Transcript Geologic Time

Geologic Time
The Rock and Fossil Record
Geologic Time Scale


time
divisions
Geologists use different _______
________
to keep track of Earth’s history.
These divisions are based on:



life forms present during the time
Type of ____________
period
geologic changes occurring at the
Type of ____________
time.
Geologic Time Scale – a record of Earth’s
history, beginning 4.6 bya, that shows events,
time units, and ages.
Geologic Time Scale

There are three types of subdivisions of
geologic time:



ERA – major subdivisions of time based on
______
differences in life-forms.
Period
__________
– subdivisions of eras based on
types of life and geologic events (mountain
building or plate movements).
Epoch
_________
– subdivisions of periods based on
types of life and geologic events. (Epochs are
found in the Cenozoic Era alone because the
fossil records and rock layers are more complete
for this time.)
Era
Phane
rozoic
Eon
Cenozoic
Period
Epoch
Quaternary
Recent
Pleistocene
Neogene
Pliocene
Miocene
Paleogene
Oligocene
Eocene
Paleocene
Mesozoic
Cretaceous
Jurassic
Triassic
Paleozoic
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Precambrian Time
Earths Story and Those Who First
Listened.

uniformitarianism to
Scientists use ____________________
interpret the past and predict the future.


James Hutton 1795 – “Theory of Earth”
present
past
The _______
is the key to the _____.

Things that happened now must have
happened in the past.
Uniformitarianism versus Catastrophism



Uniformitarianism – equal to slow change
Catastrophism – equals quick sudden
changes.
Which is right?

A little of both.
Relative Dating - Superposition




Here’s a stack of
newspapers collecting
dust in the living room.
Everyday as the paper is
read it is put in the pile.
Someone comes to you
and asks about an
article from last week.
Where do you look?
Relative Dating? Which came first?

Geologists use relative Which layer is older?
dating to determine the
____________
relative age of objects.
Principle of Superposition –
younger layers lie above
older layers in an
undisturbed sequence of
C
rocks.
A

B
A
The Principle of Superposition

In an undisturbed layer of rock the oldest rock
BOTTOM and the youngest is
is on the ___________
on the _______.
TOP
The Principle of Superposition

Exceptions to the Principle include tectonic
MOVE tilt,
forces acting on the rocks to _______,
ERODE
fault, or __________
rock layers.
Original Horizontality: sedimentary
layers are initially deposited in horizontal
layers

The forces of reality beat up on the earth and
break it into small pieces, which then wash
down rivers to the ocean and settle out on the
seafloor. In horizontal layers.
Relative Dating

Used by geologist to determine the order of
events and the relative age of rocks by their
POSITION in the stack.
___________
Relative Dating


This image shows a stack of rocks with 3
different types of rock and one fault.
Which is the oldest?
Relative Dating

Here’s the sequence: A, B, fault, C
C
B
B
A
Unconformities




Rock layers are a record of past geologic
events.
INCOMPLETE because they are
Most are ____________
missing layers.
If sediment stops being deposited and
EROSION goes to work removing material
_________
then you will have a gap in the record.
These gaps are called _________________.
UNCONFORMITIES
Angular Unconformities
Angular Unconformities
Angular Unconformities
Disconformity

No obvious evidence of missing rock layers
UNEVEN surface caused by
except for an __________
erosion.
Nonconformity
When sedimentary rock layers form above
METAMORPHIC
IGNEOUS
____________ or intrusive ___________
rock layers.
 The metamorphic or igneous rock is uplifted
and eroded.
 Sedimentary rocks are then deposited on top.

