They form from deposits that accumulate on the Earth's surface.
Sedimentary rocks often
have distinctive layering or bedding.
Many of the picturesque views of the desert
southwest show mesas and arches made of layered sedimentary rock.
Common Sedimentary Rocks:
Common sedimentary rocks include
sandstone, limestone, and shale.
These rocks often start as sediments
carried in rivers and deposited in
lakes and oceans. When buried,
the sediments lose water and become
cemented to form rock.
Tuffaceous sandstones
contain volcanic ash.
Clastic Sedimentary Rocks:
Clastic sedimentary rocks are the group of rocks most
people think of when they think of sedimentary rocks.
Clastic sedimentary rocks are made up of pieces
(clasts) of pre-existing rocks.
Pieces of rock are
loosened by weathering, then transported to some
basin or depression where sediment is trapped. If the
sediment is buried deeply, it becomes compacted and
cemented, forming sedimentary rock.
Clastic sedimentary
rocks may have particles ranging in size from
microscopic clay to huge boulders. Their names
are based on their clast or grain size. The smallest
grains are called clay, then silt, then sand. Grains
larger that 2 millimeters are called pebbles. Shale
is a rock made mostly of clay, siltstone is made up
of silt-sized grains, sandstone is made of sand-sized
clasts, and conglomerate is made of pebbles
surrounded by a matrix of sand or mud.
Biologic Sedimentary Rocks:
Biologic sedimentary rocks form when large
numbers of living things die, pile up, and are
compressed and cemented to form rock.
Accumulated carbon-rich plant material may form
coal. Deposits made mostly of animal shells may
form limestone, coquina, or chert.
Chemical Sedimentary Rocks:
Chemical sedimentary rocks are formed by
chemical precipitation. The stalactites and
stalagmites you see in caves form this way,
so does the rock salt that table salt comes from.
This process begins when water traveling
through rock dissolves some of the minerals,
carrying them away from their source.
Eventually these minerals can be redeposited, or
precipitated, when the water evaporates
away or when the water becomes over-saturated
with minerals.
-- Excerpts from:
USGS/NPS Geology in the Parks Website, 2002,
Barker, 1997, Collecting Rocks: USGS General Interest Publication,
North Dakota Geological Survey Website, 2002,
and
USGS The Learning Web Website, 2001
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Source of this FAQ:
http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/sedimentary_rocks.html
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U.S. Department of the Interior |
U.S. Geological Survey