Transcript Deforming the Earth`s Crust PPT

Relative Dating & Deforming
the Earth’s Crust
Folds, Faults and Deformities…
OH MY!!
Uniformitarianism
Geologic processes that occurred in the past can
be explained by current geologic processes
1.
a.
b.
Erosion and weathering that happens today must
have been the same millions of years ago
Very slow, gradual, predictable process
Is this the correct principle?
Catastrophism
Geologic change happens suddenly
1.
a.
b.
2.
Mountains, canyons and oceans formed during rare
events
These rare events are called catastrophes
Up until the mid-1800’s, this was the accepted
principle
Why was this the accepted principle?
Modern Geology
Uniformitarianism
Catastrophism
• Can geologic processes take a
very long time? (think Tectonic
Plates, mountains growing)
• Can events happen very quickly?
(Think meteor causing dinosaur
extinction)
• YES!
• YES!
• Mountains forming/growing
• Grand Canyon forming from
erosion
• Moving of the continents
• Large Volcanic eruptions forming
islands and causing extinctions
• Meteor strikes creating craters and
causing extinctions
• Earthquakes, Tsunamis, etc…
Relative Dating
1.
Relative Dating – a method used to determine
whether an object or event is older or younger than
another object or event
Who is Older or Younger?
Absolute Dating
Absolute Dating – any method used to determine
the age of an event or object in numbers
1.

Certain molecules called Isotopes decay over time

Isotope – An atom that has the same number of
protons, but a different number of neutrons

We can use this decay to measure how old something
is (rocks, fossils, etc…)
1.
Half-life – the amount of time it takes for half of a
radioactive substance to decay
Disturbances

Unfortunately not all rock layers are laid out
perfectly. Some things that can make things more
difficult are…
1.
Unconformity – missing part of a geologic column
a.
2.
Missing rock layer(s) due to either Erosion or Nondeposition
Deformations – Altering of rock layers after they
are laid down, but not removing them
Deformations
Deformations

Tectonic plates move, causing stress to build up in
the lithosphere
a.

Stress = force (three types)
1. Compression = squeeze together
2.
Tension = stretch apart
3.
Shearing = sliding past
Stress from tectonic plates cause rock layers in the
lithosphere to change
Folding
Definition: bending rock layers due to stress
a)
Older rock starts on bottom
Anticline = upward arch (looks like an A)
Syncline = downward, U shaped (think sink)
SYNCLINE
ANTICLINE
Folding


Folds can be large
or small
Some measured in
kilometers or
centimeters
MONOCLINE
Faults
Definition: surface along which rocks break and
slide past each other


Can be seen on the earth’s surface
Usually have a Hanging wall and Footwall
a.
b.
c.
Type of Stress?
Which wall falls down?
Are rock layers disturbed?
a.
b.
c.
Type of Stress?
Which wall falls down?
Are rock layers disturbed?
a.
b.
c.
Type of Stress?
Which wall falls down?
Are rock layers disturbed?
Relative Dating



Faults, Unconformities and Deformations raise a
very important problem…
Rock layers aren’t always perfect
So what do we do when rock layers aren’t perfect?
What happens when they look more like…
This. How do we find Relative Age?
Relative Dating

One way scientists calculate relative age is by using
a set of rules
1.
2.
3.
4.
Superposition – if undisturbed, rock layers will
always be older as you go down (youngest on top)
Original Horizontality – Layers of rock are always
formed in relatively even horizontal layers
Cross-Cutting Rule – if there is a fault cutting through
layers of rock, the rock must be older
Deformation Rule – if there is a deformation (fold,
tilt, intrusion) the rock that is deformed must be older
How do we find Relative Age?
Relative Dating

Another way geologists can figure out disturbed
layers of rock is with the Geologic Column

Geologic Column – a complete set of the rock layers
with all known fossils and layers in their proper order


Geologic Columns are used to find Unconformities
When a layer of rock is missing from Erosion or NonDeposition the geologic column can tell geologists
what SHOULD be there
Which layers
are missing
from the
second
outcropping?