epsc 116: resources of the earth lecture 19 on...

EPSc 116: Resources of the Earth

Lecture 19 on Plate Tectonics

Focal PointsPlate tectonics is a large-scale conceptualization of the mechanisms by

which the earth’s crust is formed, transported, destroyed, and re-formed.

Plate tectonics is an expression of how the earth manages to balance its

heat budget.

It is the most important scientific theory within the earth sciences.

Plate tectonics also represents the culmination of a decades-long

controversy that provides insights into “how science works.”

Last, but not least, plate tectonics explains why certain types of ore

deposits occur where they do – geologically and geographically.

Text Fig. 2.4

Overview of Dynamic Processes of Plate Tectonics

Basaltic magma rises up at oceanic ridges, cools, and forms ocean crust rock. At subduction zones, ocean crust slides beneath less-dense continental crust. Plates melt at depth, generating volcanoes and plutons; also earthquakes.

http://deskarati.com/wp-content/uploads/2012/01/010912_1054_THERENAISSA7.jpg

Alfred Wegener (1880-1930)German meteorologist who pieced together several important observations that led to his hypothesis in 1912 about moving crustal plates on earth.


Adventurous Wegener

Recognized:

-- Jigsaw puzzle of the continents

-- Match-up of fossil species, coal deposits, and geologic features among several continents

-- Questioned how (tropical) coal deposits could occur in places that now have extremely cold climates

Postulated:

-- Present continents have moved great distances over the past millions of years

-- In the past, there were much larger land bodies – essentially aggregations of the present continents


Unexpected Distribution of Fossils

http://pubs.usgs.gov/gip/dynamic/continents.html

http://www.scientus.org/Wegener-Continental-Drift.html

Match-Up of Geological Features (Mts.) and Strata

Wegener and others pointed out the remarkable similarity between geologic features along the SW coast of Africa and the eastern coast of South America.

Highly suggestive of previous continuity between those two continents (now widely separated by the Atlantic Ocean).

Poor Reception to Wegner’s Ideas-- Wegener was not a geologist, but rather a meteorologist

-- Did not convince his geological audience (except in South Africa)

-- Did not propose a credible mechanism to move whole continents

-- Died at age 50 while on his 3rd expedition in Greenland

-- US scientists were especially unconvinced; British and other Europeans gradually began to accept Wegener’s “continental drift” proposal.

-- Americans finally became convinced

in the 1960’s due to direct

observations of magnetic

properties of rocks on the ocean floor.

http://www.ucmp.berkeley.edu/geology/tecmech.html

Red = youngest

Blue = oldest

http://www.ucmp.berkeley.edu/tectonics/crustages.jpg

Ocean Floor Forms by Upwelling of Basalt along Central Rift Zones (Spreading Centers)

http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry18.html

Age of Ocean Floor

South America

Africa

N. America

https://www.youtube.com/watch?v=kwfNGatxUJI about 3 minutes

http://pubs.usgs.gov/gip/dynamic/slabs.html

Earth’s Crust Consists of Rigid Plates Encompassing Ocean Floor OR Continent + Adjacent Ocean Floor

From Holmes. 1945. Principles of Physical Geology, p. 506

Earth is a Heat Engine (extremely hot at depth)

Convection is a very effective way to move heat (remember lava lamp)

Brittle

Ductile

Mid-Ocean Rift

Make new crust

Text Fig. 2.16

Ocean spreading zones, where plates diverge, often have volcanic activity.

Formation of Black Smoker Sulfide Deposit

Hydrothermal fluids formed as seawater circulates through cracks in ocean floor

Metals and sulfur get dissolved in hot waters

Hot waters mix with seawater: cool and precipitate metal sulfide minerals

Mid-Ocean Rift

gg

e

Destroy old crust

Higher density Lower density

Same high density

Same high density

Same low density

Same low density

Create volcanoes

Create volcanic islands

Create mountains

Plate Tectonics Controls Formation of Ore Deposits

-- Need a way to concentrate elements

Convection motion caused by heat and gravity

Huge amounts of material recycled by plate tectonics

-- Heat also causes groundwaters to move and cycle through crust

Leaching certain elements from the minerals/rocks through which water passes

Cooling and precipitating new minerals

-- Subduction causes

Heated waters to form and move from subducting slab

Melting and homogenization (mixing) of eroded materials

Melts rise, cool, form individual minerals with their own specific elements

Late-stage hydrothermal fluids move outward and upward; precipitate minerals

-- Plate tectonics model is used as a means to explore for specific types of ore

Distribution of Ore Deposits

Environments for ore deposition are largely controlled by plate tectonics. Thus, specific kinds of deposits (and metals) are associated with different tectonic settings.

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