Download PlatemarginsL3and4 9.74MB 2017-03-29 12:41:32

How good is your memory?
• Structure of the Earth
• Plate tectonic theory
Today:
• What are the different types of plate
boundary/margin?
• What processes and features occur at these
margins?
There are three main types of plate
boundary.
•CONSTRUCTIVE /Divergent boundaries -- where new crust is
generated as the plates pull away from each other.
•DESTRUCTIVE /Convergent boundaries -- where crust is destroyed
as one plate moves under another.
•CONSERVATIVE /Transform boundaries -- where crust is neither
produced nor destroyed as the plates slide horizontally past each
other.
Artist's cross section illustrating the main types of plate boundaries (see text); East
African Rift Zone is a good example of a continental rift zone. (Cross section by José F.
Vigil from This Dynamic Planet -- a wall map produced jointly by the U.S. Geological
Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)
Convection Currents
• Hot magma in the Earth moves
toward the surface, cools, then
sinks again.
• Creates convection currents
beneath the plates that cause
the plates to move.
Constructive margins
• Where plates move apart in oceanic areas
they produce mid-ocean ridges.
• Where they move apart in continental crust
they produce rift valleys.
• The space between the diverging plates is
filled with basaltic lava upwelling from below.
• Constructive margins are some of the
youngest parts of the Earth’s surface, where
new crust is continuously being created.
Oceanic ridges
• The longest continuous uplifted features on the
surface of the planet.
• Have a total length of 60,000km.
• In some parts they rise 3,000m above the ocean
floor.
• Where two plates pull apart there is a weaker
zone in the crust and an increase in heat near the
surface. The hotter, expanded crust forms a ridge.
• Volcanic activity occurs along the ridge.
• If these eruptions persist, volcanoes may
develop until they reach the surface; islands
can be formed in this way.
• Iceland is the largest example, on the MidAtlantic Ridge.
Surtsey: created by
eruptions in 1963
• These volcanoes have fairly gentle sides
because of the low viscosity of basaltic lava.
(It can flow a long way before it cools)
• Eruptions are frequent but relatively gentle.
• As new crust forms and spreads, transform
faults occur at right angle to the plate margin.
• The parts of the spreading plates on either
side of these transform faults may be moving
at different rates, leading to friction and
ultimately to earthquakes.
• These tend to be shallow-focus earthquakes,
originating near the surface.
Constructive plate boundary
Rift valleys
• Occur at constructive margins in continental areas.
• The heating and updoming of the crust leads to
fracturing and rifting.
• As the sides of the rift move apart, central sections
drop down to form rift valleys.
• The largest of these is the Great East African
Rift Valley.
• 4,000m long (from Mozambique to the Red
Sea)
• From the Red Sea it extends north into Jordan,
a total distance of 5,500km.
• 50km wide
• In some areas, the inward-facing scarps are
600m above the valley floor and often marked
by a series of parallel step faults.
Step faults
Inward facing
scarps
Fracture
Volcanic activity.
• Active volcanoes are surface evidence of
volcanic activity beneath the rift valley.
Mount Kilimanjaro
Mount Kenya
The rift valley might widen still further, allowing
the sea to inundate it, possibly cutting off part
of East Africa.
Task....
• Add a copy of Fig 1.5 to the handout.
• As neat as possible, include all labels.
Destructive margins.
• There are two types of plate, so there are
three types of destructive/convergent margin:
1. Oceanic plate moves towards continental
plate.
2. Oceanic plate moves towards oceanic plate.
3. Continental plate moves towards continental
plate.
Oceanic-continental convergence.
• The denser oceanic plate is forced under the
lighter continental one, into the upper mantle.
• This process is known as subduction.
• As the oceanic crust descends, friction with
the overlying continental crust builds up and
causes major earthquakes.
Rocks scraped off the descending plate
and folding of the continental crust
create young fold mountain chains on
edge of the continental mass. E.g the
Andes.
Deep ocean trenches are found along
the seaward edge of destructive
margins. They mark where one plate
begins to descend beneath another and
can reach great depths. E.g. Peru-Chile
trench, 8km deep
As it is less dense
than the
surrounding
asthenosphere, the
molten material
begins to rise up
through fissures
and by burning their
way through
overlying rock.
Eventually these
reach the surface to
form volcanoes.
The lava has a viscous nature (flows less
easily). This creates complex , composite,
explosive volcanoes. If the eruptions take
place off shore, a line of volcanic islands
known as an island arc can appear
Benioff zone – the further the rock
descends, the hotter the surroundings
become. This, together with the heat
generated from friction, begins to melt
oceanic plate into the magma.
What do you know?
Destructive plate boundary
Example:
• The Peru-Chile Trench.
• Also known as the Atacama
Trench.
• Eastern edge of Nazca Plate being
subducted under the South
American Plate.
• Max depth: 8,065m
• Length: 5,900km
• Width: 64km
• If the eruption of magma takes place offshore,
a line of volcanic islands known as an island
arc can appear.
• Such island chains may develop over millions
of years to become major land masses.
• E.g. Japan – Pacific plate subducted beneath Eurasian
plate.
Oceanic - oceanic convergence.
• Two oceanic plates moving towards each
other.
• One is forced under the other and the
processes involved with subduction begin.
• The crust that is subducted may be marginally
the denser of the two plates or is the one
which is moving more quickly.
• Ocean trenches and island arcs are the main
features, as the interaction takes place well
offshore.
• E.g. Marianas trench – western side of the Pacific Ocean.
• Pacific plate is being subducted beneath the smaller
Philippine plate.
Mariana Islands
• Made up by the summits of 15 volcanoes.
• To the west of the Mariana Trench.
Guam
• U.S territory, population 178,000.
• “Where Americas day begins”.
Continental-continental convergence.
• As continents have similar density and thus
buoyancy, they will not be subducted.
• They collide with each other.
• Their edges and the sediments between them
are forced up into fold mountains.
• Deep roots in the lithosphere.
• No volcanic activity.
• Movement of the plates can trigger shallowfocus earthquakes.
Himalayas
Causing Mt Everest to rise
by up to 3cm a year.
Indo-Australian Plate
Moving north and east
Approx. 5.8cm/year
Eurasian Plate
(rigid)
In parts the Indo-Australian plate is being pushed under to form the mountain
roots up to 70km deep.
This movement causes great stresses which are released by earthquakes.
Often extremely violent and destructive. E.g. Sichuan, 2008 – China, 80,000
deaths.
350km wide
Extend for 3,000km
Collision plate boundary
Conservative margins.
• Occur when two plates move parallel or nearly
parallel to each other.
• There is no creation or destruction of crust.
• No subduction, therefore no volcanic activity.
• Movement of the plates creates stresses
between the plate edges. As sections of the
plates rub past each other, the release of
friction triggers shallow-focus earthquakes.
The San Andreas Fault.
• California
• Although both plates are moving north-west, the Pacific plate
moves faster, giving the illusion that they are moving in opposite
directions.
• Los Angeles could eventually be on an island off the Canadian coast.
San Francisco
1906
1989
Another big one predicted
before 2032!!
Conservative plate boundary
Task…
• Complete the summary table of the different
types of plate margin.
Homework:
• Finish any notes/diagrams from today.
• What are ‘hot spots’? (Tip: find out how the Hawaiian
Islands were/are formed).
• Are there any other examples of ‘hot spots’?
• Find and annotate diagrams to support
written notes.