Transcript Which rocks are younger?

GEOLOGIC HISTORY
THE RULES OF THE GAME
© Copyright 2005 - 2006 M. J. Krech. All rights reserved.
Time is…
Either
relative or
absolute
Time is: a measured period during
which an event, process, or
condition exists or continues.
So….How do scientists tell how old rocks are anyway?
Scientists use fossils to help determine the age of rock layers.
1. The age of a fossil can be determined by carbon dating:
1. Carbon exists in all living things
2. Carbon decomposes at a specific rate that can be calculated
3. Scientists can measure how much carbon a fossil has left
in it in order to figure out how long it has been dead.
4. This is an example of absolute age (dating).
2. Some fossils can be used as index fossils:
1. These fossils are used to determine the approximate age of the
layer of rock that they are found in.
2. In order to be an index fossil they have to:
1. Have only existed for a certain short amount of time.
2. Have to have lived in many different places across the globe.
Dating Techniques
One ways in which geologists look at
time: Relative Dating
Relative Dating is..
What happened first?
What happened next?
…the measure
of a sequence
of events
without knowing
the exact date
at which the
events
occurred.
UNIFORMITARIANISM
Relative Age: The age of a rock layer compared to
the other rock layers around it. ( Younger or older )
Absolute Age: The age in years (definite age….14 years old)
SANDSTONE
Youngest Layer
The rock layer that
forms first is found on
the bottom of the pile
and is the oldest layer.
SHALE
The rock layer that is
on top of the pile is the
youngest.
LIMESTONE
Oldest Layer
This is called:
The Law of
Superposition.
Q#1
Principle of Superposition
• Which
rocks are
older?
• Which
rocks are
younger?
When a layer cuts
across another layer
in the sequence, it has
to be younger than the
layer that it cuts apart.
Limestone
Shale
Sandstone
Granite
ConglomerateIntrusion
This igneous intrusion
is younger than the
conglomerate, sandstone
and shale because it
cuts through each of
those layers.
This is called:
The Law of Cross-cutting Relationships
Q#2
Principle of Cross-Cutting
• What is
younger?
• The rocks
or the
fault line?
Sediments laid down in layers
over time lithified into rock
Tilting by
Uplift
Erosion of exposed
Uplifted areas
3.
2.
3.
1.
5.
4.
3.
New layers of sediment
laid down on top
of the tilted and
eroded layers
Principle of Inclusions
• Rock layers which contain other rocks
are younger than the inclusion.
The
inclusion
(the other
rocks)
is older!
Q#3
Principle of Inclusions
Which is older? The rock layer or the
rocks included in the layer?
Principle of Unconformities
Unconformities are surfaces
that represent gaps in the
geologic record.
Principle of Unconformities
missing rock layer = missing time
http://www.classzone.com/books/earth_science/terc/content/visualizations/es2902/es2902page01.cfm?chapter_no=visualization
Q#4 Where's the missing sediment?
How
do
you
know?
Q#5 Where's the missing sediment?
How
do
you
know?
What happened first?
How
do
you
know?
Where is the youngest rock?
How
do
you
know?
Where is the unconformity?
How
do
you
know?
What rock layer is the oldest?
How
do
you
know?
Which is younger?
The fault or the rock layers?
How
do
you
know?
Does this diagram show
an unconformity?
How
do
you
know?
BASALT
GRANITE
LIMESTONE
SANDSTONE
SHALE
RHYOLITE
CONGLOMERATE
METAMORPHIC