Relative Age - the age of rock layers in relation to other rock layers.
You don't know the rocks exact age, just which one is older or
Actual Age -
the actual age of rocks in years.
DETERMINING RELATIVE AGE
The Law of Superposition
It's easy. When you look at the layers of sedimentary
rocks, the older layers are on the bottom, below the
younger layers. If you think back to how a sedimentary rock forms,
it makes sense.
The Law of Cross Cutting Relationships
The relative age of igneous rocks, due
to an intrusion, are younger than the rock layers they cut
across. That means that a sedimentary rock is always OLDER than the intrusive rock that rudely shoved its way into it.
The Law of Original Horizontality
All sedimentary rocks should be in flat layers.
If not something seriously went wrong. The most likely culprit
is a mountain building event that created folds, tilts and even
Remember that all faults are younger than
the rock they cut across, because the rocks must be there
first for the fault to occur.
Unconformities are erosional events that break
the sequence of deposition. On a cross section, it looks like
a thick wavy black line interrupting the parallel layers of sedimentary
Correlating Rock Layers From Different
An outcrop is where the rock layers are exposed,
such as on a cliff or road cut. Most rock layers are covered in
soil and plants so identifying the relative age of rocks can be a detective
By matching the characteristics of several
outcrops, we can determine the relative position of identical
rock layers and then figure out which is older.
Using physical characteristics Wegener, the scientist that proposed continental drift,
noticed similar rock layers in Africa and South America. One rock
layer had white diamonds in both South Africa and Brazil! Wegener
theorized that these areas must have been connected at one point
in time, and this led to his famous theory we now know as plate
Index fossils are remains of organisms that
are common, lived in a wide geographic area and existed for only
a short period of time. In other words, if you are trying to identify
the relative age of a rock, finding a fossil that only lived in
a small region or that lived for a really long period of time
Check out these index
fossils greatest hits.
Key beds are beds that have a very unique property
that is not easily confused with other rock layers. Examples include
volcanic ash deposits, coal seams or nuclear fall-out!
Another interesting key bed item would be Iridium,
a radioactive material often found in meteors, but rare on earth.
This layer is 66 million years old. Scientists theorize that a
huge meteor collided with the earth, and then sent a huge cloud
of dust rich in Iridium circling the earth, which then settled
and became part of the rock layers. It is the "smoking gun"
for the massive dinosaur extinction.
Just as a clock is broken into units of time,
so is the geologic record. Instead of being broken into minutes,
hours, and seconds, geologic time is broken into different units- eons, eras, periods and epochs.
A brief history of geologic time video
Video on Earth History
Eons are the largest geologic unit of time.
The oldest is the Archaen, or the time from the
creation of the earth to the appearance of bacteria-like multicelled
The next eon, called the Proterozoic,
was the period in which multicelled organisms ruled the world.
We don't know much about these critters however because they had
no hard preservable parts.
Combined, the Archaen and the Proterozoic are
called the Pre-Cambrian.
The next eon is called the Phanerozoic,
which we are now in. Obviously, multicelled organisms with preservable
parts are everywhere. As time went on life forms became more and
Eons are divided into eras, based on fossil records. Due to the fact that the Precambrian has no fossils,
the Archaen and Proterazoic eons have not been divided.
The Phanerozoic eon is divided into three eras:
Old Life (marine invertebrates)
Middle Life (reptiles)
Recent Life (mammals)
Eras are divided into periods, determined by
rock formations and their ages.
Periods are divided into epochs, the smallest geologic unit of time. Each period usually has three epochs, early,
middle, late. The exception is the Cenozoic Era (our era) which
is divided into all sorts of different Greek names (ex. Anthropocene-
our present epoch).
Video on Geologic History in the Earth Seience Reference Table page 8
Video on Geologic History in the Earth Science Reference Table page 9
Half Life Chart. What is going on here?
Video on Radioactive Decay
All elements are made from atoms, and most
elements have different isotopes. Isotopes are varieties
of an element with a different mass because they contain a an
unusual number of neutrons.
An example is carbon. Its isotopes
include C-12 and C-14. The number following the chemical
symbol of the element is called the atomic mass.
Most isotopes are stable, meaning they will
not change. Some isotopes, however, are very unstable. During
the process of radioactive decay, atoms split apart, releasing
energy and parts of their nucleus. The atom will release material
and energy until it becomes a stable isotope, and this atom is
no longer radioactive. This is called the decay product.
An example is Uranium-238 (U-238), which will decay into Lead-109 (Pb-109).
Radioactive decay will occur at a constant rate for each particular element. It is not affected by any factors- not pressure, not heat, nor even the neighborhood it
When rocks contain minerals with radioactive
isotopes, their radioactivity can be used as a clock. The half
life of a substance is the amount of time that one
half of the unstable radio-isotope takes to turn into a stable
For example: The radioactive half life for
Uranium-238 is 4,500,000,000 years or 4.5 billion years. If you
originally had ten grams of U-238, in 4,500,000,000 years five grams of U-238 would turn into lead called Pb-109
and five grams of U-238 will remain the same. But in the
next half-live (in another 4.5 billion years), half of the remaining
U-238 would turn into Pb-109 and half will stay the same. Now
you have a grand total of seven and 1/2 grams of Pb-109
and two 1/2 grams of U-238. It's like magic.
Video on Radioactive Decay in Earth Science Reference Table