1. Plate Tectonics
The theory --the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, are sites of intense geologic activity, such as earthquakes, volcanoes, and mountain building.
Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from the mid-ocean ridges.

2. Earth Structure

3. Mexico quake-- magnitude at 7
Mexico quake-- magnitude at 7.0, and epicenter was in the western Pacific state of Michoacan. Its depth was about 40 miles
Indonesia-- earthquake off coast of Indonesia Wednesday, April 11 with magnitude of 8.6, sparking tsunami warnings

4. Lithosphere Consists of continental, oceanic and upper part of mantle
Continents composed of granite-type rock, quartz and feldspar minerals, density+2.8g/cm3
Oceanic crust formed of basalt; basalt rich in iron/magnesium minerals, density+3.0 g/cm3
Why do continents sit higher than ocean basins? It is their composition and density. Continent = Quartz (SiO2) and Feldspars (K, Ca, Na, Al, SiO2). These are light elements. Ocean = Basalt (Fe and Mg). These are heavier elements. Asthenosphere is partially molten. It forms a lubricant to allow plates to move.
Lithosphere is rigid layer of crust and mantle overlying partially-molten asthenosphere

5. Continental Drift Evidence
Researchers noted geographic fit of continents
e.g. Africa and S. America
Atlantic formed by separation of Africa from S. America
Seuss, 1885, proposed super continent by studying fossils, rocks, mountains
Wegener and Taylor, early 1900’s, proposed continental drift and Pangaea
Seuss hypothesized a super continent “Pangaea” surrounded by oceans.
Evidence supporting the idea that the continents had drifted.
Geographic fit of continents
Fossils
Mountains
Glaciation

6. Continental Drift Geographic Fit
About 210 Ma, Pangaea began to break apart.
Continents seem to fit together like pieces of a puzzle

7. Continental Drift Fossils
Studies were done on land animals that could not swim. Once continents split, evolution occurs. Organisms begin to differentiate. Climate is different, food sources are different, predators are different.
Similar distribution of fossils such as the Mesosaurus

8. Continental Drift Mountains
If you were to put the puzzles back together, mountain chains match up, i.e., Appalachian Mtns, British Isles, and Caledonian Mtns.
Mountain ranges match across oceans

9. Continental Drift Glaciation
Glaciers typically occur near the poles. We are in a warm period, but in the past it was cool and ice caps grew. Deposits related to glaciers were deposited. You also find glacial deposits in Africa near the equator. How to you get glacial deposits near the equator?
Glacial ages and climate evidence

10. Objections to the Continental Drift Model
Wegener did not provide a plausible mechanism to explain how the continents could have drifted.
Wegener was a scientist. Scientists go to scientific meetings to present their research. When Wegener presented his research to other physicists, they did not agree. They wanted to know how do continents drift? There was no driving mechanism to support continental drift. Wegener could not answer “how” so his Continental Drift Model was not accepted.

11. Seafloor Spreading
Continental drift reexamined in 1960’s with new information
New theory developed – Seafloor spreading
Supporting evidence for seafloor spreading
World seismicity
Volcanism
The 1960’s is relatively recent. Geology is a new science compared to Physics, Chemistry and even Biology. People were doing separate studies about earthquakes, volcanoes, age of sea floor, paleomagnetism. Someone put it all together. A new theory developed – Seafloor Spreading. There were a lot of evidence to support Seafloor spreading.
Age of seafloor
Paleomagnetism
Heat flow
Theory combining continental drift and seafloor spreading termed “Plate Tectonics”

12. Seafloor Spreading
The idea of seafloor spreading is that new crust is being formed at spreading centers and old crust is being destroyed in deep trenches. For example, Mid-Atlantic ridge is a chain of underwater mountain chains where new crust is being formed.
New sea floor created at the mid-ocean ridge and destroyed in deep ocean trenches

13. Evidence for Seafloor Spreading World Seismicity
Note: Orange is shallow. At spreading centers, EQ only occur at the surface. At SZ, deep EQ occur, i.e. Japan, Indonesia, Philippines, Aleutians. How does seismic activity support plate tectonics? They occur along plate boundaries.
Earthquake distribution matches plate boundaries

14. Evidence for Seafloor Spreading Volcanism
Where do volcanoes occur? Subduction zones. Why are they associated with subduction zones? Oceanic crust is moving deep into the Earth. Volcanism also occur at hot spots. Where is the tallest mountain? Mauna Loa. Highest elevation? Mount Everest. If you were to take Mauna Loa and place next to Mount Everest, it is the highest mountain. It is measured at its base deep on the ocean floor.
Volcanoes match some plate boundaries; some are hot spots