Nonconformity
Arrow points to contact of Cambrian Tapeats Sandstone on
Precambrian Vishnu Schist.
Cross-cutting relationships: the thing
OLDER than the thing
being cut is ________
doing the cutting



You can’t cut it if it isn’t there.
Igneous intrusions:
Dikes, Sills, Batholiths, Laccoliths
Cross-cutting Relationships
Fossils
Fossils

Definition:

Fossil – the remains, imprints, or traces of onceliving organisms, usually preserved in rock, that
tell us when, where, and how those organisms
lived.
Fossils


Fossils help scientists determine when and
where plants and animals lived.
Also, how they lived.
Fossil Formation

Necessary Conditions:

The body of a dead organism must be protected
from scavengers and microorganisms.

Quick Burial (sediments from floods, landslides,
________________
etc.)
Hard Parts
________________
(bones, shells, or teeth)

Sedimentary
Rocks
_________________________
Fossils typically do not

withstand the heat and pressure involved in forming
igneous and metamorphic rocks.

Limestone, Shale, Sandstone are examples
NEARLY ALL FOSSILS ARE FOUND IN SEDIMENTARY ROCK
Fossil in Virginia
Appalachian Plateau
Valley & Ridge
Coastal Plain
Why not the Blue Ridge and Piedmont?
Fossil in Virginia
Era
Period
Epoch
Cenozoic
Quaternary
Recent
Pleistocene
Tertiary
Pliocene
Miocene
Oligocene
Eocene
Paleozoic, Mesozoic,
and Cenozoic fossils
are found in Virginia
Paleocene
Mesozoic
Cretaceous
Jurassic
Triassic
Paleozoic
Permian
Pennsylvanian
544 mya – present
Paleozoic
Mesozoic
Cenozoic
Mississippian
Devonian
Silurian
Ordovician
Cambrian
Precambrian Time
Petrified Remains

Some or all of the
original materials in the
remains have been
replaced by minerals.
Carbonaceous Films



The tissues of most organisms are made of
compounds that contain carbon.
Sometimes that’s all that is left.
Pressure and heat force gases and liquids
from the body and leave an outline of the
body.
Carbonaceous Films

Swamps and deltas where large amounts of
plant matter are areas in which these fossils
are found.
Molds and Casts





Seashells or other hard parts fall into soft sediment such as
mud and get buried.
Compaction and sedimentation turn the sediments to rock.
Pores in the rock let water and air reach the object and allow
it to decay.
The resulting void is called a mold.
Other sediments may fill the mold and solidify into a cast.
Original Remains




Sometimes the original remains are
preserved.
Insects trapped in amber (crystallized resin)
Ice men, mammoths
Tar Pits
Trace Fossils



Fossilized tracks or other evidence of animal
activity.
Dino-tracks
Coprolites
Index fossils



Species inhabit Earth for a period of time
before evolving into new species or dying out.
Paleontologists use animals that exist for a
short time period as index fossils.
Index fossil – species that existed on Earth
for a relatively ______
short period of time, were
_________, were ___________
numerous
wide-spread
geographically.
Index fossils

Index fossils are used to determine the age of
rock layers.
200
Life Span (mya)
A
500 - 200
B
400 - 200
C
400 - 300
300
400
500
Fossils and Ancient Environments


environment
Fossils also indicate the ______________of
an area long ago.
Remember Antarctic rocks contain fossils of
tropical plants
Fossils and Ancient Environments




Most VA fossils are of __________________
marine organisms
Brachiopod fossils indicate a shallow sea
environment.
Brach’s are found in the mountains of
Virginia
What can you conclude from this?
Absolute Dating


numerical
Absolute time places a _____________
age
on an event.
Radioactive decay is used to determine
_________________
the absolute age of rocks.
Radiometric Dating


Radiometric Dating – determining the age of
a material by comparing the ratio of the
___________
(undecayed) material to the
parent
___________
(decayed) material.
daughter
Example: A rock has an isotope that takes
10,000 years to decay into half of the parent
material. The half-live is 10,000 years.
Scientist examine the rock and find exactly
half of the material is “parent” material and
half is “daughter” material. Then the age of
the rock has to be _______________
years.
10,000
C