Plate Tectonics - the how and why of earthquakes and volcanoes.
Environmental Geology Lecture Notes
From Pluto, the Greek God of the Underworld, to plutonism, the production and movement of molten rock at depth, people have known the earth's interior as the source of awe inspiring and sometimes fearful forces. This is a focus of study in this part of the course.
Image to right is of Mount Ranier, one of the Cascade volcanoes that occurs in Washington State. Why is this volcano here, and what risk does it pose?
History of ideas on tectonism
Tectonism = solid earth movements.
Vertical movements are easy to document and were noted early in human history. During earthquakes they can be directly experienced, as Darwin did in 1835 along the coast of Chile, where and when portions of the sea bottom were permanently raised out of the water to make new land.
"The temple is that of Jupiter Serapsis at Pozzouli. ... supported by 46 columns, each 42 feet high, and five feet in diameter. Three of the columns are now standing, and they bear evidence of submergence for a considerable time to half their height. The lower twelve feet are smooth; for nine feet above this, they are penetrated by lithodomus or stone-bearing shells, remains of which (a species now living in the Mediterranean) were found in the holes. The columns when submerged, were consequently buried in the mud of the bottom for 12 feet, and were surrounded by water nine feet deep. The pavement of the temple is now underwater. Five feet below it there is a second pavement, proving that these oscillations had gone on before the temple was deserted by the Romans."
That the top of Mount Everest is composed of marine strata is also telling. Leonardo de Vinci had this insight concerning earth movements from marine strata exposed in the Italian mountains.
Geosynclinal theory - early idea/paradigm from 1800s to 1950s; characterized by vertical movements with secondary gravity driven slumping, and by thermal shrinkage of outer crust; stabilist paradigm. What is meant by stabilist? The crust moves up and down, but essentially stay put, so that Africa and South America were always separated. Early on it was recognized that there were many problems with this model, but it was considered the best available..
Alfred Wegener and continental drift (1910-1930): he was an early proponent of the idea that continents had moved large distances with respect to each other, had split apart and had sutured together. His ideas were largely rejected, albeit not totally, by the geoscience community. Link to Wikipedia article on Wegener and continental drift.
To the right is a photo of Alfred Wegener, a very interesting character.
1960s and the plate tectonic revolution:
Sidebar: Why was the U.S. (and other countries) doing extensive geophysical exploration, especially of the seafloor, in the 1950s? The answer to this question helps demonstrate the importance of serendipity in science.
Definition of the basic tenants of plate tectonic theory: An outer shell of the earth , the lithosphere, is composed of distinct, curved and semi-rigid plates that move large distances with respect to each other at rates of cms/year. Geologic activity of varying types is concentrated at or near plate boundaries of different types.
Evidence for Plate Tectonics
Evidence was/is multi-faceted and includes:
Some associated images exemplifying a small bit of the evidence are below:
Diagram showing both how the continents fit to form a past super continent called Gondwanaland, and also how the fossil distribution matches and make sense when you reconstruct the continents together. Image source: USGS Dynamic Earth.
In this map each do represents an earthquake and the color of the dot is the depth the earthquake started at. Note how the earthquakes are strongly concentrated, and also note the depth patterns that occur. The concentrations of earthquakes are plate boundaries, but note that earthquakes also occur within plates. Image from: USGS Earthquake Hazards Program, World Seismicity Maps, Pacific Rim. http://earthquake.usgs.gov/regional/world/seismicity/pac_rim.php
GPS geodesy, where we can track movements from year to year using high-precision GPS, have provided major confirmation that plate tectonics occurs in fundamentally the way previous work indicates it did.
This map depicts the result of the ability to actually watch and measure plate motions over a period of years. The arrows show the directions and amounts of movement in mm/yr relative to the interior of the North American continent of individual GPS station at the position of the dot. Note how the arrows to the W are much larger, and those to the E are very small. Those to the W are more attached to the Pacific plate than North America. This reminds us that plate motions and deformation can occur along fairly wide zones. The data consistently shows that the Pacific plate side is moving northward relative to North America. This type of research around the world has given us a much more detailed image of crustal movements associated with plate tectonics. It naturally has important implications for earthquake prediction. Image source: http://geomaps.wr.usgs.gov/pacnw/rescasp1.html
Conceptual framework for plate boundary classification
The classification is based on relative motion across boundary and type of crust involved.
Continental vs. oceanic crust: arguably one of the most important geologic distinctions for planet earth.
Image of bathymetry/topography from http://www.ngdc.noaa.gov/mgg/image/2minrelief.html. The different colors basically represent elevations.The deeper the blue the deeper underwater, and the green to red transition marks elevation above sea level. In this image the difference between oceanic crust and continental crust is readily evident. Notice the long linear feature of higher elevations that more or less goes down the middle of the Atlantic Ocean - this is mid-Atlantic spreading ridge, where oceanic crust is being created as we speak, although very slowly.
Lithosphere (plate material) versus asthenosphere (underlying weak material that allows plate motion). Lithosphere = crust + some 'rigid', colder (still very hot), attached mantle.
Importance of concept of rheidity (non-intuitive behavior for most):
Map summarizing some of the major plates and plate boundaries. Image source: http://vulcan.wr.usgs.gov/Glossary/PlateTectonics/Maps/map_plate_tectonics_world.html
Types of plate boundaries based on movement pattern (kinematics): divergent, convergent, transcurrent and hybrid.
Convergent plate boundaries (3 sub-types): ocean against ocean, ocean against continent, and continent against continent.
Transcurrent plate boundaries (sideways movement):
Hybrid types of plate boundaries:
Link to lecture notes on an entire 3 credit course related to plate tectonics.
Link to class notes from GEOL 3300 Plate Tectonics.
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