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Geology 15 Fall 2013 Lecture 5 Schedule:• Hazard Update• Review Lecture 4• Review Activity 1 Cover 

Material/Objectives• Elastic Rebound Theory• Origin of the Earth• Plate Tectonics• Earth Structure

• Review Class Material• Preview Next Class• One minute Paper (?) http://www.earthds.info/

Free Geology eBooks:

http://itic.ioc‐unesco.org/index.php?option=com_content&view=category&layout=blog&id=1437&Itemid=1437&lang=en

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Review Lecture 4

• Earthquakes:What are earthquakes? What causes them? What is the results of an earthquake?

• Stress and Strain (Energy Transfer): What is stress and what is strain? How do these relate to earthquakes?

San Andreas fault at Wallace Creek.

http://arrowsmith410‐598.asu.edu/Lectures/Lecture4/GLG410_598‐‐Excel_Warmup3.html

Stress: Tension vs. Compression vs. ShearTensional stress is the stress that tends to pull something apart. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.Compressional stress is stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock.

Image courtesy of Michael Kimberly, North Carolina State Univ.

http://earthquake.usgs.gov/learn/glossary/

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What is strain?Strain is the relative change in shape or size of an object due to externally‐applied forces (e.g. stress).

http://physics.bgsu.edu/~stoner/p201/shm/sld002.htm

Hooke’s Law: Stress is directly proportional to strain. >>>>>>>>

http://en.wikipedia.org/wiki/Hooke%27s_law

Activity 1

• Material/Objectives• Elastic Rebound Theory• Origin of the Earth• Plate Tectonics• Earth Structure

What is the Elastic Rebound Theory?

The elastic rebound theory is an explanation for how energy is spread during earthquakes. As rocks on opposite sides of a fault are subjected to force and shift, they accumulate energy and slowly deform until their internal strength is exceeded. At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap back to their original undeformed shape.

http://en.wikipedia.org/wiki/Elastic_rebound_theory

• The rocks accumulate energy and slowly deform until their internal strength is exceeded. 

• At that time, a sudden movement occurs along the fault, releasing the accumulated energy, and the rocks snap backto their original undeformed shape.

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Reid's Elastic Rebound Theory

• From an examination of the displacement of the ground surface which accompanied the 1906 earthquake, Henry Fielding Reid, Professor of Geology at Johns Hopkins University, concluded that the earthquake must have involved an "elastic rebound" of previously stored elastic stress.

• If a stretched rubber band is broken or cut, elastic energy stored in the rubber band during the stretching will suddenly be released. Similarly, the crust of the earth can gradually store elastic stress that is released suddenly during an earthquake.

• This gradual accumulation and release of stress and strain is now referred to as the "elastic rebound theory" of earthquakes. Most earthquakes are the result of the sudden elastic rebound of previously stored energy.

http://earthquake.usgs.gov/regional/nca/1906/18april/reid.phpThe following diagram illustrates the process. Start at the bottom. 

1. A straight fence is built across the San Andreas fault. 

2. As the Pacific plate moves northwest, it gradually distorts the fence. Just before an earthquake, the fence has an "S" shape. 

3. When the earthquake occurs the distortion is released and the two parts of the fence are again straight; but now there is an offset.

This diagram greatly exaggerates the distortion. Actually, the distortion is spread over many miles and can only be seen with precise instrumentation (e.g. GPS).

Zoback, 2006

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Reid, 1910, assumes that earthquakes occur when the strain accumulates to a given level and the stress drop and the magnitude of each earthquake are the same

i.e. Earthquakes with similar size occur at regular intervals.

Satake and Atwater, 2007

In time-predictable models, ruptures still occur at a given stress level, but the stress drop and the magnitudes vary

i.e. The time until the next earthquake can be predicted by the size of its predecessor.

In slip-predictable models, the time until the next earthquake is related to the amount of slip from the previous earthquake

i.e. The size of the next earthquake increases with the length of time since the preceding earthquake.

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Reality is more complicated, so, much like early models of plate tectonics had to be revised, so did our models of recurrence.

i.e. Earthquake recurrence may be neither slip nor time predictable.

