Plate Tectonic Notes

April 19, 2023

I. Earth’s Interior

A. Layers of the Earth

1. Crust: thin and solid outermost layer of Earth above

the mantle

a. Continental crust is less dense and thicker (20-40km)

(12-25 mi) than oceanic crust, which is more dense and

thinner (5-8km) (3.1-4.9 mi)

Lithosphere & Asthenosphere• Lithosphere (60 mi thick) is made up of the crust and the

upper portion of the mantle• Asthenosphere-hot, plastic portion of the mantle that can

flow-soft rock in the asthenosphere circulates by convection currents & this results in slow movement of plates

2. Mantle: layer of rock between the Earth’s crust and corea. Denser than the crustb. Outermost part is solid and rigid, but as you move closer to the core, the mantle becomes soft and easily deformed (like gum..hot plastic portion of mantle)

1. Called the asthenosphere• *Also, very hot• (1200 or 2280 )℃ ℉

3. Outer Core: Liquid metal outer edge of corea. Composed mainly of iron and nickel

4. Inner Core: Solid metal inner corea. Composed mainly of iron and nickel

*Temps are thought to be above 10,800℉*Earth’s center has radioactive isotopes that do fission & release a ton of heat energy

B. As you travel from crust into the core, the temperature increases1. Temperatures from 375oC at the crust to over 6000oC (10,800 )in the inner core℉

C. Thermal Energy Transfer1. Material moves with the Earth as a result of conduction (transfer thermal energy from individual atom to

individual atom) and convection currents (hot material rises & cool material sinks back to the core)

D. Evidence for Characteristics of Layers1. P-waves and S-waves (see below) *P Waves travel through liquids & solids but S Waves cannot travel through liquids

II. Continental Drift

A. Theory that Earth’s surface is made of large movable plates

1. Alfred Wegener was a German scientist who first proposed the idea that the continents of the Earth were once one large supercontinent

a. Called this supercontinent Pangaea

In 1910 Alfred Wegener begins to wonder.

•All the continents were stuck together at one time (300 million years ago?).

•Continental drift = slow movement of continents over Earth’s surface.

He thought that:

Supercontinent called PANGEA

Pangea means all lands

Wegener’s summary was based on various careful observations:

Wegener’s Evidence

#1 - All of the coastlines of the

continents seemed to fit together like

puzzle pieces.

Wegener’s Evidence

#2 - Fossil evidence:Identical fossils found on different continents

Wegener’s Evidence

•Large ice masses carve grooves in the rocks over which they flow.

•Such masses tend to flow outward (generally downhill) from a central locality.

#3 - Glacial Evidence:

Wegener’s Evidence#4 - Rock Ages:the framework of rock ages showed

strong correlation across the Atlantic, as did mountain ranges of similar age.

B. Early evidence for Continental Drift

#5. Migration patterns of animals have not changed since it was supercontinent.

C. Recent evidence for Continental Drift1. Magnetic polarity bands near mid-ocean ridge [Fig 6, p.703]

a. Alternating magnetic bands, normal versus reversed, on new ocean floor

2. Age of rocks relative to the mid-ocean ridge: the farther away from the mid-ocean ridge, the older the rock

III. Plate TectonicsA. Lithosphere is made up of the crust and the upper

portion of the mantle1. Divided into 7 major plates and several smaller plates called tectonic plates2. Continents are embedded into the tectonic plates3. Plates “float” on the asthenosphere

-hot, plastic portion of the mantle that can flow -soft rock in the asthenosphere circulates by convection currents & this results in slow movement of plates

Movement of the plates

B. Plate Tectonics: the theory that explains how the outer parts of the Earth change through time1. Also explains the relationships between continental drift, sea-floor spreading, seismic activity, and volcanic activity

IV. Plate BoundariesA. Divergent Boundaries: two plates move apart creating a

gap between them 1. Causes mid-ocean ridges to form

a. Less dense magma rises up in between the two separating plates creating

new ocean floorb. Gap between the

two plates is calleda rift valleyc. Most studied mid-ocean ridge is Mid- Atlantic Ridge

Sea-Floor Spreading – (Divergent)

B. Convergent Boundaries: two plates are coming together

1. Forms ocean trenches, volcanoes, &/or mountainsa. Subduction zone: process by which one

lithospheric plate moves beneath another as a result of tectonic forces

1. Ocean trench: forms between 2 oceanic plates or oceanic plate and continental plate

a. Older plate is denser, so it sinks below the other plate b. Can be extremely deep

2. Volcanoes are formed as the subducting plate is melted and the magma rises to surface on other plate

b. Colliding tectonic plates create mountains1. Two continental plates collide and the land is pushed upward creating

mountains2. Example: Himalayas

Common Events at Plate Boundaries:Volcanoes

Then how can there be volcanoes that are not on plate boundaries?

= Volcano

?

Hot Spots•A hot spot is an area of persistent volcanic activity that comes from unusually hot areas of the mantle-core boundary.

•Hawaii, Iceland, and Yellowstone are examples of hot spots.

C. Transform Fault Boundaries: two plates sliding past each other 1. Fault: crack in Earth created when rocks on either side of a break move2. One cause of earthquakes3. Example: San Andreas Fault (from Mexico to California)

V. EarthquakesA. As plates move past each other, pressure

builds up and eventually the stress becomes so great it causes the rock to crack1. Focus: exact point inside Earth when an earthquake originates 2. Epicenter: point on Earth’s surface directly above an earthquake’s focus

B. Energy is transferred by waves1. Longitudinal waves (P Waves): Move faster through rock, so first waves to reach recording stations 2. Transverse waves (S Waves): Move more slowly, so arrive second to the recording station3. Surface waves: seismic waves that can only move through solids

Common Events at Plate Boundaries:Earthquakes

Earthquake Effects: Tsunami

A tsunami is a giant wave or sequence of waves

produced by large water displacements. These are most

commonly caused by underwater earthquakes.

Tsunamis can be destructive long distances from their

sources.

C. Seismograph: machine that records the movements produced by an earthquake1. Use seismographs to investigate the Earth’s interior2. Differences in velocities of the waves indicate the type of material the waves travel through

a. Scientists constructed the model of Earth [Fig. 19, p.712]

D. Richter Scale: a scale that expresses the magnitude of an earthquake1. Range from 2.0 to 10.0 magnitude2. No earthquakes have ever been recorded at 9.0 or 10.0