Important
Facts
Uniformitarianism

The Present is the KEY to the
PAST.

Laws of nature are the same now
as they have always been.

Past events explained and
estimated by modern processes.

Charles Lyell credited with this
concept. Explained in his book
“The Principles of Geology.”
Other Important Words…

Lithification: the consolidation of a
loosely deposited sediment into a
hard sedimentary rock.

Strata: rock layers
Original Horizontality

When sedimentary rock layers form, they
are “created” in a horizontal fashion.
And, most importantly…
Relative vs. Absolute Age of
Rock

Relative Dating: determination of the age of a rock or
geologic event by comparing with other rocks/geologic
events
Can you infer who is older?
She is.
Law of Superposition
In undisturbed
layers of
sedimentary
rock or lava
flows…
Top Layer = Youngest
Bottom Layer = Oldest
Cross-Cutting Relations

Any feature that cuts across a rock or layer of sediment is
younger than the rock or layer it cuts
Law of Included Fragments

Sediments are older than the cement and rock
they are part of
Fossil Correlation

The same group
of fossils are of the
same age, and
therefore, strata
containing them
are also of similar
age.
Uncomformity

A gap or
missing layers
of rock due to
erosion.
Angular Unconformity

Older rock under the line of erosion are tilted at
an angle, while the younger strata above are
horizontal.
Disconformity

Strata above AND below the line of
erosion are horizontal.
Nonconformity

Unconformity that separates igneous or
metamorphic rock from a layer of
sedimentary rock.
Relative vs. Absolute Age of
Rock

Absolute: A form of geologic dating that gives an actual
date/age in history that the item was formed or died.
6 months
82 years
Phenomenon of
Radioactivity

Some elements, such as uranium (U) and
thorium (Th) are unstable.

They decay spontaneously.
The Atom
Radioactive Decay

238 U
spontaneously emits a particle from its
nucleus called an alpha particle (2 protons + 2
neutrons).
Radioactive Decay
Uranium-Thorium Decay
Beta Particle Emission

But, 23490Th is also unstable, and it emits a beta
particle …
Thorium-Protactinium
Decay
U – Pb
Series

This process is
called radioactive
decay, and
eventually
uranium (parent)
decays to lead
(daugher
product)
U – Pb
Series

The rate at which
this process
occurs is
measured in
terms of a “half
life”.
Half Life= Number of years for ½ of the original number
of atoms to decay from U to Pb.
Radiometric Dating

Assume: Rate of decay (half life) of long-lived elements has
not varied since the Earth was formed.

This is an example of uniformitarianism.
Half-Lives of Radioactive
Isotopes
Examples of very old rocks
found on Earth

3.4 billion year old granites – South Africa

3.7 billion year old granites – Greenland

Oldest terrestrial rocks ~ 4 billion years

Central Canada and Central Australia

These are deep in the interiors of the continents

Core drilling aids in excavation of these rocks.