Download Mountain Belts formed at Divergent and Convergent Boundaries

Mountain Belts formed at
Divergent and Convergent
Boundaries
Mountains Formed at Divergent
Boundaries
Divergent Plate Boundaries
Mid Ocean Ridges
• Continuous underwater
mountain chains that extend
up to 60,000 km around the
globe.
• Generally around 1500km
wide and have peaks around
3km high.
• Site of most volcanism on
earth
How are mountains at mid
ocean ridges formed
Mid-ocean ridges result
from convective upwelling
of mantle beneath thin
oceanic lithosphere. Hot
bouyant material in the
mantle rises causing the
lithosphere above it to form
a dome.
They are formed along
structurally weak zones
created where the ocean
floor is being pulled apart
lengthwise along the ridge
crest. New magma from
deep within the Earth rises
easily through these weak
zones and eventually erupts
along the crest of the ridges
to create new oceanic crust.
As the crust moves away
from the ridge it gets older
and becomes thicker and
more dense, sinking deeper
into the asthenosphere,
therefore lowering
elevation.
Rate of spreading of mid ocean ridges
Slow spreading produces stepper flanks and prominent rift valleys
Fast spreading produces gentle slopes and lack of prominent rift valley
Structure of oceanic lithosphere
Features
• Volcanic activity
Basaltic magma
• Pillow basalt fissures
• Shallow earthquakes
• Faulting (normal) but
no folding (rocks have
been deformed by
extension forces not
compression)
• Abyssal hills
Shallow earthquakes (<10km deep)
Notice that earthquakes also are found at transform faults
across mid ocean ridges.
Normal faults are a result of
tension forces
Transform faults near
divergent boundaries
• Most transform faults are
found on the ocean floor.
They commonly offset the
active spreading ridges,
producing zig-zag plate
margins, and are generally
defined by shallow
earthquakes.
Continental Rifts
• How are they formed?
Young rift zones occur
within continental
landmasses and are caused
by convective upwelling of
mantle beneath weak
continental lithosphere.
When continental crust
stretches beyond its limits,
tension cracks begin to
appear on the Earth's
surface. Magma rises and
squeezes through the
widening cracks, sometimes
to erupt and form
volcanoes.
Normal faults are a result of
tension forces
•Faults- normal faults.
faulting produces large elongated down-dropped blocks
(grabens) and uplift blocks (horsts)
Rock Types
Magma - Basaltic
• Rock Types - Mostly
basaltic.
• However continental
crust is much richer in
silica, therefore can get
Ryolite at zones of
continental rifting.
Andersite is less
common.
• Old divergent
continental margins are
the largest stores of
sediment on earth.
Therefore are very rich
in oil deposits etc.
Mountains at convergent boundaries
Ocean / ocean boundaries
• Mountains formed at
ocean/ocean boundaries are
of the volcanic island arc
type. They form on an
oceanic plate that has
another oceanic plate
subducting under it.
Ocean / ocean
• Accretionary
wedge elongated
mounds of ocean
floor sediments that
have been tightly
folded and
chaotically mixed in
the trench by the
faulting and folding
caused as they are
scraped from the
down-going oceanic
plate
Ocean / ocean
• Those formed of chains
of explosive volcanoes.
These volcanoes form
from andesitic magmas
that are generated as the
subducted plate partially
melts when it comes in
contact with the hot
asthenosphere. Steam
and other volatile
substances find paths
upwards, creating vents
for magma to reach the
surface to create the
violent volcanoes.
Main Features
Ocean / continent boundaries
• As an oceanic plate is subducted beneath a continent,
the sediments on the upper surface of the lower plate
will be scraped off to produce a wedge of sediment called
an accretionary wedge. Where the accretionary wedge
is forced directly against the leading edge of continental
crust, the subducting plate will be forced down steeply
into the asthenosphere where the plate will be partially
melted. Steam produced in the process also partially
melts the upper mantle. Andesitic magmas are produced
from these processes. Mountains will be produced in the
continental plate from the compression and uplift of the
low density wedge sediments and the sediments and
rocks of the continent, and from the intrusion of magma
produced from the partial melting in the subduction
zone.
Rock types
• Volcanic and plutonic
rocks (rich in silica)
• Andersitic lava
• Granites and dolerites
found in plutons
(underground)
• Metamophic rocks
(schists/ gneisses)
Features
•
•
•
•
Trench
Forarc basin
Accretion wedge
Violent volcanic
activity
• Fold and thrusts
• Sedimentary basins
Continent / continent
boundaries
• When two continents
collide, the ocean
between them has been
subducted under one of
them (consumed). The
continents will have been
flanked by accreted
sediment from the ocean
floor that was scraped off
from the subduction. This
sediment forms into a
huge wedge as it is
folded, compressed and
uplifted.