Section 3

## Plate Tectonics - Lecture Notes

Gross Features of the Earth (REarth=6370 km)
(Moving from a high altitude above the surface to the center of the earth.)
Ionosphere - ~100 km (Layers of the atmosphere are usually separated by changes in temperature.)
Mesosphere - ~50 km (Ozone layer exists between the mesosphere and stratosphere)
Stratosphere - ~12 km
Troposphere - clouds and water vapor

Crust - Is the outer layer of the earth with an average thickness of about 15 km.  It has an average density of 3,000 kg/m3.  (The density of iron is 7,900 kg/m3 and marble is 2,700 kg/m3.)
Mantle - Is the largest part of the earth by volume (80%), extends between 15 km to 3,000 km in depth (going towards the center), and has an average density of 5,000 kg/m3.
Core - This part of the earth is usually divided into an inner core and an outer core.  Both parts have an abundance of nickel and iron.  The average density is 11,000 kg/m3.  The outer core is liquid (molten rock) but the inner core is considered a solid metallic material.

How do we know of this internal structure?

Most of what we know comes from examining the propagation of seismic waves (ground vibrations) and modeling the structure of the earth so that it is consistent with the seismic studies (and other experimental observations).

It is sometimes instructive to examine the earth's crust and mantle in a slightly different way.  It can be broken into a top, solid (or rigid) layer called the Lithosphere.  At a depth of about 100 km, the mantle becomes capable of flowing.  From this depth to about 500 km is called the Asthenosphere.
Note:  The Lithosphere includes the crust and the very top part of the mantle.  The Asthenosphere is part of the mantle.

Plate Tectonics

During about 1912 Alfred Wegener, a Meteorologist, proposed a theory of Continental Drift.  He suggested that the earth's crust was composed of solid "plates" that were in motion (albeit slow, ~ 6cm/yr) and about 200 million years ago all the continents were connected into a super-continent called Pangea.  This hypothesis was not well received when it was first proposed.

"Wegener's hypothesis in general is of the foot-loose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.  Its appeal seems to lie in the fact that it plays a game in which there are few restrictive rules and no sharply drawn code of conduct." (R.T. Chamberlain - A Geological Miscellany, Princeton Press, 1982)

This theory is true (such that, it fits almost all observations and most scientist agree with it).  Plate Tectonics integrates continental drift, sea floor spreading, and seismic zones.

Plate Tectonics - Related Web Links

Plate Tectonic Animations from UC-Berkeley
On-line Book This Dynamic Earth: The Story of Plate Tectonics by W. Jacquelyne Kious and Robert I. Tilling
USGS historical perspective on Tectonic Plates
Paleomagnetism and the reversal of earth's magnetic field
Google - Search for Plate Tectonics

On-line Lecture Notes

GOOGLE search for plate tectonic lecture notes
Dr. Andy Frank's Physical Geology Plate Tectonics
Dr. Pamela Gore's Earth's interior lecture notes
Dr. Susan DeBari's Plate Tectonics lecture notes

## Earthquakes - Lecture Notes

"Civilization exists by geologic consent....subject to change without notice."
- Will Durand, Historian

The earth is dynamic.  It has internal processes that are in motion.  Huge forces produced by the internal processes of the earth are constantly pushing and pulling at the earth's Crust.  These forces produce stress within the solid rock layers.  When sufficient stress is built-up a threshold is reached and the rock layers will "snap".

Earthquakes are vibrations of the earth caused by a rapid release of energy.  This energy comes from the stress built up along fault lines (or underground nuclear explosions).  The so-called "Elastic/Rebound" theory.  Seismology deals with the measurement of these ground vibrations.

Seismographs consist of inertia member, transducer, and a recorderHere is an example of the readout from 3 small earthquakes.  (Real time seismogram readouts from UC-Berkeley)
Body Waves:
P-Waves (or primary waves) travel quite fast (~7 km/sec), shakes the ground in a compression/expansion mode, and can travel through the outer core of the earth.
S-Waves (or secondary waves) travel a bit slower (~4 km/sec), shakes the ground in a transverse mode, and cannot travel through the outer core.
Surface Waves shakes the ground in a transverse mode and travels much slower than S-Waves.

