Transcript 3. Radiometric Dating

Geologic Time
Mr. Litaker
4/2/2016
1
Objectives

State the principle of uniformitarianism.

Explain how the law of superposition can be used to determine the relative
age of rocks.

Apply the law of crosscutting relationships to determine the relative age of
rocks.
4/2/2016
2
Uniformitarianism

In the late 1700’s, James Hutton
published his Theory of the Earth.
Hutton put forth the idea of…

UNIFORMITARIANISM: the same
forces which change Earth’s
surface now, such as volcanism
and erosion, were at work in the
past
4/2/2016
3
Earth’s Age

Hutton’s ideas raised serious
questions about Earth’s age. Until
that time, most scientists thought
Earth was only 6,000 years old,
and all Earth’s features had
formed at the same time.

Geologists now estimate that
Earth is about 4.6 billion years old
4/2/2016
4
Relative Dating

Principle of Original Horizontality:
sedimentary rocks left undisturbed
will remain in horizontal layers

Law of Superposition: in
sedimentary rock, each layer is
older than the one above it and
younger than the one below it.


Relative Dating: tells us the
sequence of events, NOT how
long ago they occurred. What
came 1st, 2nd, 3rd…
5
4/2/2016
6
Cross-Cutting Relationships

Principle of Cross-Cutting
Relationships: when a fault cuts
through a layer of rock, the fault is
younger than the rocks affected

The rock layers must have been
there 1st in order to fragment or
break, so intrusions (faults) must
be younger than the layer they
intrude upon

Relative Dating Video
Absolute Dating Methods


Absolute Age: numeric age of an
object, as established by a
process, such as radiometric
dating
1. Rates of Erosion: scientists
use this method to estimate
absolute age

only practical for geologic features
which formed within the past 10,000
to 20,000 years

less dependable for older
features because rates of
erosion can vary
8
Absolute Dating Methods

2. Rates of Deposition: estimate
of absolute age by calculating the
rate of sediment deposition.


not always accurate because not all
sediment is deposited at the same
rate
provides only an estimate of absolute
age.
Absolute Dating Methods



3. Radiometric Dating:
comparing the remaining
percentage of an element’s nuclei
with one which has not decayed
Nuclei of the same element emit
particles at a constant rate
Half-Life: time required for half of
an element’s nuclei to decay
10
Absolute Dating Methods

Scientists measure the
concentration of an original
element’s nuclei against one
which has decayed in a rock, and
use the known decay rate of that
element to determine the absolute
age of the rock.

Radiometric Dating Video
4/2/2016
11
4. Carbon Dating

Atoms of the same element which
have different numbers of
neutrons are called isotopes

All living organisms contain
carbon-12 and carbon-14 isotope.

So, living organisms contain
carbon atoms with 12 neutrons
and other carbon atoms with 14
neutrons
Carbon Dating

Once a plant or animal dies, the
ratio of 14C to 12C begins to
change.

To find the age of a sample of
non-living organic material,
scientists take the ratio of 14C to
12C and then compare this with
the ratio of 14C to 12 C known to
exist in a living organism.
Carbon Dating Videog

The
Geologic
Time Scale
14