GEOSC 10: Geology of the National Parks

Plate Tectonics I:Making Mountains and Earthquakes

Presented by Dr. Sridhar AnandakrishnanThe Pennsylvania State University

GEOSC 10 - Geology of the National Parks

Overview

•Death Valley National Park

‣ Spreading apart

‣ Heat within the Earth drives that spreading

•Yellowstone National Park

‣ Massive volcanic caldera

‣ Earthquakes, and what they tell usabout the Earth’s interior

GEOSC 10 - Geology of the National Parks

Death Valley National Park

•Eastern CA, near the Nevada border

•Lowest (-200ft), hottest (135F), driest (2”/yr) spot in US.

•Evaporation of water leaves behind lots of minerals - Borax mined for detergent

GEOSC 10 - Geology of the National Parks

Mining

GEOSC 10 - Geology of the National Parks

Death Valley is Spreading

•Pull-apart fault

•Block in between drops down

•We can measure the spreading...

GEOSC 10 - Geology of the National Parks

Death Valley Spreading Zone...

•Pull-apart fault continues South to the Gulf of CA.

•Baja & Mexico separating

GEOSC 10 - Geology of the National Parks

Mid-ocean Spreading Ridges

•The world’s oceans have many spreading ridges.

•They look a little like the seams on a baseball.

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

GEOSC 10 - Geology of the National Parks

Heat Drives Plate Tectonics

GEOSC 10 - Geology of the National Parks

What is heat?

•Vibration of Atoms

‣ What are atoms? The smallest unit of matter. Break up anything into smaller and smaller pieces - you get to atoms eventually. Each element (oxygen, carbon, nitrogen) has a fixed number of protons (atomic weight).

- Usually, the number of neutrons equals the number of protons and electrons. If not, we have isotopes.

‣ At -273C, there is no vibration - absolute zero.

GEOSC 10 - Geology of the National Parks

Moving heat

•Move heat around (i.e., get other atoms to vibrate more)

‣ Radiation (like the sun or a tanning booth)

‣ Conduction

‣ Convection

Don’t do Don’t do it! Skin it! Skin

cancer is cancer is bad!bad!

GEOSC 10 - Geology of the National Parks

Heat (cont.)

•Convection is when a heated material itself moves to a new place, carrying heat with it.

•Hot things are less-dense

‣ When you mix less-dense and more-densethings together, the less-dense will rise.

•Heated rocks deep inside the earth rise up... cool at the surface and then sink down.

•This is a convection cell.

Density = Density = mass/volumass/volu

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GEOSC 10 - Geology of the National Parks

Convection Cell Inside Earth

•Heated rocks rise (because they are less-dense)

•They cool at the surface of the Earth.

•Then sink back down.

GEOSC 10 - Geology of the National Parks

Convection Drives Plate Tectonics

•Rising rocks push aside the cool rocks at the surface.

•Those cool rocks travel sideways, and will move the overriding plates with them.

GEOSC 10 - Geology of the National Parks

Heat (2)

•The heat inside the earth is from radioactive decay.

‣ Atoms (remember them?) are usually stable, but sometimes can break apart (changing into other elements).

‣ They release heat when they do this. (make a lot of them do it at the same time - atom bomb).

•Some of the heat is from the original formation of the planet... but most from radioactive decay

GEOSC 10 - Geology of the National Parks

Truth with a T

• In 1897, Lord Kelvin calculated that Earth was 24 million years old, based on how much heat left over.

•Darwin, geologists said Earth much older.

•??

GEOSC 10 - Geology of the National Parks

Truth with a T

•Marie Curie (pictured here on a visit to Pittsburgh, 1921) discovered radioactivity, which is the source of the heat.

•Geologists right...

GEOSC 10 - Geology of the National Parks

Summary (1)

•Heat inside the earth from radioactive decay drives convection cells.

•The cells move crustal plates (there are 8 major plates and some small ones) around on the surface of the Earth.

•Death Valley is on a pull apart fault... follow it south and you come to oceanic spreading ridges.

GEOSC 10 - Geology of the National Parks

Yellowstone National Park

•First National Park (est. 1872).

•Washburn expedition documented geysers, hot spring, and earthquakes.

•Huge volcanic caldera (crater left over after eruption) 30-40 miles across.

‣ Eruptions 1.8 million, 1.2 million, and 600 thousand years ago

‣ Each one 1000 times as big as Mt. St. Helens

GEOSC 10 - Geology of the National Parks

Earthquakes

•Elastic rebound (“stick-slip”) earthquakes occur when strain builds up over time (bottom and middle)

•Eventually the fault breaks (slips) and an earthquake occurs

GEOSC 10 - Geology of the National Parks

Earthquake Locations

•Earthquakes occur where two plates come together

‣ When they slide past each other (San Andreas Fault)

‣ When they over-ride each other (subduction zone)

•Sometimes they occur within a plate

GEOSC 10 - Geology of the National Parks

Earthquake Locations

GEOSC 10 - Geology of the National Parks

Heat Drives Plate Tectonics

GEOSC 10 - Geology of the National Parks

Tsunami

GEOSC 10 - Geology of the National Parks

Earthquake Energy Travels

•When the fault slips, the rocks nearby get deformed... they deform the next bit of rock... and so on.

‣ Grab a rope and shake it...

•Two kinds of waves are generated

‣ P-waves (compressional, “Push”)

‣ S-waves (shear, “side-to-side”) - can’t happen in water or air or other fluids.

GEOSC 10 - Geology of the National Parks

Seismic Energy

•The energy that travels away from the earthquake is called seismic energy.

•Travels right through the Earth to the other side (for big earthquakes).

•Outer core is fluid because no shear waves go through it.

GEOSC 10 - Geology of the National Parks

Earthquake Size

•Earthquake magnitude (“Richter scale”) goes from 1 (small - won’t notice it) to 9 (Sumatra event).

•Going up by 1 number in magnitude (4→5) means the motion of the ground goes up by 10 times and energy up by 30 times!

•True, there are fewer large events, but because the bigger ones are so much morepowerful, most earthquake energyis released by a few large earthquakes

GEOSC 10 - Geology of the National Parks

Summary (2)

•Earthquakes mainly occur at plate boundaries

•Faults stick, then slip suddenly

•The energy from the slip can spread far away...

‣ Causing destruction near the fault

‣ Can be measured far away to give information about the structure of the Earth

‣ Magnitude scale is non-linear

‣ Few large events release most energy