The Richter Magnitude Scale is based upon the formula  M=log(A)+CM is the magnitude, A is the amplitude of ground shaking, and C is the distance away from the focus.  This implies that a magnitude 4 quake has 10 times the amplitude of a magnitude 3 quake measured from the same location (More Detail from USGS).  A rather subjective scale also exists called the Modified Mercalli Intensity Scale.

The best scientist can do in predicting earthquakes is to assign probabilities.  An earthquake can be more or less likely to occur in a certain region during a specified time.  Compression and tension builds up within rock structures in a cyclical fashion.

The date corresponding to position (a) in the graph has a much lower probability of an earthquake happening than position (b).

Armenian earthquake of 1988 was a M=6.9 and 25,000 people died.
Loma Prieta (Oakland, CA) earthquake of 1991 was a M=7.1 and 100 people died.
What is happening here?

Effects of Earthquake Ground-Shaking on Structures

 Effects of Earthquakes1.  Fire - gas lines break and create fires, water lines have also been ruptured (so much for fighting the fire)2.  Damage to Structures3.  Seismic Sea waves (Tsunami)4.  Landslides5.  Land Movement (surface shifting, sometimes produce visible cracks in the ground)6.  Liquefaction - Shaken soil or sediments will become like "quick sand" and loose their ability to support structures.  This behavior can be demonstrated by placing a weight onto a pan full of loose sand and shaking the pan.  The weight will sink and tilt.  (More on liquefaction during the Kolbe, Japan, earthquake.)7.  Sound? Tsunami evacuation route sign in Long Beach, WA.

USGS Response to an Urban Earthquake - Northridge '94, The Causes and Effects of Liquefaction, Settlements, and Soil Failures
Virtual Earthquake, a nice interactive site that provides a good tutorial
SeismoCam
, Cooperative project between Caltech and USGS (Real-Time view of a seismograph)
JAVA Simulations of a model Seismograph
FEMA Earthquake Fact Sheet (Adobe Acrobat format)
National Earthquake Information Service Near-Real-Time Global Earthquake Events
National Earthquake Hazard Reduction Program (FEMA, NSF, USGS, NIST)
Reducing Earthquake Losses Fact Sheet from the USGS
Northern California Earthquake Data Center
Advanced Research on Earthquakes from Harvard
USGS On-line Earthquake Resources
On-line Book on Earthquakes by Kaye M. Shedlock & Louis C. Pakiser (USGS)
Surfing the Internet for Earthquake Data (collection of web links)

On-line Lecture Notes

Dr. Andy Frank's Physical Geology Earthquakes
Dr. Pamela Gore's Earthquake lecture notes
Dr. Susan DeBari's Earthquake lecture notes
Seismology and The Earth's Interior - Susan Schwartz, UC-Santa Cruz
Google - Search for Earthquake Lectures

Geophysical Exploration: Geophysical Exploration (surveys include: gravity, magnetic, refraction
seismicity, DC resistivity). Steven Roecker's Applied Geophysics class (lecture notes and links,
includes gravity, seismic, electric, and magnetic measuring techniques). Dick Gibson's primer on using
gravity and magnetism to explore subsurface geology.
Google - Search for Geophysical Exploration

## Volcanoes - Lecture Notes

Unlike earthquakes, volcanic eruptions can be predicted.

Evidence that a volcanic eruption is imminent:
Dramatic increase in earthquake activity in the area
Tilt meters (usually laser surveying) indicate the mountain is bulging
Volcanic gases emitted from fissures in the ground

Three general ways to characterize volcanoes:
Extinct - no signs of volcanic activity, lots of erosion has occurred since the last eruption.
Dormant - not much erosion, "sleeping" and is capable of erupting, little to no signs of volcanic activity.
Active  - Some historical record of an eruption or is currently erupting.

