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-Volcanoes are formed
when molten rock, or
magma, from the
earth’s mantle, bursts
through a weakness in
the earth’s crust.
- The eruption of a
volcano is usually
spectacular: The redhot molten rock
released is called lava.
The Ring of Fire lies along several of the Earth’s plates in
the Pacific Ocean. The presence of so many active
volcanoes suggests that the ring of fire is a giant weakness
in the Earth’s crust.
Volcanoes are caused by a
process called SUBDUCTION.
During this process, the oceanic
crust and the continental crust
push against each other.
The oceanic crust is heavier, so it
passes below the continental
crust.
Cracks in the continental crust
are formed, and hot magma
from the mantle is expelled out
in the form of lava!
http://www.youtube.com/watch?v=ANeH9W-HMPc
http://www.youtube.com/watch?v=gmwylbF3-CA&feature=related
For many years,
geologists have used
evidence found in rocks
to try and establish the
ago of the earth.
There are many kinds of
rocks, but all rocks can
be classified into three
basic groups.
Igneous rocks are formed
above or below the Earth’s
surface from hot, molten
rock called magma which
comes from inside the
Earth, and has cooled
down and become solid.
Lava from volcanoes cools
down to become igneous
rocks.
Igneous means ‘fire-like’.
The appearance of igneous rocks depends
on two major factors:
• Which substance it contains
• How quickly it cools
Granite
Examples include:
Basalt – Dark coloured rock with small
crystals formed when lava cools quickly.
Basalt
Granite – White pink or grey with flecks of
black, large crystals, often used for
buildings and even gravestones.
Obsidian – Smooth, glassy rock with no
visible crystals and usually black. Formed
when lava cools too quickly for visible
crystals to form.
Obsidian
Sedimentary rocks are the result of
erosion. A river scouring its way through
rock and soil carries tiny particles called
sediment suspended in water.
The particles are carried to a lake or the
sea, and slowly settle to the bottom,
building up layer after layer.
Over millions of years, these layers
become compressed by further layers of
sediment.
Minerals in the water cement the particles
in the layers together.
These rocks often contain fossils of
shellfish and other creatures.
Metamorphic rocks are rocks
that have changed by heat, or
by heat and pressure together,
into a new kind of rock.
Limestone – Sedimentary rock
This can happen when plates
of the Earth’s crust collide, in
the formation of mountains.
Limestone can be changed to
marble in this way.
Marble – Metamorphic rock
‘Metamorphic’ mean changed.
http://www.youtube.com/watch?v=SRaInMDNyE8
http://www.youtube.com/watch?v=v3yJArifULo