EARTH’S HISTORY Unit 12 Review 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 type composition color thickness fossils

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 ALocation BLocation C € ш€ Ж€ € ص€ Ж€ ш € Ω€ Ж ш€ €€ Ж€ ص

€ widespread geologically widespread geographically Ж widespread geologically limited geographically صlimited geologically 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. U 238 is used to determine the age of rocks that are hundreds of millions of years old.

C 14 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.