Transcript Plate Tectonics

Plate Tectonics
Prepared by Betsy Conklin for
Dr. Isiorho
Plate Tectonics
• plate tectonics: the idea that the earth’s surface
is divided into a few large, thick plates that
move slowly and change in size
• continental drift: the idea that continents move
freely over the earth’s surface, changing their
positions relative to one another
• sea-floor spreading: a hypothesis that the sea
floor forms at the crest of the mid-oceanic
ridge, then moves horizontally away from the
ridge crest toward an oceanic trench
The Early Case for Continental Drift
• continents can be made to fit together like pieces
of a picture puzzle
• Alfred Wegener purposed that the continents were
originally one giant supercontinent which he
called Pangaea. Pangaea then split into two parts Laurasia which is now North America and
Eurasia, and Gondwanaland which is now the
southern-hemisphere continents and India
• polar wandering: an apparent movement of the
earth’s poles
• the distribution of fossils on various continents
Skepticism about Continental Drift
• fossil plants could have been spread from
one continent to another by winds or ocean
current
• polar wandering might have been caused by
moving poles rather than by moving
continents
Sea-Floor Spreading
• subduction: the sliding of the sea floor beneath a continent or
island arc
• convection: a circulation pattern driven by the rising of hot
material and\or the sinking of cold material
Plates and Plate Motion
• plate: a large, mobile slab of rock that is
part of the earth’s surface
• lithosphere: the relatively rigid outer shell
of the earth of which the plates are a part
• asthenosphere: a zone of low seismic-wave
velocity that behaves plastically because of
increased temperature and pressure
• transform fault: the portion of a fracture
zone between two offset portions of a ridge
crest
Divergent Plate Boundaries
• divergent plate boundary: a boundary between plates that
are moving apart
Transform Plate Boundaries
• transform plate boundary: a boundary between
plates that are moving horizontally past one
another
Transform boundaries between two ridges
Transform boundaries between
a ridge and a trench
Transform boundaries between
two trenches
Convergent Plate Boundaries
• convergent plate boundary: a boundary between plates that
are moving toward each other
• ocean-ocean convergence: when two plates capped by sea
floor converge and one plate subducts under the other
• island arc: a curved line of volcanoes that form a string of
islands parallel to the oceanic trench
Convergent Plate Boundaries (cont.)
• ocean-continent convergence: when a plate captured by oceanic crust
is subducted under the continental lithosphere and an accretionary
wedge and forearc basin form an active continental margin between
the trench and the continent
• magmatic arc: a broad term used both for island arcs at sea and for
belts of igneous activity on the edges of continents
Convergent Plate Boundaries (cont.)
• continent-continent convergence: when two continents
approach each other an collide
What causes Plate Tectonics
• mid-oceanic ridge crests are hot and
elevated, while trenches are cold and deep
• ridge crests have tensional cracks
• the leading edges of some plates are
subducting sea floor, while the leading
edges of other plates are continents (which
cannot subduct)
Mantle Plumes and Hot Spots
• mantle plumes: narrow columns of hot mantle rock that
rise through the mantle, much like smoke rising from a
chimney
Continental breakup caused by a mantle plume. (A) A dome
forms over a mantle plume rising beneath a continent.
(B) Three radial rifts develop due to outward radial flow
from the top of the mantle plume. (C) Continent separates
into two pieces along two of the three rifts, with new ocean
floor forming between the diverging continents. The third
rift becomes an inactive “failed rift” (or aulacogen) filled
with continent sediment.
Pictures
All pictures used in this power point presentation
were taken from the following:
Carlson, Diane H., David McGeary and Charles C.
Plummer. Physical Geology: Updated Eighth
Edition. New York City, McGraw-Hill Higher
Education, 2001.