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SEDIMENTARY ROCKS
Sedimentary rocks are formed from pieces of pre-existing rock or pieces
of once living organisms that are deposited, become compacted and then
cemented together. Sedimentary rocks form from deposits that accumulate
on the Earth’s surface and within bodies of water during the process of
sedimentation. Although the amount of sedimentary rock that covers the
Earth’s surface is extensive,it is estimated to be only about 5-8% of the
total volume of crust.
How Sedimentary Rocks Form
Prior to deposition, sediments are formed by the weathering and erosion
of rock from a source area. These sediments are carried away from the
source area by water runoff, ice or wind and then deposited in a basin on
the Earth’s surface or on the bottom of an ocean, lake or stream where it
becomes trapped. This sediment load may also include pieces of shell and
bone from once-living organisms as well as trace amounts of minerals
precipitated from the water.
Over time, these sediments will stratify or form
distinct layers similar to what you see in the
drawing to the right. Compaction caused by the
effects of gravity pushing down on the strata and
chemical interactions between the water and
elements present in the sediments will lead to
the cementation of the sediments and formation
of sedimentary rock layers.
Kinds of Sedimentary Rock
Source: National Park Service
Clastic
Clastic sedimentary rocks are made up of pieces or clasts
of pre-existing rock. Clastic sedimentary rocks may have
particles that range in size from microscopic to that of
huge boulders. These particles are named based on their
grain size. The smallest grains are clay, then silt and
then sand. Grains larger than 2 millimeters are often
called pebbles. Examples include breccia, sandstone, shale
and siltstone.
Chemical
Breccia
Source: Wikipedia
Chemical sedimentary rocks are formed by the chemical
precipitation of minerals followed by their deposition,
compaction and cementation as the water evaporates away.
The rock salt that table salt comes from is formed this
way as is limestone, chert, flint and iron ore.
Biological
Biological sedimentary rocks are formed when large numbers
of living things die and pieces of their bodies
including shell and bone accumulate and are then compacted
and cemented to form rock. Examples of biological
sedimentary rock include some forms of limestone, coal and
coquina.
Why Study Sedimentary Rocks?
Rock Salt
Source:
Mineralology4kids
Coquina
Source: Wikipedia
Sedimentary rocks and the rock strata they are formed from provide us
valuable information about the subsurface Earth that can be used to help
us construct houses, roads, commercial buildings, tunnels, canals and
other structures. Sedimentary rocks are also important sources of
natural resources such as iron ore, fossil fuels (coal, petroleum and
natural gas) and even drinking water (aquifers). Studying sedimentary
rock layering can also help us to understand events in geologic time such
as flooding, coastal deposition and climate changes that may have taken
place in a given location over time.
FEATURED PINTEREST PINS
Miller’s Science Spot
I thought this was a very nice and well drawn illustration of sedimentary
rock formation that should help you get started in understanding the
process.
Starburst Rock Cycle
A great article on how to use Starburst candies to demonstrate rock
formation- not just of sedimentary rock, but also igneous and metamorphic
rock.
Puddingstone Conglomerate
I had to include this pin for personal reasons. In 1985, my family and
I were living in Smith’s Creek, Michigan and our next door neighbor was
Mrs. Helen Neumann who had named the Puddingstone. A large Puddingstone
remains in front of the home to this day. The article includes great
decriptions of other unique sedimentary conglomerates.
Geology.com
Geology.com is a great website with lots of great images of sedimentary
as well as igneous and metamorphic rocks.
Sedimentary Rock Gallery About.com
This is a nice gallery of sedimentary rock types and examples.
RECOMMENDED INTERNET RESOURCES
Exploring Earth Animation of Sedimentary Rock Formation
A nice, brief visual that illustrates the basic functions of deposition,
compaction and cementation that form sedimentary rocks.
USGS Schoolyard Geology
Most appropriate for elementary and middle school students, this website
provides a more through description of the processes of sedimentary rock
formation.
Mineralology for Kids
Nice description and categorization of clastic sedimentary rocks by grain
size. Explore the site further for information about chemical and
biological sedimentary rocks.
USGS: What is Geologic Time
The concept of geologic time is an important one to master in your study
of rocks and minerals, geologic mapping and many other subcategories of
Earth Science. This USGS site should provide you with a great
introductory level description of geologic time.
Video: Sedimentary Rock Formation in Streams and Oceans
A great You-Tube visual that illustrates fundamental concepts of
sedimentary rock formation both in nature and the laboratory.
Formation of Petroleum
A great excursion video to enhance your knowledge of sedimentary rock
formation.
Australian Video: Formation of Petroleum
Another great video illustrating the formation of petroleum.
Sedimentary Rocks: The Key to Past Environments
A lengthy, more formal video discussion of sedimentary rock stratification
in the Grand canyon as it relates to the composition formation of
sedimentary rocks.
Principles of Stratigraphy
Stratigraphy is another fundamental concept of geology students should
understand and this has always been one of my favorite videos for
teaching the principles of geologic stratigraphy. The author does a great
job using Adobe Illustrator to illustrate each principle with a brief
audio description.
Cochise College 12 Common Sedimentary Rocks
What I like about this website is its categorization of common types of
sedimentary rock as well as the nice, crisp images it provides.
Modes of Sedimentary Rock Classification
This site provided by James Madison University provides several different
modes for classifying sedimentary rocks that you may find useful.
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Mark A. Van Hecke
Instructional Design Consultant
www.vanheckescience.com
[email protected]