Download 1. There is a link between WHY they occur and - DP

What processes lead to
Volcanic Activity ?
In other words why do they occur ?
1. There is a link between WHY they
occur and WHERE they occur.
2. There is also a link between their
LOCATION and the TYPE of volcano..
Volcanoes tend to produce either:-
A lot of ash, gas and dust
from violent explosions...
Or:
Lots of fluid basalt lava
which can flow for miles !
To understand where volcanoes
occur we need to understand how
our planet is made:-
Essentially the thin surface of our
planet (the CRUST) is broken into
about 12 large pieces called plates.
These plates sit on a
layer of denser, and
hotter rocks known as
the MANTLE.
The top 60km of MANTLE is solid and
behaves in the same way as the CRUST.
Together they are called the
LITHOSPHERE.
The lower MANTLE rocks are very hot.
Rocks close to their melting point become
soft or ‘plastic’ and are able to flow
even though they are still solid. This
layer is called the ASTHENOSPHERE.
The vast heat energy from the liquid
OUTER CORE generates rock flows in
the ASTHENOSPHERE called
convection currents.
In some places the LITHOSPHERE is
being drawn together and in others it
is being pulled apart by these slow
moving convection currents
Where the plates are drawn together
you get a DESTRUCTIVE MARGIN,
where they are pulled apart you get a
CONSTRUCTIVE MARGIN.
DESTRUCTIVE PLATE MARGIN !
At a destructive margin one plate is
drawn under another. Usually thinner and
denser oceanic crust is pushed under
lighter continental crust.
As it is reabsorbed into the mantle the rocks
break up and melt. Oceanic crust contains a
lot of water and this lowers the melting point
of the rocks and makes the melt ‘sticky’.
There is also a lot of steam……...
So volcanoes at destructive
margins are...
VIOLENT AND
EXPLOSIVE !
CONSTRUCTIVE PLATE MARGIN!
At a constructive margin the
plates are being drawn apart. This
allows hot rocks to rise upwards.
As pressure is reduced the rocks
melt to form magma. This bursts
through cracks to form lava flows.
Almost all constructive margins are
at the bottom of the Oceans so…..
The lava is cooled very quickly into
‘lumps’ known as Pillow Lava...
...which heaps up into huge mountain
ranges known as Mid Oceanic Ridges.
Because new crust is being created
at these ridges the Oceans get
wider. This is known as...
SEA FLOOR
SPREADING
The Atlantic Ocean is widening
at the rate of 2cm a year !
•The Earth differs from an electric power plant in that
there is no permanent magnet in the center of the
Earth;
•at the very high temperatures of the core, permanent
magnets can't exist, because thermal energy makes
atoms vibrate and tumble so fast that their tiny
magnetic dipoles can't lock into alignment.
•Instead, the Earth is a "self-exciting dynamo,"
meaning that the magnetism is produced by electric
currents in the outer core.
•This is the magnetism that led to the generation of an
electric current in the flowing iron alloy in the first
place—this system perpetuates itself.
In 1963, F. Vine and D.H. Matthews reasoned
that, as basaltic magma rises to form new
ocean floor at a mid-ocean spreading center, it
records the polarity of the magnetic field
existing at the time magma crystallized.
As spreading pulls the new oceanic crust apart,
stripes of approximately the same size should
be carried away from the ridge on each side
(Fig. 5).
Basically, as basalt cools, it ‘records’ the
magnetism of the earth.
This magnetism switches and so we get
alternate stripes.
This is called Paleomagnetism.
Basaltic magma forming at mid-ocean ridges
serves as a kind of "tape recorder", recording
the Earth's magnetic field as it reverses
through time. If this idea is correct, alternating
stripes of normal and reversed polarity should
be arranged symmetrically about mid-ocean
spreading centers.
The discovery of such magnetic stripes
provided powerful evidence that sea-floor
spreading occurs.
This helps explain
CONTINENTAL DRIFT.
There is one exception……….
This is the East African Rift Valley.
This is a constructive margin
beneath a continent.
Diverging convection currents are
splitting Africa in two. Volcanoes form
where cracks appear in the crust.
Eventually a new Ocean will form !
Such volcanoes (like NYIRAGONGO)
will produce lava flows.
So most volcanoes
occur at plate margins.
Violent explosive ‘grey’ type
volcanoes tend to be found at
Destructive margins.
Volcanoes that produce lava
flows are associated with
Constructive margins.
BUT….
What about Hawaii ?
It has much volcanic
activity but it is not
near a plate margin !
Scientists believe that there is a
very hot plume or convection
current in the mantle below Hawaii.
This ‘blow torches’
its way through the
overlying crust to
make volcanoes.
This is called a
HOT SPOT.
The ‘hot spot’ remains fixed
but the plate moves over it.
This means that there is a linear chain of
volcanic islands stretching many hundreds
of miles to the north west of Hawaii. The
further away the older the island !
A brand new volcanic island
is now forming just to the
south east of Hawaii.
Random facts - Measuring Water Depth
Today's oceanographers use sonar instruments to
generate a sound signal that is bounced or "echoed" off
the sea floor and then recorded on board the ship.
The speed of sound in water is 1,500 m per second, four
times faster than the speed of sound in air.
By carefully measuring the round-trip time of the sound
waves and taking into account the variables of
temperature and salinity, the depth of the water and the
distance to another object can be measured accurately