Download Superposition of Rock Layers

Relative Ages of Rocks
Lesson Objectives
Explain Steno’s Laws of Superposition and Original Horizontality
Based on a geologic cross-section, identify the oldest and youngest formations
Explain what an unconformity represents
Know how to use fossils to correlate rock layers
Natures Doesn’t Change
Geologists use present-day processes to interpret the past.
If you find a fossil of a fish in a dry, desert environment did that fish flop around on
land to get there?
Since fish don’t flop around on land today, we can assume that fish didn’t in the past
and the rock moved from an ocean environment to a dry environment.
Superposition of Rock Layers
In 1666 Nicholas Steno noticed that a great white shark tooth looked similar to fossils
found in inland mountains and hills.
Steno was one of the first to realize that fossils were once living organisms and he
sought to explain how fossils seashells could end up in rocks and mountains far from
the ocean.
Superposition of Rock Layers
Steno proposed that if a rock contained the fossils of marine animals, the rock
formed from sediments that were deposited on the seafloor.
The rocks were then uplifted to become mountains.
Based on these ideas Steno developed several Laws
Original Horizontality
Sediments are deposited in fairly flat, horizontal layers.
If a sedimentary rock is found tilted, the layer was tilted after it was formed.
Lateral Continuity
Sediments are deposited in continuous sheets that span the body of water that they
are deposited in.
When a valley cuts through sedimentary layers, it is assumed that the rocks on either
side of valley were originally continuous.
Superposition
Sedimentary rocks are deposited one on top of another.
The youngest layers are found at the top of the sequence, and the oldest layers are
found at the bottom.
The Grand Canyon
The Grand Canyon provides an excellent illustration of the principles we just
discussed. The many horizontal layers of sedimentary rock illustrates the principle of
original horizontality.
The youngest rock layers are at the top and the oldest are at the bottom, which is
described by the law of superposition.
Distinctive rock layers, such as the Kaibab Limestone, are matched across the broad
expanse of the canyon. These rock layers were once connected, as stated by the rule
of lateral continuity.
The Colorado River cuts through all the layers of rock to form the canyon. Based on
the principle of cross-cutting relationships, the river must be younger than all of the
rock layers that it cuts through.
Determining the Relative Ages of Rocks
These principles are essential for determining the relative ages of rocks and rock
layers.
In the process of relative dating, scientists do not determine the exact age of a fossil
or rock but look at a sequence of rocks to try to decipher the times that an event
occurred relative to other events represented in that sequence.
Relative Age of Rocks
The relative age of a rock then is its age in comparison with other rocks.
If you know that relative ages of two rock layers
1) Do you know which is older and which is younger?
2) Do you know how old the layers are in years?
The principle of cross-cutting
relationships states that a fault
or intrusion is younger than the
rocks that it cuts through.
The fault cuts through all three
sedimentary rock layers (A, B,
and C) and also the intrusion
(D). So the fault must be the
youngest feature.
The intrusion (D) cuts through
the three sedimentary rock
layers, so it must be younger
than those layers. By the law of
superposition, C is the oldest sedimentary rock, B is younger and A is still younger.
The Full Sequence of Events
1)
4)
2)
5)
3)
6)
Uniformitarianism
James Hutton, the founder of modern geology, formulated the idea of
uniformitarianism: The present is the key to the past.
The same processes that operate on Earth today operated in the past as well.
Earthquakes, volcanoes, erosion, weathering, ect, all happen today and all happened
deep in the past.
Matching Up Rock Layers
Superposition and cross-cutting are helpful when rocks are touching one another and
lateral continuity helps match up rock layers that are nearby, but what about rocks
separated by great distances?
There are three kinds of clues.
Matching Rock Layers 1
Distinctive rock formations may be recognizable across large regions.
The famous White Cliffs of Dover in southwest England can be matched to similar
white cliffs in Denmark and Germany.
Matching Rock Layers 2
Two separated rock units with the same index fossil are very similar in age.
To be an index fossil the organism must have
1) Been widespread to that it is useful over large areas
2) Existed for a brief period of time so that the age is easy to determine
Matching Rock Layers 3
A key bed can be used like an index fossil since a key bed is a distinctive layer of rock
that can be recognized across a large area. A volcanic ash unit could be a good key
bed.
One famous key bed is the clay layer at the boundary between the Cretaceous Period
and the Tertiary Period, the time that the dinosaurs went extinct. This thin clay
contains a high concentration of iridium, an element that is rare on Earth but
common in asteroids
The Geologic Time Scale
With the information they collected from fossil evidence and using Steno’s principles,
they created a listing of rock layers from oldest to youngest.
Then they divided Earth’s history into blocks of time with each block separated by
important events, such as the disappearance of a species of fossil from the rock
record.
In the geologic time scale the youngest ages are on the top and the oldest on the
bottom.