Xenoliths - hitchhikers from the deep
A Xenolith tells us it's story.
A xenolith is a piece of rock picked up and carried along within a
lava or magma which was not part of the original magma but has been
introduced from elsewhere, either at depth or especially the surrounding
solid country rock
The greenish rounded rock in the middle of the image below is a piece of a
rock called peridotite that has been chipped off and carried
upwards by the black surrounding rock made of basalt lava.
Peridotite xenolith ( a piece of the upper Mantle) in basalt lava
The Crust of the Earth is the solid rock we live on and is about
about 30 to 50km thick.
Beneath this is the Mantle. The Mantle is about 2900km thick and
make up about 84% of the earth's mass.
My Story - the Xenolith
That piece of rounded greenish rock is me a piece of Peridotite,
a "hitchhiker from the deep earth" - I come all the way from the Upper
Mantle - that is a region starting at the bottom of the
Crust an continuing to a depth of about 400km.
Much of the time the enormous pressure on the Upper Mantle of the
overlying rocks tens of kilometres of rock keeps it squeezed together as a
solid. But if the pressure is released (via a crack or movement in the
crust) the Mantle turns to liquid. So how did I get from so deep in the earth all the way to the
surface - I hitchhiked!
Since liquids are lighter (less dense) than the surrounding solid rock ,
the liquified rock, being lighter, will move upwards if it can
find a pathway either some of the way or all other the way to the surface.
This is the process "powers" volcanoes and magma intrusions, not "secret
pumps" but just lighter stuff moving on top of heavier stuff - simple!
The mantle is thought to be primarily composed of ultrabasic rocks (rocks
rich in magnesium and iron, and poor in silica; mostly peridotites).
The liquid rock is called magma when it is beneath the surface and
lava when it reaches the surface. How did I pick up a ride to the surface?
On the magma's upwards journey it broke me off, piece of solid rock
and carried me all the way to a volcanoe where I came out surrounded
by basalt lava.
Most magmas (molten rock) are generated in the Upper Mantle (measured from
the base of the crust down to 400 km).
Though we rarely see Upper Mantle rocks they comprise about 10 % of the
Earth's total mass. So a "hitchhiker" like me is very special like a deep
sea diver bringing up treasure. Xenoliths like me give people a
valuable look at the rocks found at very great depths in our planet.
The Upper Mantle's Composition: mostly Peridotite (made up of the minerals
olivine + pyroxene) but there is even more we can learn from xenoliths...
Additional minerals in the peridotite can tell us about how deep the rock
the addition of the mineral plagioclase tells us the peridotite
originates at < 30 km depth
the addition of the mineral spinel tells us the peridotite
originates between 30 km - 70 km depth
the addition of the mineral garnet tells us the peridotite
originates at > 70 km depth)
in tectonically active regions, eclogite (amphibole + garnet) is a
major component of the peridotite
Why am I rounded and in a hollow?
I am rounded and surrounded by a hollow around because the red hot basalt
lava (at 1200 degrees celsius) has been trying without success to melt me
all up! But I am a pretty tough rock!
Another example of a xenolith is shown below:
In this case the light coloured piece of rock is a xenolith of the
intrusive rock gneiss stuck to a lava bomb that has been thrown out
of a volcanoe.