G 208 Lecture Notes 04/02/2005

• Geology is the study of the Earth

• 2 kinds of crusts:

1) Continental crust (made of felsic rocks)- most mean elevation of 840m above sea level- made up of variety of rocks, but almost all are fairly rich in silicon and low in iron

2) Oceanic crust (made of mafic rocks)- most mean depth of 3795m below sea level- made up of one type of rock that is richer in iron and lower in silicon that the rocks of the continents

• Earthquakes occur the same places as volcanoes

· The answer to the following questions is the theory of plate tectonics

1) Why do volcanoes occur only at certain locations and why are they often found in chains?

2) Why are earthquakes associated with some volcanoes? And why are they located in linear belts?

• Development of the theory:

1) Continental Drift- 1915

2) Sea Floor Spreading- 1962

3) Plate Tectonics- 1965- 67

(lithosphere vs. crust)

• Continental Drift (Wegener, 1915)

- Various evidence suggests super continent (pangea) existed 250-180 million years ago

- All continents in one mass, break apart, and drift away

- Edges of continents fit together (first commented on by Sir Francis Bacon in 1620)

• Fossil Distribution

- Mesosaurus

• Mountains and rock ages

- Appalachian and Caledonian mountains (300 million years old)

• Distribution of Glacial Features

- 300 million years old

• Wegener unable to persuade colleagues
• Lacked a mechanism for movement
• Publicly ridiculed in 1928

• Earth’s magnetic field

- Can be used to tell magnetic North & South and latitudes

- Sometimes field orientation is preserved in rocks

- Field flips polarity (North & South direction) routinely (based on continental rocks)

• Rocks can save a record of Earth’s magnetic field
• Examined rocks on continents- found magnetic field reverses polarity from time to time
• Used radiometric dating to determine ages of volcanic rocks on continents and correlated age with magnetic polarity
• Magnetic time scale developed using continental volcanic rocks that had been dated using radiometric techniques

• Sea Floor Spreading

- Make new ocean floor at mid-ocean ridges

• Seismologists were using earthquakes data to model the Earth’s interior (temperature and pressure)

• Earth

- Composed of layers

- Interior is hotter than the exterior

- Consists of:

1) Core

2) Mantle

3) Crust

• Five layers of physical properties

1) Inner core (metallic solid)

2) Outer core (molten liquid)

3) Mesosphere (plastic solid)

4) Asthesosphere (plastic)- where melting occurs

5) Lithosphere (ridgid)- consists of crust and upper mantle

• Plate Tectonics

- Lithosphere of Earth divided into plates that move relative to one another

- Explains most of Geology seen at the surface of Earth- most earthquakes, volcanoes, mountains form at boundaries between two plates

- Earthquakes mark the location of plate boundaries

- Lithosphere is broken into pieces (plates) that ride atop a convecting mantle

• Convection- amount of materials get heat up & rise and dense when cool
• Rocks are not good conductor of heat

• Three types of plate tectonics:

1) Divergent boundaries

- Plates are moving apart

2) Convergent boundaries

- Plates are moving together

3) Transform boundaries

- Plates are moving sideways

• Subduction leads to arogeny

• Appalachians

- Mountain belt produced by (in part) continent- continent plate collision

• Convergent Plate Boundaries

1) Ocean- continent

- Volcanic arc

2) Ocean- ocean

- Island arc

3) Continent- continent

- Mountain belt

• Hot spots

- Track past plate tectonics

- Stay still but plate moves dragging off the spots