From Pluto, the Greek God of the Underworld, to plutonism, the production and movement of molted rock at depth, people have known the earth's interior as the source of awesome and sometimes fearful forces. This is the focus of study in this second part of the course.

Tectonism - earth movements.

Vertical movements are easy to document, and during earthquakes can be experienced, as Darwin did in 1835 along the coast of Chile, where portions of the sea bottom were permanently raised out of the water.

"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 Meditteranean) 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.

Geosynclinal theory - early idea from 1800s to 1950s; characterized by vertical movements with secondary gravity driven slumping, and by thermal shrinkage of outer crust; stabilist paradigm. Many problems

Alfred Wegener and continental drift (1910-1930).

1960s and the plate tectonic revolution:

• geophysical exploration of the seafloor.
• polar wandering.
• global seismic networks.

Sidebar: Why doing all this geophysical exploration in the 1950s? Demonstration of 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 various types is concentrated at or near plate boundaries of various types.

Continental vs. oceanic crust:

• oceanic crust:
  • composed of basalt, richer in Mg and Fe, hence denser.
  • thinner crust (5-12 km), rides lower and hence the floor to deep oceanic basins.
  • created at sea floor spreading centers and recycled at subduction zones.
• continental crust:
  • much greater compositional diversity, richer in lighter elements such as Si, Al, K.
  • can't be subducted and recycled - so very old with complex history.
  • thicker crust (average 35-40 km), which since lighter tends to ride higher.

Importance of concept of rheidity (non-intuitive behavior).

Types of plate boundaries:

Divergent:

• continental rifts: East African Rift zone, Lake Baikal
• young oceanic rifts (seafloor spreading centers): Red Sea.
• mature oceanic rifts (seafloor spreading centers): MAR with Iceland as anomalous emergent part, and EPR in Pacific.

Convergent boundaries:

• subduction - island arc: wastern Aleutians
• subduction - continental arc: Andes, Cascades of U. S..
• collisional: Himalayas.

Transcurrent:

• oceanic transforms: Romanche transform (in S Atlantic).
• continental transforms: San Andreas fault system.

Hybrid:

• transpression - South Island, New Zealand.
• transtension - Dead Sea Rift.

• earthquake frequency and types are a function of plate setting.
• type of volcanic behavior is function of plate setting.
• plate movement rates determine many other rates, such as rate of tectonic uplift or subsidence and associated local shoreline movement. These can be important in modeling.
• economic resource distribution can be correlated with plate motion history, and thus plate tectonics is part of resource exploration models.
• locally, geomorphology and associated surface processes (mass wasting) are controlled by tectonism (e.g. California and the Iceland).
• there are possible connections between plate tectonic behavior and climate change.
• gives a much better framework for understanding geology overall.

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