LECTURE 11. PLATE TECTONICS; DIASTROPHISM
Continental Drift
The theory of continental drift formulated by Alfred
Wegener in the early 20th century is the forerunner to the
modern theory of plate tectonics. It was based on the
observation that many continents "fit together like a
jigsaw puzzle" (e.g. S. America and Africa) and,
if joined back together, many similarities exist in rock
types, geologic structures and plant and animal fossils.
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North and South America, Europe and Africa all fit
together like pieces of a jigsaw puzzle. Mountain ranges,
rock types and fossils also match up.
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PANGAEA
Wegener proposed that 200 million years ago all
continents were joined together in a supercontinent he
termed PANGAEA (“All land”, consisting of N. America
and Eurasia to the north - LAURASIA; and the southern
continents to the south - GONDWANALAND). Pangaea
broke apart and the continents drifted, like icebergs in
the sea, "bulldozing" their way through the denser
oceanic crust below.
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Pangaea
The continents
today
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There were two major criticisms of Wegener's
theory - firstly, no explanation was given for the driving
force behind continental drift, and secondly, the oceanic
crust is far too dense and rigid to allow the continents to
"push" their way through it. The theory was largely
dismissed. The explanation for the drifting of the
continents had to wait until the 1940's and 1950's when
exploration of the ocean floors (offshoot of submarine
detection) resulted in the new theory of plate tectonics.
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PLATE TECTONICS Recap: the upper 70 km of
MANTLE and CRUST = the rigid LITHOSPHERE;
beneath this is the partially molten ASTHENOSPHERE.
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The lithosphere is not continuous, but is fractured into a
number of LITHOSPHERIC PLATES (about 7 major plates).
The plates are moving, as shown by the arrows, at typical
velocities of 1 -10 cm/year (fingernail growth). It is the
movement of these plates and the interaction between them
that is known as PLATE TECTONICS.
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SEA FLOOR SPREADING BY CONVECTION
The exact driving force of plate motion still isn't certain, but
the most likely explanation is that the plates are driven by
convection currents in the asthenosphere. Plates spread
outwards from mid-oceanic ridges where new ocean floor is
created, and return into the earth at subduction zones were
plates sink and melt.
Subduction zone
Mid-oceanic ridge
Subduction zone
Sea-floor spreading
Convection
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Much important geologic activity, including earthquakes
and volcanoes, is found at
PLATE BOUNDARIES
3 types:
Divergent – plates move away from each other.
Convergent – plates move towards each other.
Transform fault – plates slide past each other.
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Divergent. Mid-oceanic ridges; shallow earth quakes, volcanic
eruptions -> new sea floor consisting of BASALT (magma from
asthenosphere forms basalt). Plate movement is initiated at midoceanic ridges; continents are carried along on top of plates;
supporting evidence from "hot spots".
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A hot spot is a stationary plume of rising magma from the
Mantle, as this periodically erupts through the overlying
plate, a chain of volcanoes is formed in the direction of plate
motion.
E.g. Hawaiian islands
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Convergent. Subduction zones; earthquakes occur down the
subduction zone; volcanism from melting of subducted plate e.g.
Cascades, Mt. St. Helens. Possible collisions include: ocean plate ocean plate > island arcs e.g. Japan, Aleutian Islands. Ocean plate continental plate collision > volcanic arc e.g. Cascade Range;
Andes. Continent-continent collision > mountains e.g. Himalayas.
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Island Arc. E.g. Japan.
Earthquakes take place
down the WadatiBenioff zone (part of
subduction zone where
earthquakes occur).
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Juan de
Fuca plate
Volcanic Arc: Cascade
Range.
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Continent-continent collision;
suture zone.
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Transform
fault - plates
slide past
each other;
large
earthquakes
are common
e.g. San
Andreas
fault.
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1989 Loma Prieta
earthquake.
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Distribution of magnitude 5+ earthquakes, 1980-1990. The pattern
closely matches plate boundaries, particularly subduction zones and
transform faults.
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Distribution of major volcanoes. The pattern closely matches plate
boundaries, particularly subduction zones. Note the Pacific “Ring
of fire”.
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Diastrophism
Central to the theory of plate tectonics is the idea that the
crust is subject to both vertical and horizontal movements.
These movements can deform the crust by tilting it,
breaking it and bending it.
Tilted Beds: angle
of tilt is the dip ranges from 0
(horizontal) to 90o
(vertical).
Orientation of the
outcrop is the
strike e.g. northsouth, east-west
etc.
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Steeply dipping rock strata.
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Folded Beds: Rocks at depth under great pressure tend to bend
rather than break when subjected to stress, producing folds.
Main types = syncline, anticline, monocline, overturned fold.
Folds are often exposed at surface by erosion of overlying rocks,
typically ridges and valleys are formed e.g. Appalachians.
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The ridges and
valleys of the
Appalachians
are exposed
folds.
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Faults: Brittle rock (e.g. near surface) tends to break rather
than bend when subjected to stress -> fault.
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(Same as a transform
fault)
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Sometimes, the vertical movement of rock in a fault can result
in a cliff-like feature on the surface of the Earth - a FAULT
SCARP.
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Blocks of crust can be uplifted and depressed along faults
forming GRABENS and HORSTS. e.g. Death Valley is a
graben surrounded by horsts on either side.
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Death Valley, California. The flat valley floor
occupies a graben and is filled with sediment. The
horst rises abruptly on the far side of the valley.
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