Transcript Day 11 Relative Dating & Principles Notes 11

Determining Relative Age
The beginning of modern
geology
James Hutton (1726 – 1797)
• Father of Modern
Geology
• Formulated
Uniformitarianism
• Also formulated
Plutonic Theory
• Thought that the
Earth was very
old.
Uniformitarianism
• A principle that assumes geologic processes
that occurred in the past can be explained
by current geologic processes.
– Examples:
• Past volcanoes act just like modern
volcanoes.
• Erosion occurs today just like it did
long ago.
• Mountains are forming and eroding
just like they did millions of years ago.
Hutton and the Earth’s Age
• In Hutton’s day most
people thought that
the Earth was about
6,000 years old
• Hutton observed
geologic changes on
his farm were very
slow and determined
that the Earth must
be infinitely old.
MOST GEOLOGISTS TODAY AGREE THAT THE EARTH IS
ABOUT 4.6 BILLION YEARS OLD
Relative Age
• Layers of rock, called
strata, show the
sequence of events
that took place in the
past.
• Relative age indicates
that one layer is
older or younger
than another layer.
RELATIVE AGE DOES NOT GIVE A LAYER’S AGE IN YEARS
Law of Superposition
• A sedimentary rock layer, or bed, is older than
the layers above it and younger than the layers
below it if the layers are not disturbed.
Principle of Original Horizontality
• Sedimentary rocks left undisturbed
will remain in horizontal layers.
• Sedimentary rock layers that are not
horizontal must have been acted on
by forces within the Earth.
• Sometimes these forces can force
older rock on top of younger rock.
Law of Superposition and the Principle of
Horizontality
Clues to Original Horizontality
• Graded Bedding
– Larger particles should be on the bottom of
the layers.
• Cross Beds
– Caused by soft sediments, like sand,
slumping to form curved layers with respect
to the bedding plane. The bedding plane is
the boundary between two layers, or beds.
• Ripple Marks
– Ripple marks, formed by water, point up in
undisturbed layers.
Clues to Original Horizontality
Unconformities
• A break in the geologic record created
when rock layers are eroded or when
sediment is not deposited for a long
period of time.
• There are three types of unconformities
– Nonconformity
– Angular unconformity
– Disconformity
Nonconformity
•
When stratified rock (sedimentary)
rests on unstratified rock (igneous).
Laws of Crosscutting Relationships
•
•
•
A fault is a break or crack in the Earth’s crust
along which rocks shift their position.
An intrusion is a mass of igneous rock which is
injected into rock and then cools and solidifies.
A fault or igneous intrusion is always younger
than the rocks it cuts through.
Laws of Crosscutting Relationships
Laws of Crosscutting Relationships
Angular Unconformity
•
When horizontal layers of rock rest
on top of tilted layers of rock.
Disconformity
•
Occurs when younger rock is
layered on top of older rock that has
already been eroded.
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization
Principle of Inclusions
• Included rock fragments must be older than
the rock in which they are found.
Principle of Faunal Succession
• Fossils in rocks
occur in a fixed,
worldwide
sequence.
• These fossils
can be used to
correlate and
relatively date
rocks in which
they are found.
Index Fossil – widespread and is
associated with a specific span of
geologic time