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The Laying of Sedimentary Rocks Over Time
Sedimentary rock forms from bits of rocks deposited over time by
wind and or water. Rocks occur in all sizes. It is the tiny amounts
of mud, pebbles and sand grains and smaller rock particles that can
form sedimentary rock. Sediment can also contain the remains of
once living things. This may be bones, shells, droppings, leaves or
stems. Wind and water carries sediment and deposits it in layers
usually in a lake or ocean bottom. The processes that turn
sediments into solid rock are compaction and cementation.
Erosion
Destructive forces are
constantly breaking up and wearing away the rocks of Earth. The
forces include grinding ice, waves, rain, and heat. Erosion occurs
when these agents loosen and carry away the fragments of rock or
once living things somewhere else.
Deposition Environments: a place where sediments collect.
Eventually, wind or water will drop these fragments. They will
accumulate in low valley or river bottom. Deposition is the
process by which wind or water sediment settles out in layers.
Furthermore, over time the remains of living things may
harden in this sediment and change into fossils. After sediment
is deposited the processes of compaction and cementation
change the fragments into sedimentary rock.
Compaction
At first the fragments of rock lie loosely together. But gradually, over many,
many years these thick layers build up. This build up becomes heavy and
presses down on the layers beneath it. Compaction is the pressing down of
layers forcing the sediments to fit closer together. Over millions of years this
process can squeeze fragments tightly together. The layers often remain visible
in the rock and may be used to identify sedimentary rock.
Cementation
During the process of compaction the minerals in the rock are dissolving. The minerals fill in the spaces
between sediment particles. Cementation is the process of sediment being glued tightly together. The processes
of erosion, deposition, compaction and cementation may occur over millions of years transforming rock
fragments into solid rock.
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Relative Dating
Read the following story carefully. Then determine the correct order for the sentences. Be aware that the order
has been mixed up!
I got out bread, peanut butter and jelly.
I ate the sandwich.
I put the two pieces of bread together.
I spread a layer of jelly on another piece of bread.
I was hungry.
I spread a layer of peanut butter on a piece of bread.
How did you determine the order of the sentences? If you did it like many people, you based your sentences on
what you know normally happens when you make a lunch. This is like what geologists do to determine age.
The process of putting things in a "correct order" based on experience is called relative dating. Relative dating
is determining whether an object is older or younger than other objects or events. It doesn’t give an exact age
but a relative (or comparison) age. This means that you know which events came first, but not the exact time
when they happened.
*Imagine that throughout your life you’ve taken many pictures. Rather than putting them into a photo album
they are stacked up in a box. Where are the oldest pictures, by year, going to be in your stack? At the bottom or
the top? What about the newer pictures? Where are they in the stack? Rock layers are just like the stack of
pictures. The layers at the bottom are the oldest. Each layer will be successively younger or more recent. This is
called superposition. Superposition is the principle that says younger rocks lie above older rocks in an
undisturbed sequence.
*Scientists use the fossils of animals to help determine relative age. Certain groups of fossil animals and plants
occur in the geologic record in a specific order. If a scientist finds one of those fossils they can then assume the
age of the rock based on the age of the fossil.
*Often igneous intrusions will cut across layers of rock. The principle
of cross-cutting relationships states that the intrusions are younger than
the rock they cut. Therefore, the igneous intrusion may be below a layer
that is older than it.
The USGS has a good introduction of relative dating to have you
practice putting historical events in the correct order. Rock layers
represent "historical events," so a rock layer is like a page in the book
of Earth's geologic history. Here is a sample:
Consider six historical events: the Wright brothers' flight, the
bicentennial of American independence, the First and Second World
Wars, the first astronaut landing on the moon, and when television
became common in homes. First, let's try to put these events in order.
Our knowledge of the words first and second tells us that the First
World War came before the Second World War. We may know or may
have been told that the landing of Neil Armstrong on the moon was
seen by many people on television, but there was no television around when the Wright brothers flew at Kitty
Hawk. Thus, we can order these three events: first Wright brothers' flight, then television common in homes,
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then the landing on the moon. By a process of gathering evidence and making comparisons, we can eventually
put all six events in the complete proper order: Wright brothers' flight, First World War, Second World War,
television common in homes, landing on the moon, and American bicentennial.
Because we have written records of the time each of these events happened, we can also put them in order by
using numbers. The Wright brothers' flight occurred in 1903, the First World War lasted from 1914 to 1918, and
the Second World War lasted from 1939 to 1945. Televisions became part of our homes in the 1950's, Neil
Armstrong walked on the moon in 1969, and America celebrated 200 years of independence in 1976.
The following activity lets you be the geologist. There are five different pictures of one rock sequence
throughout time. Number the pictures from one to five with one being the oldest and five being the youngest.
Below each picture choose the process that is taking place in the box above. (Hint: for help numbering the
boxes refer back to the last three paragraphs that start with an asterisk.) Write the term that describes the event
that happened in the space below the drawing.
Now, examine the drawing below and answer the
questions:
Analysis:
1. Is Rock layer "A" younger or older than the
other rocks? What evidence explains your
answer?
2. Is the fault (dotted line labeled "L") older or
younger than layer D? Explain your answer.
3. List the rock layers in order from oldest to youngest. (include the lines "K" and "L").