Weldon et. al., 2004

e.g. San Andreas fault

Origin of the Earth?

http://www.baluskin.ru/products/aist‐malchikhttp://commons.wikimedia.org/wiki/File:The_Earth_seen_from_Apollo_17_%28AS17‐148‐22727%29.jpg

Origin of the Earth http://www.natgeoeducationvideo.com/film/1170/the‐origin‐of‐earth

http://lunarscience.nasa.gov/articles/nasa‐scientist‐jen‐heldmann‐describes‐how‐the‐earths‐moon‐was‐formed/

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RELATIVE TIME

Principle of Superposition – Sediment arranged in order

• Youngest beds “superposed” towards top

• Oldest at base

• IF not disturbed

• Fossils are helpful

Based on the Principle of Superposition, which one is oldest?

2

1

3

1. 12. 23. 3 4. All are the same

ABSOLUTE TIME

• Actual # years before present• Radiometric age control

– Radioactive elements naturally DECAY(change into another stable element)

– Parent & Daughter elements– Certain atoms decay at steady rate

Rock paintings of elephant hunt, S. Africa (~40,000 yrs ago)

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HALF LIFE• Time required for ½ original “parent” element to

decay into “daughter” element – Carbon-14 Nitrogen-14 5730 years– Uranium-238 Lead-206 4.468 billion years

http://www.earth‐time.org/trollart.html

http://www.geosociety.org/science/timescale/timescl.pdf

Plate Tectonics

• What is Plate Tectonic Theory?• What are “Plates?”• Where are they?• What are in between the plates? Why?• Do plates move? If so, why?

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Bathymetry and Topography

NOAA, 2012

Where are the plates?

De Mets, et.al. 2010

Where (Why) Are There Earthquakes?

Earthquake epicenters (mostly along plate boundaries)

• magnitude >3 in black and >5.5 in red

De Mets, et.al. 2010

Plate Boundaries Plate Boundaries and Their Motions

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Types of Plate

Boundaries

Divergent

Convergent

Transform

(More Vectors!)

A and B are on the sametectonic plate.

1. Draw this map in your notes.2. Label how plates interact at A and B.

3. Answer how many individual tectonic plates are in the diagram?

WHY DO THE PLATES MOVE ????

Aliens?

Wind?

Emotions?

Energy! How?

Heat‐flow and density changes?

Possibly due to processes involved in thermal convection

What is “thermal convection?”

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Thermal Convection:  Thermal Convection: 

https://vimeo.com/35213650Crustal Age Bathymetry

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Depth = 2.5 km + 0.33 x (sq root age in Ma)

• Average depth ocean ridges = 2.5 km• As plates cool, they get more dense and sink

Thermal Control on Crustal Depth Bathymetry and Topography

Differences in physical properties and strength.

Differences in mineral and chemical composition.

How do we “know” about the structure of the earth’s interior?Seismology!

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Refraction (Snell’s Law)Can you think of a refraction example?

Refraction Example

Why are there different layers?

http://earthquake.usgs.gov/research/structure/crust/download.php

Oceanic Crust Mean = 7 km Continental Crust Mean = 40 km

Why is some crust above sea level?

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• Isostasy is the vertical movement of the crust to attain “buoyancy”in the mantle.

• The height a block of wood floats in water depends on it’s densityand thickness.

• The “height” of the earth’s crust also depends on it’s densityand thickness.

~2.7 g/cc

~3.3 g/cc

~4.5 g/cc

Geology 15 Fall 2013 Lecture 5 Review:

• Origin of the Earth• Plate Tectonics• Earth Structure

http://www.earthds.info/

Free Geology eBooks:

http://itic.ioc‐unesco.org/index.php?option=com_content&view=category&layout=blog&id=1437&Itemid=1437&lang=en

Tsunami Email Notifications:

Next Class• Plate Tectonics• Plate Interactions

2/3/2014

One Minute Paper, that lasts three minutes.

I want to read about what you do understand. What is the most exciting thing you learned today?

I want to read about what you do not understand. What may have been confusing?

What is one question that you have about today’s lecture?

We will cover these issues during our review of today’s class at the beginning of our next class.