Geologic History
Earth is very, very old…
Earth’s History
• The history of Earth and the ages of rocks can
be investigated and understood by studying
rocks and fossils.
• Evidence of ancient, often extinct life is
preserved in many sedimentary rocks.
Fossils
• Fossils—remains, imprints, or traces of prehistoric
organisms
– 1. Fossils can form if the organism is quickly buried by
sediments.
– 2. Organisms with hard parts are more likely to become
fossils than organisms with soft parts.
Fossils
• Fossil evidence
indicates that life
forms have
changed and
become more
complex over
geologic time.
Relative Ages of Rocks
• Principle of superposition—process of reading
undisturbed rock layers
– 1. oldest rocks in the bottom layer
– 2. younger rocks in the top layers
Relative Ages of Rocks
• How old something is in comparison with
something else is its relative age.
– 1. The age of undisturbed rocks can be determined by
examining layer sequences.
– 2. The age of disturbed rocks may have to be determined
by fossils or other clues
Relative Ages of Rocks
• The same rock layers can be found in different
locations; fossils can be used to correlate those
rock layers.
Absolute Ages of Rocks
• Absolute age—age, in years, of a rock or other
object; determined by properties of atoms
Absolute Ages of Rocks
• Unstable isotopes break down into other isotopes
and particles in the process of radioactive decay.
Absolute Ages of Rocks
• Calculating the absolute age of a rock using the ratio of parent
isotope to daughter product and the half-life of the parent is called
radiometric dating.
– 1. Potassium-argon dating is used to date ancient rocks millions of years old.
– 2. Carbon-14 dating is used to date bones, wood, and charcoal up to 75,000
years old.
– 3. Earth is estimated to be about 4.6 billion years old; the oldest known rocks
are about 3.96 billion years old.
Absolute Ages of Rocks
• Uniformitarianism—Earth
processes occurring today
are similar to those which
occurred in the past
The Geologic Time Scale
• Geologic time scale is a record of Earth’s history
from 4.6 billion years ago to the present
• Based upon rock and fossil records.
Eons
• An eon is the largest unit of time and is
measured in billions of years. There are four
Eons in Earth’s History.
Eons
• 90% OF Earth’s history falls in the Archean and
Proterozoic Eons. This is referred to as
Precambrian Time.
Eras
• An era is the next longest span of time. It is
measured in hundreds of millions of years.
• Eras are defined by differences in life forms
found in rocks.
Periods
• Periods are defined by the
life forms that were
abundant or became extinct
at the time in which specific
rocks were deposited. They
are measured in tens of
millions of years.
Epochs
• The fossil record of the Cenozoic Era is complete
enough to allow further more precise divisions.
• This is the smallest unit of geological time.
• Not all periods have epochs.
In Virginia…
• Fossils are found mainly in the Coastal Plain, Valley
and Ridge, and Appalachian Plateau provinces.
• Most fossils in Virginia are of marine organisms,
indicating that at one time Virginia may have been
periodically covered by sea water.
• Paleozoic, Mesozoic, and Cenozoic fossils are found
in Virginia.
In Virginia…
• Virginia has 5 physiographic provinces with unique
physical characteristics resulting from geological
past.
Coastal Plain
• Flat area composed of young sediments over
older crystalline base rocks.
• Layers of sediment produced by erosion of the
Appalachian Mountains and Piedmont.
• Deposited when sea levels were higher.
Piedmont
• Area of rolling hills above ancient igneous rock and
metamorphic rock.
• Igneous rocks are the roots of past volcanoes formed
from an ancient episode of subduction prior to the
formation of the Appalachian Mountains.
Blue Ridge
• High ridge separating the Piedmont from the
Valley and Ridge.
• The billion year old igneous and metamorphic
rocks of the Blue Ridge are the oldest in the
state.
Valley and Ridge
• An area with long parallel ridges and valleys above
ancient folded sedimentary rocks.
• Folding occurred during a collision between Africa and
North America during the Paleozoic Era.
• This collision produced the Appalachian Mountains.
Appalachian Plateau
• Has a rugged, irregular topography above
ancient, flat-lying sedimentary rock.
• This is where most of Virginia’s coal resources
come from.
Vocabulary
Write down these vocabulary terms and define from notes of
your text book. Quiz next class!!!
• Law of
Superposition
• Relative Dating
• Absolute Dating
• Half-life
• Radiometric
Dating
• Fossils
• Uniformitarianism
• Eons
• Eras
• Period
•
Quarternary
Period
• Epoch
• Precambrian
•
•
•
•
Coastal Plain
Piedmont
Blue Ridge
Valley and
Ridge
• Appalachian
Plateau