Sunday Rocks!A Presentation of Rocks Discussed by Helen Claire Gould,

author & geologist,

on Luke Chillingsworth’s Sunday Show

On Sunday 12th October, 2014.

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.

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.

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.

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.

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.

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.

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.

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.

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.