Types of volcanoes

Shield volcanoes are relatively quiet, gently sloping, low viscosity magma (usually low in SiO2), largest in overall size, spews the least amount of gases
Example: Kilauea

Composite (or Stratovolcano) are explosive, steeply sloped, high viscosity magma, intermediate in size, intermediate gas content
Example: Mt. St. Helens, Pinatubo, Mt. Shasta

Cinder Cone volcanoes are somewhat explosive (not as much as Composite), have the steepest sloping, high viscosity magma, smallest in overall size, highest volcanic gases (another Cinder Cone picture)
Example: Paricutin

Volcano Relative Sizes

Click on picture to enlarge.  This is a composite of three pictures.

Aspects of volcanic eruptions
Pyroclastic Debris is the material ejected from an erupting volcano.  Usually classified as (small to large) ash, cinders, blocks, and bombs.
Pyroclastic Flow (or fiery cloud) is a cloud of super-heated gas and ash that flows down slope from an erupting volcano.  It can reach speeds of 100 mi/hr and has a temperature of ~800oC (hot enough to melt glass).  It burns anything combustible.
Volcanic Gases include CO2, CO, HCl, HF, H2S, SO2, H2, and H2O (many of these are toxic in sufficient concentrations)

 Pictures taken during a trip to the Craters of the Moon National Monument in Idaho in August, 2002. Volcanoes of the Cascade Mountain RangePictures taken in 2003 (Mt. Rainier and Mt. St. Helens) and 1992 (Crater Lake).  Select speaker notes in the slide show if you want more informatiion.

Some Interesting Volcanoes
Vesuvius erupted in 79AD and buried the cities of Herculaneum and Pompeii (it was 10 times the power of Mt. St. Helens).
Krakatoa is the considered the world's greatest explosive eruption.  Before the eruption is was an island that stood 800 m above sea level.  After the eruption it became submerged 300 m below sea level.
Mt. St. Helens erupted in 1980.  One of the best documented eruptions in history.
Pinatubo erupted in 1991.
One of the most recent eruptions was Montserrat in the West Indies

On-line Books from the USGS (contains:  VOLCANIC AND SEISMIC HAZARDS ON THE ISLAND OF HAWAII, VOLCANIC HAZARDS AT MOUNT SHASTA, CALIFORNIA, VOLCANOES, VOLCANOES OF THE UNITED STATES, MONITORING ACTIVE VOLCANOES, ERUPTION OF HAWAIIAN VOLCANOES -- PAST, PRESENT AND FUTURE, ERUPTIONS OF MOUNT ST. HELENS -- PAST, PRESENT, AND FUTURE)

Volcano World
Smithsonian Institute's Global Volcanism Program
Virtual Field trip to the Hawaiian volcanoes
Take a virtual hike up Mt. St. Helens (this link provides lots of information on Mt. St. Helens)
Volcanoes on other worlds
On-line video clips of volcanoes

On-line Lecture Notes

GOOGLE search for volcano lecture notes
Dr. Andy Frank's Physical Geology Volcanoes
Dr. Pamela Gore's lecture notes on Volcanoes
Dr. Susan DeBari's lecture notes on Volcanoes
Google - Search for Volcanoes, Lectures

Review Quizzes
Section 3

Review Quiz Section 3
Tarbuck and Lutgens, Essentials of Geology, Self-Quiz (Select Chapters Earthquakes and Earth's Interior, Minerals, Volcanoes and Igneous Activity, Plate Tectonics)
Volcanoes
- Dr. Andy Frank's Practice Exam
Earthquakes - Dr. Andy Frank's Practice Exam
Plate Tectonics - Dr. Andy Frank's Practice Exam
Earthquakes and Seismicity - North Dakota State University Self-Test, Geology 120
Plate Tectonics - North Dakota State University Self-Test, Geology 120
Deformation and Structure - North Dakota State University Self-Test, Geology 120
Internal Structure of the Earth - North Dakota State University Self-Test, Geology 120