15. Evidence for Seafloor Spreading Age of Seafloor
Age of Earth is 4.5 Ga. Oldest oceanic crust is only 180 Ma. Why is the ocean so young? Old oceanic plate is being subjected or destroyed.
Youngest sea floor is at mid-ocean ridge
Oldest sea floor away from mid-ocean ridge

16. Evidence for Seafloor Spreading Paleomagnetism
Today, Earth’s magnetic field points to the North. Why? They believe that the magnetic field is caused by the liquid outer core moving around the solid inner core by Earth’s spin. The iron (Fe) content in the core produces a magnetic field. Magnetic field is important to us in that it prevents harmful radiation of the Sun to reach us. Every 700,000 years or so, the Earth’s magnetic field reverses. These reversals are recorded in molten magma. As the magma cools, its iron-rich minerals tend to line up with Earth’s magnetic north.
Earth has a magnetic field - Probably caused by rotation of solid inner core in liquid outer core (both mostly Fe)
When rocks cool at the Earth’s surface, they record Earth’s magnetic field (normal or reverse polarity)

17. Evidence for Seafloor Spreading Paleomagnetism
The record of these alternating reverse and normal polarity helps support the idea of seafloor spreading.
Paleomagnetic studies indicate alternating stripes of normal and reverse polarity at the mid-ocean ridge.

18. Seafloor Spreading Heat Flow
Magma was found were new seafloor was being made.
In 1960, convection currents were proposed as driving force to move continents

19. Theory of Plate Tectonics
Heat flow provided the mechanism to move the lithospheric plates.
John Tuzo Wilson combined ideas of continental drift and seafloor spreading into “Plate Tectonics”

20. Principles of Plate Tectonics
Earth’s outermost layer composed of thin rigid plates moving horizontally
Plates interact with each other along their edges (plate boundaries)
Plate boundaries have a high degree of tectonic activity
mountain building
earthquakes
volcanoes

21. Plate Boundaries Three types
There are three ways that plates can move relative to each other. They can move apart – divergent plate boundary, they can come together – convergent boundary, or they can slide past one another – transform fault.

22. Plate Boundaries Divergent
Plates move away from each other
New crust is being formed

23. Divergent Plate Boundaries Examples
East African Rift
Mid-Atlantic Ocean Ridge

24. Plate Boundaries Convergent
Three Types:
Ocean-continent
Ocean-ocean
Continent-continent
Plates are moving toward each other
Crust is being destroyed

25. Convergent Plate Boundaries Examples
Mount Fuji, Japan
Mount Lassen, California
Andes, South America

26. Plate Boundaries Transform
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Plates slide past one another
Crust is neither created nor destroyed

27. Transform Plate Boundaries Examples
Calexico, California
San Andreas Fault
Carrizo Plains, Central California

28. Stop Here

29. Origin of the Earth Gravitational compression
Meteors and Asteroids bombarded the Earth
Gravitational compression
Our early Earth grew from a barrage of extraterrestrial impacts, increasing in mass over time. In the early stages of planetary accretion, the Earth was much less compact that it is today. The accretionary process led to an even greater gravitational attraction. Gravitational energy converted into heat. Radioactive elements are unstable, and over time become more stable. Radioactive decay processes releases heat. U-238 to Pb Initial accretion led to a homogeneous sphere that eventually became density stratified with the heavier iron and nickel sinking to the center of the earth, and the lighter silicates (rocky) floated to the top.
Density Stratified planet

30. Earth’s Interior Core dense Nickel and Iron Inner core – solid
Outer core – liquid
Mantle
Less dense than core
Iron and Magnesium silicates
Partially molten
Crust
Outermost layer
Continental – granite-type, density=2.8 g/cm
Oceanic – basalt, density=3.0 g/cm
Rigid

31. Evidence of Internal Structure
Granite
Basalt
Density
calculate density of Earth
Speculate on probable compositions
Meteorites
Use composition and age to determine composition and age of Earth
By calculation, the Earth’s density is 5.8 gm/cm3. In the Sierras, composed mostly of granite, represents the thin part of Earth’s interior, i.e. skin of an apple. If you calculate the Earth’s surface using granite, it comes to 2.8 gm/cm3. What is this telling us? There are heavier rocks in the Earth. We know the age of the earth is 4.5 Ga. People who study meteorites determined their age to be about 4.5 Ga – same as Earth. Meteorites are composed of Iron and Nickel. It is hypothesized the Earth’s interior is composed of the same elements. P-waves travel 4x faster than S-waves and also travel through different material. These also give us some clue on the properties of the Earth’s interior.
Seismic waves
Travel times and direction give indication of internal structure of Earth

32. Types of Seismic Waves P waves Primary, pressure, push and pull
Travel fastest of the seismic waves (average 6 km/sec)
Travels through solids and liquids
S waves
Secondary, shaking, side-to-side
Slower (average 4 km/sec)
Travel through solids only

33. Seismic Waves Through Earth