Download Rocks and Fossils

Geologic Time
The Foundation: Fossils and Relative Time
Understanding how both are used to get a better
understanding of our Earth’s past (and its future)
*The Beginning: 3 Basic
Rock Types*
 Sedimentary Rock- Rocks made by erosion of
rock;( limestone, sandstone, shale and they
are water soluble)
 Igneous Rock – rocks made by magma;
granite, marble, and basalt and are water
resistant)
 Metamorphic rock- sedimentary rocks that due
to being under pressure and heat turn into
more igneous nature
Determining the age of
rocks: 2 ways
 Absolute age -Actual Age (determined
by Carbon dating)
 Relative Age -Age compared to other
objects (gives you an estimate of its age;
index fossils or depth provide this)
*Absolute Age*
 It tell the actual age or how long ago an
even occurred
 They use Radioactive dating
 Radioactive dating-measures the age of
a material by comparing the amount of a
radioactive form of an element in a rock
or fossil with the amount of its decay
product
 Precise
*Relative age*
 It tells if something happened earlier or
later than others without giving a definite
date
Did these Rocks fall before
or after the rock layers
formed?
*Law of superposition*
 Formed by undisturbed sedimentary rock
layers
 Older rock layers lie beneath younger
rock layers: (Grand Canyon at first
glance)
Exceptions to the Rule: 4




Overturned Bed
Angular Bed
Intrusions
Unconformity
Law of Superposition
EXCEPTIONS: Overturned
bed
 Sedimentary Rock layers totally
misplaced
 Commonly caused by Compression
forces and found in mountains
Law of Super Position EXCEPTIONS:
Angular bed
 Horizontal rocks are younger than the
tilted rocks
 Tilted rock caused plate-tectonic forces
Law of Superposition
EXCEPTIONS : Intrusion
Caused by Magma
 Igneous rocks are younger than any
sedimentary rocks they cut
*Law Of Superposition
EXCEPTIONS*:
Unconformity
 Missing layer or gap in the rock record:
caused by magma intrusion or erosion
 Makes it hard to understand how Earth
has changed during a specific period of
time
Missing Layer destroyed
by magma
*2 Types of Erosion of
Rocks*
 Mechanical
Weathering :
 water freezes and
unfreezes causing
Rocks to split open
from ice the
expanding in the
cracks;
 also wind blowing
sand against rocks
*2 Types of Erosion of
Rocks: Chemical Weathering
*
 Chemical
Weathering- Acid
rain dissolves away
the rock
Why do We Care about
Rocks?
 Rocks provide us
with a “Picture” of the
Earth’s Past History
 Fossils
*Fossils*
 Important to understand the history of life
on Earth
 Older rock—simpler organisms and
Younger rock-more complex organisms
 Shows how species have changed:
EVOLUTION
 Shows how species are related to
another or changed to environmental
change
*How Fossils Form*
 1 Quick Burial of organism in soft sediment:
Mudslide, volcanic ash, quicksand, mudpits, tarpits
 2 Organism remains undisturbed for long period of
time
 3. Water slowly enters area soft tissue is dissloved
and water slowly eats away bone/shell
 4. If water flow is “fast” complete organism will be
eaten away creating “hole” in the rock. Leaves
basic shape of organism
 5. If water eats away very slowly minerals in water
can replace bone creating a Cast. Minerals
recrystalize as soon as bone is dissolved creating
a PERFECT COPY!
 6. Fossil is then found millions of years later:
FACT-Only 17 T-rex Skeletons have ever been
found
Ways Fossils form
Fossil
Process
Notes
Molds and casts
Acids eat away the skeleton or
shell and leave an
impression in the rock
If the mold fills with minerals it
becomes a cast
Tracks and trails
Imprints are left in mud which
later hardens
Dino tracks are common
Carbonization
Oils leave the plant and the
remaining matter becomes
a layer of carbon
fossilized plants
Petrification
Minerals in plant cells
crystallize; minerals enter
openings or cavities in
shells or bones
Common with plants
Replacement
Object buried in mud has its
molecules replaced by
minerals
rare
Recrystallization
mineral aragonite in shells turns
then into calcite
Preserves the general shape of
the animal
Soft tissue preservation
Mummification, frozen in ice
Fossils are relatively you and do
not last long
Traps
Whole organisms locked in tar
pits, asphalt, amber etc…
Molds
 Hollow area in sediment in the shape of
an organism or part of organism
 Hard part of organism buried in sediment
 Water carries dissolved minerals and
sediment may seep into the empty space
of the mold
Cast/Recrystalization
 Solid copy of the shape of an organism:
bones replaced by minerals in water.
EXTREMELY ACCURATE; VERY RARE
Petrified
 Fossils where minerals replace all or part
of an organism
 Turns into stone
 Water rich with mineral seep into spaces
Carbonization
 Oils from organism
turn into carbon
imprint on rock
 Common in plants
and Fish
Trace fossils
 Fossilized footprint or trail
*INDEX FOSSILS*
 Index fossilcommonly found
fossil worldwide that
can give you an
approximate age
and environment of
the area then.
(Trilobites)