SAN ANDREAS FAULT
The San Andreas Fault marks the junction between the
North American and Pacific Plates. The fault is 1300
km long, extends to at least 25 km in depth, and has a
north west-south east trend. It is classified as a right
lateral (dextral) strike-slip fault.
Although both plates are moving in a north westerly
direction, the Pacific Plate is moving faster than the
North American Plate, so the relative movement of the
North American Plate is to the south east. The Pacific
Plate is being moved north west due to sea floor
spreading from the East Pacific Rise (divergent margin)
in the Gulf of California. The North American Plate is
being pushed west and north west due to sea floor
spreading from the Mid Atlantic Ridge (divergent
margin).
Movement along the fault is not smooth and
continual, but sporadic and jerky. Frictional forces lock
the blocks of lithosphere together for years at a time.
When the frictional forces are overcome, the plates
slip suddenly and shallow focus earthquakes are
generated. Landscape and manmade features (eg
rivers, fences and roads) are displaced across the fault as movement occurs. San Francisco has historically
suffered significant earthquakes, notably in 1906 and 1989.
The average rate of movement along the San Andreas Fault is between 30mm and 50mm per year over the last
10 million years. If current rates of movement are maintained Los Angeles will be adjacent to San Francisco in
approximately 20 million years.
California 2003 earthquake:
Population of California 35.25 million , area affected population 24,000
Magnitude of 6.5 - 3 people died
Epicentre not dear densely populated area
Happened along a conservative plate margin of two plates that run parallel to the San
Andreas Fault
80 buildings were damaged
, OCEANIC/CONTINENTAL: THE ANDES
The Nazca Plate is moving eastwards, towards the South American Plate, at about 79mm per year.
Where the two plates meet, the denser oceanic lithosphere of the Nazca Plate is forced down and under the
more buoyant continental lithosphere of the South American Plate, descending at an angle into the mantle in a
process called subduction. This is marked on the ocean surface by the presence of the Peru-Chile (or Atacama)
Trench.
The friction between the plates prevents the subducting oceanic plate from sliding smoothly. As it descends, it
drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus
earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as
a Benioff zone.
As subduction of the Nazca Plate continues, some of the sediments that accumulated on the ocean floor
(together with some of the ocean crust) are scraped off and forced (accreted) onto the South American Plate
in a process called “obduction”. This forms an accretionary wedge (or ‘prism’), where layers of the deformed
and metamorphosed sediments and ocean crust are thrust onto the South American Plate along faults – or
thrust planes – adding to the size of the continent.
Cross section of an accretionary wedge
The effect of the collision of the two plates deforms the leading edge of the South American Plate by folding
the rocks. This crustal shortening increases the vertical thickness whilst reducing the width of the lithosphere
in the collision zone (imagine a car hitting a solid wall) and so produces the fold mountains of the Andes.
Continued subduction of the Nazca Plate brings sea water, locked in the ocean crust, deep into the mantle. As
the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting.
This produces magma, which rises and may be erupted explosively as andesite at the surface.
Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000 oC. It is
viscous, trapping gases as it rises. The water and
gases in andesitic magma account for the explosive
activity of andesitic volcanoes, which typically lie
dormant for many hundreds or thousands of years.
These volcanoes typically produce ash and
pyroclastic flows, as well as small amounts of
andesitic lava.
Andean volcanoes such as the stratovolcano Láscar,
in northern Chile, are a good example of this type of
activity. Láscar erupted ash and pyroclastic flows in
1993 and was still active in 2012.
The San Andreas Fault marks the junction between the
North American and Pacific Plates. The fault is 1300
km long, extends to at least 25 km in depth, and has a
north west-south east trend. It is classified as a right
lateral (dextral) strike-slip fault.
Although both plates are moving in a north westerly
direction, the Pacific Plate is moving faster than the
North American Plate, so the relative movement of the
North American Plate is to the south east. The Pacific
Plate is being moved north west due to sea floor
spreading from the East Pacific Rise (divergent margin)
in the Gulf of California. The North American Plate is
being pushed west and north west due to sea floor
spreading from the Mid Atlantic Ridge (divergent
margin).
Movement along the fault is not smooth and
continual, but sporadic and jerky. Frictional forces lock
the blocks of lithosphere together for years at a time.
When the frictional forces are overcome, the plates
slip suddenly and shallow focus earthquakes are
generated. Landscape and manmade features (eg
rivers, fences and roads) are displaced across the fault as movement occurs. San Francisco has historically
suffered significant earthquakes, notably in 1906 and 1989.
The average rate of movement along the San Andreas Fault is between 30mm and 50mm per year over the last
10 million years. If current rates of movement are maintained Los Angeles will be adjacent to San Francisco in
approximately 20 million years.
California 2003 earthquake:
Population of California 35.25 million , area affected population 24,000
Magnitude of 6.5 - 3 people died
Epicentre not dear densely populated area
Happened along a conservative plate margin of two plates that run parallel to the San
Andreas Fault
80 buildings were damaged
, OCEANIC/CONTINENTAL: THE ANDES
The Nazca Plate is moving eastwards, towards the South American Plate, at about 79mm per year.
Where the two plates meet, the denser oceanic lithosphere of the Nazca Plate is forced down and under the
more buoyant continental lithosphere of the South American Plate, descending at an angle into the mantle in a
process called subduction. This is marked on the ocean surface by the presence of the Peru-Chile (or Atacama)
Trench.
The friction between the plates prevents the subducting oceanic plate from sliding smoothly. As it descends, it
drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus
earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as
a Benioff zone.
As subduction of the Nazca Plate continues, some of the sediments that accumulated on the ocean floor
(together with some of the ocean crust) are scraped off and forced (accreted) onto the South American Plate
in a process called “obduction”. This forms an accretionary wedge (or ‘prism’), where layers of the deformed
and metamorphosed sediments and ocean crust are thrust onto the South American Plate along faults – or
thrust planes – adding to the size of the continent.
Cross section of an accretionary wedge
The effect of the collision of the two plates deforms the leading edge of the South American Plate by folding
the rocks. This crustal shortening increases the vertical thickness whilst reducing the width of the lithosphere
in the collision zone (imagine a car hitting a solid wall) and so produces the fold mountains of the Andes.
Continued subduction of the Nazca Plate brings sea water, locked in the ocean crust, deep into the mantle. As
the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting.
This produces magma, which rises and may be erupted explosively as andesite at the surface.
Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000 oC. It is
viscous, trapping gases as it rises. The water and
gases in andesitic magma account for the explosive
activity of andesitic volcanoes, which typically lie
dormant for many hundreds or thousands of years.
These volcanoes typically produce ash and
pyroclastic flows, as well as small amounts of
andesitic lava.
Andean volcanoes such as the stratovolcano Láscar,
in northern Chile, are a good example of this type of
activity. Láscar erupted ash and pyroclastic flows in
1993 and was still active in 2012.
