Geologic+History

Alban Hoxhaj

Earth History

It is estimated that the Earth formed along with the solar system 4.6 Billion years ago (4,600 MYA)

Geologic Time Scale A summary of the major events in Earth's history

EON- largest segment of geologic time

ERA

PERIOD

EPOCH- smallest segment of geologic time

Reading The Rock Record

__Relative Time vs. Absolute Time__ Relative Time places events in a sequence but does not identify their actual date of occurrence.

Absolute Time identifies the actual dates of geologic events.

FINDING THE AGE OF ROCKS WITH RELATIVE TIME

Law of superposition- in undisturbed sedimentary rocks the oldest rock layers are at the bottom and the youngest are at the top

LAW OF CROSS-CUTTING RELATIONSHIPS- An igneous intrusion is younger than the rock it has intruded into. (Look at the Whiskers)

LAw of included fragments- If fragments of one type of rock are found in another rock layer the rock fragments must be older than the rock layer in which they are found.

Faulted and Folded layers- Layers of rock that have been faulted or folded must have been present before the actions of faulting or folding took place.

Unconformity- A place in the rock record where layers of rock are missing because of uplift and erosion. The result can be a large age difference between the rocks above and those below the erosional surface (it appears like a squiggly line in a cross-section)

Rock Correlation

Matching of rock layers that can be seen at the Earth's surface, over a large area.

An OUTCROP is exposed rock layers at the Earth's surface.

A KEY BED is a thin, widespread layer, usually of volcanic ash, that can be used to correlate an exact point of time.

FOSSILS

A FOSSIL is any evidence of earlier life preserved in the rock.

ORIGINAL REMAINS (RARE)- The actual unchanged remains of the plant or animal are preserved.

REPLACED REMAINS- The soft parts of the original animal have disappeared and the hard parts have been replaced by mineral material. (PETRIFIED WOOD)

MOLDS AND CASTS- Fossil shells or bones are dissolved completely out of the rock leaving a hollow depression in the rock. New mineral materials fills the mold it forms a cast of the original fossil.

Trace Fossils- Evidence of life other then remains, which includes any impressions left in the rock.

INDEX FOSSILS

Easily identifiable Short-Lived Widespread occurrance

Measuring Absolute Time Tree Rings

Each ring represents a single year (spring/fall) the width of the ring depends upon the temperature and rainfall.

Varves

Glacial lake deposits. a thick light colored layer in the summer and a thin dark layer in the winter.

Radioactive Dating

Using to date far back in time. certain rocks contain radioactive isotopes

Radioactive Isotopes are atoms of elements that give off radiation from their nuclei

Radioactive decay is the process by which a radioactive isotopes changes into a new stable element.

Half-Life The rate at which a radioactive element decays. it is the time it takes for half of the atoms of the radioactive element to decay to a stable end product (see page 1 of the ESRT)

At the end of each Half-Life, Half of the radioactive material remains

Parent Isotope= The radioactive isotopes that begins.

Daughter Isotopes = The stable isotopes that has been changed.

Radiocarbon Dating- Uses the radioactive isotopes carbon- 14 found in all living things. Because carbon- 14 is continually absorbed by food and water it stays constant in living things. When the living thing dies the percentage of carbon- 14 decreases at the rate of its Half-Life. Can be used to date beak about 100,000 years.

Uranium lead Method-is useful todate rocks older tha 10 million year. can be used only on igneous rocks that contain the right kind of uranium.

Rabidium strontium method can also be used to date older rocks because of its long half life it is also very commonly found in igneous rocks.

Poassium argon method is very useful since potassium 40 can be found in metamorphic sedimentary and igneous rocks itcan date oldr rocks but may also date rocks as young as 50,000 years.