Transcript Secrets of the Layers of Rock

Law of Superposition
 Chapter 8 Lesson 2
Secrets of the Layers of Rock
Law of Superposition
 In layers of horizontal sedimentary rock,
each layer is older than the layer above
it and younger than the layer below it
 “Oldest on bottom, youngest on top”
Age of Rocks
Pages 304-305 in Ch. 8
 The relative age of a rock is its age
compared to the ages of other rocks.
 Ex: You are a 6th grader. Most 6th
graders are about 11 - 12 years old.
 The absolute age of a rock is the
number of years since the rock formed.
 Ex: You are 11 years, 7 months old
Relative age Activity
 As a class you are going to line
yourselves up by your relative age
 (from oldest by the door to youngest to
the other side of the room)
Absolute age
 Now you are going to line yourselves up
by your relative age
(from oldest by the door to youngest to
the other side of the room)
Ex: You are 11 years, 7 months old
Fault: A break in the earths crust
* They are always younger than
the rock it is cutting through
Intrusions: Magma
pushes bodies of rocks
below the surface.
It cools and hardens into
a mass of igneous rock
* Intrusions are always
younger than the rock
layers it around it
Extrusions:
Lava that hardens on the surface
and forms igneous rock
*extrusions are always younger
than the rocks below it
How Rock Layers Can Change
Pages 308-309 in Ch. 8
Unconformities
A gap in the
geological time
record. It shows were
rock layers have been
lost due to erosion
Folding
Layers are overturned
completely. The
youngest layer may
end up as the top
layer
How do fossils help us to tell
time?
Index Fossils
 Fossils of widely distributed organisms
that lived during only one short period.
 Example: ammonite
Methods for Dating the Ages of Rocks:






Relative Age
Index Fossils
Extrusion (igneous)
Intrustion (igneous)
Law of Superposition
Folding
Uncomformity
*The age of rock compared to
the age of other rocks
*Sedimentary rock ( usually)
Absolute Age
•
•
•
•
Radioactive decay
Radioactive dating
Half-life
Carbon-14 dating
*The number of years
since the rock
formed
*Igneous Rock
Radioactive Decay
 Elements in rocks can break down, or decay,
over time. Scientists study the amount of decay
to discover the age of rocks.
 During radioactive decay, the elements of one
atom break down to form atoms of a different
element.
 This rate (or amount/speed) of decay is
measured by the half-life (how long it takes half
the atoms to decay).
Radioactive Dating
Half-Life
The half-life of a
radioactive
element is the
amount of time it
takes for half of
the radioactive
atoms to decay.
What pattern do
you see in the
graph?
Radioactive Dating
 Studying radioactive decay (breakdown of rocks)
to determine the age of a rock.
 That information helps scientists calculate how old
a rock is.
 In Carbon-14 dating, scientists see how much of
the element carbon-14 is left in a rock. When
carbon-14 decays, it turns into Nitrogen-14, so
how much carbon-14 is left will tell scientists how
long it has been decaying, therefore telling them
the age.
Radioactive Dating
Elements Used in Radioactive Dating
Geologic Time
The Geologic Time
Scale: A record of the geological
events and the evolution of life forms
as shown in the fossil records
The divisions of the geologic time
scale are used to date events in
Earth’s history.
It helps make it easier to understand
how things have changed over time,
or evolved
Eras
Divided time between the
Precambraim Time and the present
time into 3 long units
Periods
Eras are subdivided into units of
geological time
* The Precambrian Time covers about
88% of Earth’s History and it ended
542 million years ago. Very few fossils
have survived this time period
How has Earth Changed over time?
 Natural processes such as weathering, erosion and
plate tectonics have reshaped Earth’s surface.
 The distribution of land and water on Earth has
changed over time. Pangea is the large landmass that
use to be on earth. Overtime, the landmass broke up
into different smaller landmasses that have become our
continents
 Uniformitarianism – geologic processes that we
have today we also had in the past.
 This helps scientists understand why and how the Earth
changed. The Earth is always changing or evolving. By
studying Earth’s changes we see in the present,
scientists can make inferences about the past. (like
changes in rocks, weather, etc)
Geologic Time
The Evolving Earth
The timeline shows how Earth has changed over time.
Pangea
The name of
the large
landmass when
the continents
were formed
into one large
landmass.