Transcript Chapter2 PlateTectonics

Chapter 18
Plate tectonics
History of plate tectonics
• The earth’s surface is divided into
several major and minor plates and
the interaction between these
plates is known as plate tectonics.
There are 8 major and about 30
minor plates.
–N. American
–S. American
–African
–Eurasian
–Indian
–Pacific
–Australian
–Antarctic
• Intense geologic activity occurs at
the plate boundaries where plates;
–Collide with one another
–Move away from one another
–Slide past one another
• The concept of plate tectonics was
developed in 1960s by combining
two theories;
–Continental Drift
–Sea floor spreading
1. Continental Drift: proposed by
Alfred Wagener in 1912.
• He found similarities in S. America,
Africa, India, Antarctica and
Australia.
• On this basis he proposed;
–All the continental landmasses
were once joined together in one
supercontinent—Pangea. All the
oceans formed one super
ocean—Tetheys.
–The Pangea then split into a
northern Laurasia and a southern
Gondwanaland.
• Revival of continental drift:
Wegener’s mechanism of
continental drift was not very
convincing and his theory
remained discarded till 1960s.
• It was revived with the advent of
paleomagnetism, which confirmed
that the continents have moved
relative to one another.
• Additional evidence for cont. drift:
–The continents fit like a jigsaw
puzzle
–Rock similarity
–Fossils and age similarity
2. Sea floor spreading: proposed by
Harry Hess in 1962.
• He proposed that the sea floor
moves away from the crest of a
mid ocean ridge and finally
disappears beneath a continent or
an island arc (subduction).
• Mantle convection is responsible
for ocean spreading.
Plates and plate boundaries
• Rigid lithospheric plates move
over plastic asthenosphere.
• Plate boundaries: 3 types;
1. Divergent plate boundary: plates
move away from one another.
Also known as Constructive P.B.
or spreading center.
2. Convergent P.B: plates move
towards one another. Also known
as Destructive P.B.
3. Transform P.B: plates move
horizontally past one another.
Also known as Conservative P.B.
Do plates really move?
• Plate motion is very slow but
predictable. 1-10 cm/year.
• Movement is measured by
satellites, lasers and GPS.
Do plates really move?
• The magnetic anomalies at the sea
floor and movement along a
transform fault indicate plate
motion.
Divergent Plate Boundaries
• Two plates move away from one
another.
• Can occur within a continent or an
ocean. If it occurs in a continent—
rifting.
Divergent Plate Boundaries
• Generally basaltic magma erupts
and spreads on either side of the
fractures causing the plates to
push away from one another.
• Examples:
–Red Sea.
–East African Rift
• A passive continental margin
forms when one of the moving
portions of the plate are covered
by sediments.
• A mid ocean ridge is formed when
divergent P.B. occurs in the middle
of an ocean—mid Atlantic ridge.
Convergent P.B.
• Two plates move towards each
other and collide forming
mountains.
Convergent P.B.
• Three types of convergent p.b;
–Ocean-ocean convergence
–Ocean-continent convergence
–Continent-continent convergence
• Ocean-ocean convergence: one
plate subducts under the other, a
trench and a volcanic island arc
are formed.
• Ocean-continent convergence:
oceanic plate subducts beneath
the continent and an active
continental margin is formed.
• Continent-continent convergence:
neither plate subducts and a
collisional mountain chain is
formed.
3. Transform P.B: plates slide past
one another an no significant
material is created or destroyed.
• Transform faults generally
connect two divergent p.b. or two
trenches.
Why plates move?
• Mantle convection.
• Ridge Push
• Trench Pull
Mantle plumes and Hot Spots
• Mantle convection moves towards
the earth surface in the form of a
plume and appear on the surface
as a Hot Spot.
Plate tectonics and ore deposits
• Valuable metallic ores are
associated with divergent
boundaries and volcanism on the
sea floor.