EARTH’S HISTORY Unit 12 Review Book: Topic 13. I. Determination of Age.

EARTH’S HISTORY

Unit 12Review Book: Topic

13

I. Determination of Age

A. Uniformitarianism• The belief that the present geological events are the same as past events.

• This allows us to understand, and make inferences about, our geologic past

B. Principle of Superposition

• relates to the original horizontality of deposited sediments

• It is a determination of the relative age of a rock or event.

• It states that the youngest rock layers are found on the top of a series of rock strata and that rock age increases with depth.

• A fold or a fault is younger than the rock it disturbed.

• An intrusion is younger than the rock it cuts through

• An extrusion is younger than the rock it cuts through, but older than the rock layer that formed on top of it.

• An inclusion is older than the rock it is in.

• A joint is younger than the rock it is in

• The metamorphic rock formed when an intrusion cuts through pre-existing rock is younger than the pre-existing rock, but older than the igneous intrusion.

• This is called contact metamorphism and is indicated by a hachured line along the interface between the pre-existing rock and the igneous rock

• A vein (formed when dissolved minerals solidify in a crack in a rock) is younger than the rock it is in.

D. Correlating Rock Layers

• A process by which rock strata are matched

• A rock outcrop is an exposed section of bedrock

1. Visual correlation:matching rock layers based on similarities in:rock typecompositioncolorthicknessfossils

2. Index Fossils

• Fossils of organisms that have lived for a relatively brief amount of time and have existed over a large geographical area.

• Index fossils are• found only in one rock layer

• and in a variety of locations

• Index fossils are:

–limited geologically

–widespread geographically

Location A Location B Location C

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€ widespread geologically widespread geographically

Ж widespread geologically limited geographically

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Ш limited geologically widespread geographically

3. Key Bed

• A volcanic eruption results in the distribution of a thin layer of ash over a large geographical region.

• This makes it effective in determining relative age and correlating rock layers

• Rock particles and debris resulting from the impact of an asteroid can also cover a large region in a very thin layer

• This is equally effective in correlating rock layers and determining relative age.

4. Unconformity

• A buried erosional surface indicates that uplift, weathering and erosion has occurred in that region

• This destroys a portion of the rock record

• which results in a gap in geologic time!

• An unconformity is represented by a wavy line (an irregular surface) between two rock layers:

C. Absolute Age

• refers to the measured age of a rock or event in years.

• It is generally determined by comparing the amount of radioactive material to the amount of stable decay product found in a sample

• Radioactive decay data is used to determine the absolute age of rock and rock layers

• An isotope is a variation of an element in which the atomic mass differs

• Radioactive decay occurs when an isotope is unstable and releases particles in order to become stable.

• A radioactive isotope decays into a stable decay product.

1. Half-life

• The half-life of a radioactive isotope is the amount of time it takes for one half of the atoms to become stable decay product.

• The half-life of a substance does not change!

• It is independent of size, mass, temperature, pressure and location!

• The half-lives and stable decay products of common radioactive isotopes are listed on the front page of your ESRT

• When determining the age of a rock, the older the rock is presumed to be, the longer the half-life of the radioactive isotope must be. U238 is used to determine the age of rocks that are hundreds of millions of years old.

• C14 is used to determine the age of relatively recent materials and all organic remains.

II. Evolution

• The evolution of conditions on Earth is recorded in the rock record.

• The presence of marine fossils in NYS indicate that it was once covered by a shallow sea.

• The presence of warm climate fossils in NYS indicate that it once had a climate similar to that of low latitude regions.

• Large coal deposits indicate that conditions in the region were wet (swampy)

1. Organic evolution shows how life forms change through time.

• Climate and environmental changes result in variations within a species making it better able to survive.

• These variations are passed on to offspring and are preserved in the rock record. The fossil record offers support to this theory indicating gradual changes from an older species to a newer one.

2. Rapid (punctuated) evolution

• occurs when cataclysmic events such as volcanic eruptions, collisions of comets or asteroids often result in immense changes in the environment

• This often spurs rapid evolutionary changes and extinctions.

• Evidence exists to support the belief that the extinction of the dinosaurs resulted from the impact of an asteroid.

III. Earth’s Past

• The earth is estimated to have formed ~4.6 billion years ago.

• Heat from impact events, radioactive decay and gravity caused the earth to melt.

• Melting resulted in the separation of the earth into density zones: core, mantle, crust, atmosphere.

• It is estimated to have solidified with a solid crust ~4.2 billion years ago.

• Gases from the interior seeped out through the crust (outgassing) and created a second atmosphere of water vapor, carbon dioxide, nitrogen…

• The cooling of the earth resulted in the precipitation of water to form the oceans ~4 billion years ago.

• Ocean salts accumulated due to the chemical weathering of the ocean crust.

• ~3.5 billion years ago, stromatolites (colonies of algae and bacteria) formed.

• They used carbon dioxide and released oxygen (photosynthesis) changing the atmosphere to one of nitrogen and oxygen.

• Oxygen reacted with the iron in the crust creating iron oxides…resulting in an appearance similar to that of Mars

• ~2.8 billion years ago this reaction ended…

• allowing more oxygen to accumulate and the protective ozone layer to form.

• Life evolved to sexually reproducing, hard-bodied life forms during the Cambrian period.

THE END