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Helen Gould Rocks
1. Sunday Rocks!
A Presentation of Rocks Discussed by Helen Claire Gould,
author & geologist,
on Luke Chillingsworth’s Sunday Show
On Sunday 12th October, 2014.
2. Basalt
Basalt is the commonest rock in
the Solar System. It forms the
ocean floors, and also occurs in
many island settings. Hawaii,
Iceland and Madeira are islands
made of basalt. It is erupted at
mid-ocean ridges.
It’s a dark rock, and is very heavy
because it’s very dense. It also is
quite a fluid rock, so it’s able to
form lava tubes, rolling down
volcanic flanks for sometimes many
miles. As the flow front progresses
the lava drains forwards, keeping
the tube growing.
Because it’s so dense it can be
subducted under continental rocks.
3. Vesicular Basalt
A vesicle is a hole in the rock, and
this has loads! They’re formed by
escaping gas bubbles as the
basaltic magma comes to the
surface.
This rock has sharp edges, and also
feels quite heavy.
When the vesicles become infilled,
over time, with minerals, they are
known as amygdales.
4. Biotite Granite
Biotite is a type of mica, and as all
rocks are made of minerals such as
mica, knowing what they are can
help to identify them.
This rock is black, white and grey,
with a speckly appearance, and
relatively coarse-grained, as the
mineral particles can be seen with
the naked eye.
The minerals in this rock are mica
(black), albite (white), and quartz
(grey).
If basalt is the rock of the ocean
floors, granite is the rock of the
continents. Compared with basalt
it’s light in weight and colour,
which is why basalt gets subducted
below it at oceanic trenches.
5. Gabbro
Gabbro is a very coarse-grained
volcanic rock – you definitely can
see the mineral grains with the
naked eye!
It often occurs in situations where
a new island is being formed. In
Lanzarote it formed the basement
rocks of the island of La Graciosa,
at the top of the main island.
It contains 50-60% plagioclase
feldspar, with olivine, quartz and
other minerals making up the rest.
6. Obsidian
Obsidian is a black volcanic glass
which can be knapped in the same
way as flint to produce knife-edges,
spear tips, hand axes and so
on.
The curving fracture surfaces you
can see are typical of both
obsidian and flint, as well as being
a distinctive feature. This is called
conchoidal fracture.
It is very shiny, a feature known as
vitreous lustre.
7. Snowflake Obsidian
Obsidian is a glass, which means it
cools very quickly, too quickly for
minerals to grow in the liquid.
Snowflake obsidian is volcanic glass
in which, over time, minerals have
grown. These can be seen as white
or grey flecks and patches in the
black glassy matrix.
The mineral is cristobalite.
8. Pumice
Pumice is well-known as the stuff
used to get rid of hard skin on the
feet.
It’s such a light rock that it floats
on the sea surface when erupted
beside an ocean or undersea.
Compare it with basalt and you’ll
really notice the difference.
The lightness of this rock is due to
it being, effectively, a froth of
glass. The magma contains lots of
gas, which escapes on eruption,
and the remaining material cools
very quickly, before crystals can
form.
9. Agglomerate
Magma can be erupted as a variety
of sizes of solid particles, ranging
in size from fine ash to blocks and
bombs.
The larger particles can be as big
as a house. But smaller blocks and
bombs can form into a rock layer,
similar to the way some sediments
become sedimentary rocks. The
particles fuse together due to the
heat as they fall back to the
ground, and layers can build up on
top of one another during
successive eruptions.
10. Banded Iron Formation
This slice of tumbled and polished
rock is about 2.5 billion years old –
over half the age of the Earth.
The red bands are chert, a type of
silica not unlike flint, and the grey
bands are iron.
Due to our atmosphere, free iron
isn’t found on Earth, though it is
found on the Moon.
This rock was laid down during the
formation of our present
atmosphere.
