Environmental Geology Lecture
Outline: Plate Tectonics - the how and why of earthquakes and
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
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.
taken from Dana, 1894, Manual of Geology, 5th edition, American
"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
That the top of Mount Everest is composed
of marine strata is also telling.
- 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
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.
Conceptual framework for plate
boundary classification: Based on relative motion across boundary
and type of crust involved.
Continental vs. oceanic crust:
- oceanic crust:
- composed of basalt, richer in Mg and Fe,
- 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
Types of plate boundaries:
- continental rifts: East African Rift zone,
- young oceanic rifts (seafloor spreading
centers): Red Sea.
- mature oceanic rifts (seafloor spreading
centers): MAR with Iceland as anomalous emergent part, and EPR
- subduction - island arc: wastern Aleutians
- subduction - continental arc: Andes, Cascades
of U. S..
- collisional: Himalayas.
- oceanic transforms: Romanche transform
(in S Atlantic).
- continental transforms: San Andreas fault
- transpression - South Island, New Zealand.
- transtension - Dead Sea Rift.
Significance of plate tectonics
in environmental geology?
- 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
Below are two maps showing the configuration of continents
and oceanic basins 237 Ma ago and 100 Ma years ago. Compare them
to each other and the present configuration and you begin to get
some idea of how dynamic the outer shell of the earth is. These
maps were generated by C. R. Scotese (email@example.com).
© Harmon D. Maher Jr.. This page
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