1. Earth’s Structure
View of Earth from moon. We know what the surface of the Earth looks like. What does the inside look like?

2. Origin of the Earth 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

3. Earth’s Interior Core Mantle Crust dense Iron and Nickel
Inner Core - solid
Outer Core - liquid
Mantle
Less dense than core
Iron and Magnesium silicates
Mostly solid
Upper mantle is partially molten
Core – composed of two parts. The core is very dense and composed mostly of Iron and Nickel. The inner core is solid. This is because there is so much pressure exerted that it can only exist in the solid form. The outer core is liquid iron and nickel. Again, it is very dense but because it is in its liquid form it is less dense than solid and forms the outer core.
Mantle is composed of Iron and Magnesium silicates (Si + O). It is mostly solid, but the uppermost part of the mantle is partially molten (slush – where you do have liquid and solid mixed). Now, this is very important in plate tectonics.
Crust –This is the outermost layer. It is very thin and rigid. Rigid means that it is a solid. If you can imagine an apple. It would have the thickness of the skin of an apple. There is two types of crust – continental crust and oceanic crust.
Important notes:
Thin crust + uppermost mantle is rigid. This forms the “lithospheric plates”.
partially molten mantle acts as a lubricant. This enables the plates to move freely along the surface of the Earth.
Crust
Outermost layer
Very thin and rigid
Continental – granite
Density = 2.8 g/cm3
Oceanic – basalt
Density = 3.0 g/cm3

4. 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 4.6 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

5. Types of Seismic Waves P waves S waves Primary waves
Push and pull movement
Travel fastest (~ 6 km/sec)
Travel thru solids and liquids
S waves
Secondary waves
Move side-to-side
Slower (~ 4 km/sec)
Travel thru solids only

6. Seismic Waves Through Earth
Scientists use waves generated by earthquakes to determine the Earth’s interior. that the outer core of the earth is liquid.
Earthquakes generate P-waves and S-waves within the earth. Shadows occur on the opposite side of the earth from the earthquake epicenter because the outer core reflects S-waves, and bends P-waves. S-waves are reflected because they cannot travel through liquids, and they cast a larger shadow than the bent P-waves. Geologists and seismologists determined the size of the outer core by using the 154-degree arc of the S-wave shadow and measurements taken on the surface of the earth.

7. 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

8. 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

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

10. 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

11. 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

12. 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

13. Continental Drift Model Problems
Alfred Wegener
Presented research to professionals
Did not provide a plausible mechanism to explain how continents 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.

14. 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”

15. 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

16. 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

17. 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

18. 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

19. 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)

20. 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.

21. Seafloor Spreading Heat Flow

22. Seafloor Spreading Convection Currents
Magma was found were new seafloor was being made.
In 1960, proposed as driving force to move continents

23. 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”

24. 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 high degree of tectonic activity
mountain building
earthquakes
volcanoes

25. 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.
Divergent
Convergent
Transform

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

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

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

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

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

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