Transcript Plate Tectonics, Isostasy, and Paleogeography

From Hypothesis to Theory
CONTINENTAL DRIFT AND PLATE TECTONICS
CONTINENTAL DRIFT
In the 19th and early 20th centuries, several scientists suggested
that the continental masses had been slowly but steadily moving
across the Earth's surface.
First proposed by A. Snider in 1858 and developed by F.B. Taylor
(1908), this idea was later articulated by Alfred Wegener (1915) in
his Continental Drift Hypothesis.
These early notions of continental drift were based on the following
evidence:
• Some continents seem to fit together like a jigsaw puzzle.
• Some geologic features, rocks and fossils found on the East
coast of North and South America are similar to formations
found on the West coast of Africa and Europe.
Geographic,
Paleomagnetic,
and Paleo geographic Evidences
• Wegener’s
Supercontinent –
Pangaea
– later divided into
Gondwana and
Laurasia
– and eventually
into the present
continental
configurations
and positions.
• In the late 1950s and 1960s, scientific research in oceanography,
geophysics, and other Earth Sciences (aided by sonar, radioactive
dating), provided further support for Continental Drift
• Later, detailed mapping and study identified the process of seafloor
spreading, and pointed to the Theory of Plate Tectonics
PLATE TECTONICS
Plate tectonics is the study of the lithosphere, the outer portion of the earth
consisting of the crust and part of the upper mantle.
 According to the Theory of Plate Techtonics, the lithosphere is
divided into about 30 lithospheric or techtonic plates –13 large
and 17 smaller – which move and interact with one another
 Most plates include both continental crust and oceanic crust
 These plates move very slowly over a semi-molten or plastic
asthenosphere, only 2 to 5 centimeters (1-2 inches) a year
 Continents and oceans move and change in shape as a result of
these plate movements
 These movements also create earthquakes, volcanic activity,
mountain ranges, ocean trenches and many other features.
 These movements
of the lithosphere
result from heat
energy derived
from the earth’s
interior –
heat-driven
convection cells
in the mantle.
Seafloor Spreading
at an oceanic ridge produces new seafloor
In some places the plates pull away from each other (divergence), in
other place they push together (convergence), and elsewhere they slide
alongside each other (lateral/transform movement)
These interactions, thus, create three major plate boundary types:
1. Divergent Plate Boundaries 2. Convergent Plate Boundaries 3. Transform Plate Boundaries
DIVERGENT PLATE BOUNDARIES
Also called Constructive Plate
Margins because they create
crustal ridges and new ocean floors
Where are most divergent plate
boundaries?
• In the middle of oceans –
e.g., Mid-Atlantic Ridge
• Occasional “oceanic” volcanoes,
as in Iceland, mark divergent
plate boundaries (Iceland is a
volcanic island , and part of the
Mid-Atlantic Ridge)
• Divergent Plate Boundaries on
Land – Rift Valleys
e.g., East African Rift Valleys
CONVERGENT PLATE BOUNDARIES
Often known as Destructive Zone as plate subduction takes place
How many ways can plates converge?
 Ocean-Continent
 Ocean-Ocean
 Continent-Continent
What Goes on at Ocean-Continent Convergent Plate Boundaries?
 Earthquakes occur as the
ocean plate subducts
 An ocean trench forms as the
sea floor bends
 Volcanoes form as magma
erupts from the subduction
zone
 Mountains form as sediments
are folded and faulted
What Happens at Ocean-Ocean Convergent Plate Boundary?
 Earthquakes occur as one ocean plate subducts
 An ocean trench forms as the sea floor bends
 A volcanic island arc forms as magma rises above
the subduction zone
Continent-Continent Convergent Plate Boundary
Also called a Collision Zone
• Mountain building occurs as the continental crust is folded
and faulted
• Earthquakes occur as two continents collide
• NO volcanic activity – e.g., India crashes into Asia
Himalayan Mountains and Tibetan Plateau
TRANSFORM PLATE BOUNDARIES
San Andreas Transform Fault
Plate Boundary
Earthquakes only
Also, Juan de Fuca Plate –
subduction –volcanos on the
mainland
Plate boundaries
SUMMARY
Scientists now have a fairly good understanding of how the plates move and
how such movements relate to earthquake activity. Most movement occurs
along narrow zones between plates where the results of plate-tectonic forces
are most evident.
•continental drift evolved into the theory of plate tectonics
•earthquakes & volcanic activity, mountain building and Folding and Faulting
occurs along tectonic plate boundaries
•Divergent boundaries/onstructive boundaries -- where new crust is generated
as the plates pull away from each other. Rift Zones
•Convergent boundaries -- where crust is destroyed as one plate dives under
another. convergent boundaries: subduction zones and continental collisions,
ocean to ocean convergence
•Transform boundaries -- where crust is neither produced nor destroyed as the
plates slide horizontally past each other.