The Rock and Fossil Record Studying the rock and fossil record Download

The Rock and Fossil
Studying the rock and fossil record helps
us understand Earth’s history and the
history of life on Earth.
The Principle of Superposition
(Law of Superposition)
• Which layer is the oldest?
• In Sedimentary rock, the oldest is
on the bottom the youngest in on
the top, this is Superposition. The
rock’s age is based on the position
of the rock.
The Principle of Superposition
• Applies mainly to sedimentary rock layers!!!
• This principle states that the younger
rocks are located above the older rocks.
The rock layer must be undisturbed!
• Sketch
Disturbing Forces
• Forces within Earth can disturb the rock layers
(rock layers are also know as rock sequences).
• Examples of Events (plate boundaries
converging, diverging, transforming)
1. Folding 2. Tilting
• Examples of Features that Disturb:
1. Faults (found along all plate boundaries)
2. Intrusion (potential volcanic activity)
Fault (Disturbed Rock Layer)
Intrusion (Disturbed Rock Layer)
• Unconformities: An unconformity is a gap in
the geological record that can occur when
erosion wears away rock layers and other
rock layers form on top of the eroded
• Just remember these are usually as a result
of erosion and nondeposition (Oops!! The
sediments did not get deposited!!)
The Geologic Column
• Simply think of the Grand Canyon:
• An arrangement of rock layers in which
the oldest rock layers are at the bottom
Dating Rock Layers
• 2 Ways to Measure the Age of Rocks:
1. Relative Dating (Age)
1. Absolute Dating (Age)
Relative Age
 The relative age of a rock is its age
compared to other rocks.
Use words like: “older or younger”
(For example--think of your relatives and
your age:
I am younger than my cousin Scott,
older than my sister Sandy and older
than my sister Rhonda. My age
compared to my relatives.)
 Now you do an example of relative age!!
Relative Dating: “When Igneous Rock
Meets Sedimentary Rock”
 REMEMBER: Igneous Rock
 Lava that cools at the surface is
called an extrusion. Rock below an
extrusion is always older.
 Magma that cools beneath the surface
is called an intrusion. An intrusion is
always younger than the rock layers
Can you find the intrusion of
igneous rock?
Absolute Age
• The absolute age
of a rock is the
number of years
since the rock
was formed.
• (mya—million years ago)
Why is the study of fossils so important?
Consider the following?
The Beartooth Range
of Montana is just north
of Yellowstone. In this
location you will find
Beartooth Butte. Partway
down from its 10,500 foot
(3200 m) summit, within the dark red zone of rocks,
geologists have found fish fossils as old as 400 million
How is this possible? (Give a good scientific hypothesis.)
One Answer:
Plate tectonics push up the rock making a
mountain, weathering and erosion move the dirt
and expose the marine fossil.
Fossils can give us clues of what Earth’s
surface looked like in the past.
Fast Fossil Facts
(now say that three times in your HEAD!)
Fossils are the preserved remains of
past life on Earth.
Why are all dead organisms not
preserved as fossils?
Why are all dead organisms not preserved
as fossils?
1. Hard parts of organisms are most likely
to be fossilized.
2. Conditions must be perfect to preserve
the fossil.
In other words….
 When an organism dies, its soft parts
often decay quickly leaving only the hard
parts to fossilize.
 Ex. Bones, Shells, Teeth, or Seeds
*Fossils are found in rocks
deposited in the environment
in which they lived.
Fossils provide clues to the past
Fossils give clues of past climate and
surface changes.
Fossils of palm leaves in Antarctica could mean
that Antarctica was once in a warm tropical area.
Marine fossils found on
mountain tops or land well above
sea level. (surface change)
Revisiting the Rock Cycle :.
1. Fossils are generally most abundant in
marine sedimentary rocks.
2. Fossils generally are not found in igneous
and metamorphic rocks.
3. Most fossils found in sedimentary rock.
Fossil Types:
1. Trace fossil: provide evidence of
the activities of ancient organisms.
Examples: footprints, animal trails, or
animal burrows.
• 2. Mold: a hollow area in sediment in the
shape of an organism or part of an organism.
• 3. Cast: a copy of the shape of an
Other Types
 Petrified Fossils: fossils in which minerals
replace all or part of the organism. Ex:
petrified wood
 Carbon Films: an extremely
thin coating of carbon on rock
that forms when materials
that make up an organism
become gases and escape
leaving only carbon behind.
 Preserved Remains are formed when an
organism is preserved with little or no
change. {For example when organisms
become preserved in tar, amber (tree
sap), and freezing.}
Index Fossils
• An index fossil must be widely distributed
and represent a type of organism that
existed only briefly.
• One example of an index fossil is a trilobite.
• Trilobites were a group of hard-shelled
animals whose bodies had three distinct
• They evolved in shallow seas more than 500
million years ago.
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