PLATE TECTONICS
Plate Boundaries and Geologic Processes
- The lithosphere is fragmented into fourteen major tectonic
plates.
- Plates are composed of either oceanic crust or both continental
and oceanic crust.
- Plates are moving across the Earth's surface in different
directions and at different velocities.
- Many geologic processes, such as plutonism, volcanism,
and earthquakes result from the interaction of plates along their boundaries.
Evidence from the continents:
The first maps of Europe, Africa, and the Americas led
early scientists such as Sir Francis Bacon to speculate about ancient connections
between the continents.
This is most obvious from the maps of Africa and South
America.
Evidence of South America-Africa connection:
- the close fit of their coastlines
- Carboniferous glacial till in south Africa (below 12°S)
and in Uruguay and southern Brazil
- Carboniferous glacial marine deposits in central Africa
(between 0 and 12°S), and in Argentina, Paraguay, southern Bolivia,
and eastern Brazil
- Permian fold belts in Africa (Atlas Mt) and South America
(Cordillera de Merida)
- Jurassic evaporites between Rio de Janeiro and Recife,
and along west coasts of Cameroon, Gabon, & Angola
PLATE BOUNDARIES
We can define three types of plate boundaries on the
basis of their relative directions of movement:
- divergent boundaries: formed between two plates
that are rifting apart
- convergent boundaries: formed between two plates
that are moving toward one another
- transform boundaries: formed between two plates
that are moving in parallel but opposite directions
DIVERGENT BOUNDARIES
Divergent boundaries are the sites of the creation of
new oceanic crust by volcanism:
- magma (molten rock) rises upwards out of the asthenosphere
into widening gap (spreading center) between two diverging plates
- magma erupts (as lava) from volcanoes and
fissures, crystallizes into volcanic rocks
- spreading centers are marked by continental rift
valleys, ocean basins, mid-ocean ridges
MID-OCEAN RIDGES
The mid-ocean ridge is a chain of submarine volcanic
seamounts and subaerial volcanic islands that encircles the Earth.
The mid-ocean ridge is the largest mountain chain in
the world:
- 80,000 km long
- average of 1500 km wide
- average of 3 km high
- covers 20% of sea floor
Mid-ocean ridges consist of a central ridge crest
bordered by ridge flanks. The ridge flanks dip away from
the crest, and eventually disappear beneath the sediments of the abyssal
plain.
The ridges are covered by volcanic seamounts and
smaller (less than 1000 m) abyssal hills.
The ridges are also broken and offset by cross-cutting
fracture zones. They form deep linear gouges and steep scarps upon
the sea floor.
SUBDUCTION BOUNDARIES
- The edge of one plate plunges into asthenosphere beneath
leading edge of second (overriding) plate.
- The subducted plate melts to generate magma, which rises
upwards into the overriding plate
- Some magma accumulates in magma chambers (plutons)
in the overriding plate, where it crystallizes into plutonic rocks.
- Some magma erupts on the Earth's surface, where it crystallizes
into volcanic rocks.
Subduction boundaries are characterized by several physiographic
features, including:
- oceanic trench: narrow and elongate depression
in the sea floor that marks the line of subduction
- magmatic arc (a.k.a. arc massif): chain
of volcanic mountains (and subterranean plutons) that parallels the oceanic
trench:
- volcanic arc: land-locked arc (e.g., Andes)
- island arc: sea-bound arc (e.g., Japan)
COLLISION BOUNDARIES
- Two converging plate edges collide head-on, but there
is no subduction.
- The plate edges are deformed by folds and faults.
- The heat and pressure of collision alters the rocks of
the plate edges into metamorphic rocks.
- This deformation forms a fold-and-thrust mountain
belt along the line of collision (e.g., Himalayas)
TRANSFORM BOUNDARIES
Plates are moving past one another in parallel but opposite
directions.Plates are separated by one or several strike-slip fault
planes.
Example: San Andreas fault, which separates the
Pacific plate (moving northwest) and the North American plate (moving south).
INTRAPLATE SITES
Plate interiors are far less active than plate margins.
Tectonism is apparently due to either:
- movement of magma in the asthenosphere
- reactivation of ancient faults
One common geologic process is volcanism, which
occurs when a magma plume rises upwards through asthenosphere and
breaks through crust.
- i.e. the Hawaiian Islands
Earthquakes are rare, but they do happen.
EARTHQUAKES
Earthquakes:
- generated by the movement of rock and magma
- most common to plate boundaries
- plates often move in sudden quick lurches that send shock
waves (seismic waves) through the crust
- rising magma also generates seismic waves as it pushes
its way upward through the crust
- point of earthquake generation is called the focus
Where do they occur?
- 90% occur in the Benioff zones of subduction zones
due to the friction between the subducted and over-riding plates
- the remaining 10% occur largely:
- along the fracture zones of mid-ocean ridges
- along the faults between transform margins
- in the fold-and-thrust belts of collision zones
- occur rarely (but not never) in intraplate settings due
to movement along ancient deeply-buried faults
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