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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
€ ш € Ж €
€ ص € Ж € ш
€ Ω € Ж ш €
€ € Ж € ص
€ widespread geologically widespread geographically
Ж widespread geologically limited geographically
limited geologicallyص limited geographically
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