Geology

Introduction to GeologyGeology literally means "study of the Earth."

• Historical geology examines the origin and evolution of our planet through time.

• Physical geology examines the materials and processes of the Earth.

Geological Time

© NASA

Geologic Time• Relative Dating: Putting geologic events into

proper order (oldest to youngest), but without absolute ages. We use a number of principles and laws to do this:

• Law of Original Horzontality - Sedimentary units and lava flows are deposited horizontally.

• Law of Superposition - the layer below is older than the layer above.

• Principle of fossil succession - life forms succeed one another in a definite and determinable order and therefore a time period can be determined by its fossils.

• Law of Cross-cutting Relationships - A rock is younger than any rock across which it cuts.

Geologic Time

• Absolute (Radiometric) Dating: Using radioactive decay of elements to determine the absolute age of rocks. This is done using igneous and metamorphic rocks.

Radiometric dating: Discoveryhttp://en.wikipedia.org/wiki/Image:Henri_Becquerel.jpg

Henri Becquerel (1852-1908)

• In 1896, Discovery of radioactivity paved the way for the precise dating of events in the geological record

Radiometric dating: Decay

en.wikipedia.org/wiki/Image:Alpha_Decay.svg

• Radioactive ‘parent isotopes’ spontaneously emit protons and neutrons and decay into ‘daughter isotopes’• E.g., Uranium-238 decays into Lead-206

Radiometric dating: Half life

• The rate of decay from parent to daughter isotope depends on its half life. The half life is the amount of time needed for half the parent isotope to decay to daughter isotope.

Half life: 0

Half life: 1

Half life: 2

Linear

Exponential

Radiometric dating: Clocks

Different radioactive isotopes have different half lives Isotopes with long half lives are useful for dating old

rocks.

Decay series Half life

40K to 40Ar 1250 Ma

147Sm to 143Nd 1060 Ma

235U to 207Pb 704 Ma

238U to 206Pb 4468 Ma

14C to 14N 5370 years

Radiometric dating: Meteorites

• Radiometric age of meteorites date the formation of the Solar System and Earth (4550 million years old)

en.wikipedia.org/wiki/Image:Canyon-diablo-meteorite.jpg

http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg

Crab Nebula

Canyon Diablo meteorite

Geologic Time The oldest rocks that have been dated on Earth

are merely 4.03 billion years old (gneisses in Canada). How, then, do we know that the Earth is 4.6 billion years old? There are a number of pieces of evidence:

Some grains of sand in Australia have been dated at 4.2-4.4 billion years - this suggests that there were rocks around at that time even if they are no longer there,

Rocks collected from the Moon have yielded ages as old as 4.6 billion years old,

Nearly every meteorite that has the same composition as the Earth has an age of 4.6 billion years.

Geologic Time The concept of geologic time is new

(staggering) to many nongeologists.

The current estimate is that the Earth is ~4,600,000,000 (4.6 billion) years old.

As humans we have a hard time understanding the amount of time required for geologic events.

Radiometric Dating: History

dinosaurs

humans

first life

origin of Earth

http://en.wikipedia.org/wiki/Geologic_time_scale

en.wikipedia.org/wiki/Image:Eopraptor_sketch5.png© World Health Org.

© NASA

first complex cells

Geologic Time We have a good idea of how long a

century is. One thousand centuries is only 100,000 years. That huge amount of time is only 0.002% of the age of the Earth!

An appreciation for the magnitude of geologic time is important because many processes are very gradual.

Geologic Time Geologic time is divided into different types of units.

Eon Era Period Epoch

Note that each Eon, Era or Period represents a

different amount of time. For example, the Cambrian period encompasses ~65 million years whereas the Silurian period is only ~30 million years old.

The change in periods is related to the changing character of life on Earth and other changes in environment.

Geologic Time The beginning of the Phanerozoic

represents the explosion of life.

The time before the Phanerozoic is commonly referred to as the PreCambrian and represents over 4 billion years of time. The Phanerozoic eon (abundant life) represents only the last 13% of Earth time.