Igneous Rocks

Towering nearly 400m above the tropical stillness of the Sunda Strait in Indonesia, one of the most terrifying volcanoes the world has ever known has begun to stir once more. 126 years since Krakatoa first showed signs of an imminent eruption, stunning pictures released in July 2009 prove that the remnant of this once-enormous volcano is bubbling, boiling and brimming over. Last time, the bang when it erupted was heard 4,000km away.This time there are thousands of people living under its shadow…

When volcanoes erupt and the liquid rock comes up to the earth’s surface, then new igneous rock is made. Igneous means made from fire or heat – from the Latin ‘ignis=fire‘. When the rock is liquid and inside the earth, it is called magma. When the magma gets hard inside the crust, it turns into granite.
Most mountains are made of granite. It cools very slowly and is very hard.

When the magma gets up to the surface and flows out, like what happens when a volcano erupts, then the liquid is called lava.  Lava flows down the sides of the volcano.  When it cools and turns hard it is called obsidian, lava rock , basalt or pumice – depending on what it looks like.

  • Igneous rocks form when molten lava (magma) cools and turn to solid rock.  The magma comes from the Earth’s core which is molten rock .
  • Obsidian is nature’s glass. It forms when lava cools quickly on the surface. It is glassy and smooth.
  • Pumice is full of air pockets that were trapped when the lava cooled when it frothed out on to the surface.  It is the only rock that floats.

There are 5 kinds of igneous rocks, depending on the mix of minerals in the rocks.

  • Granite contains quartz, feldspar & mica
  • Diorite contains feldspar & one or more dark mineral. Feldspar is dominant.
  • Gabbro contains feldspar & one or more dark mineral. The dark minerals are dominant.
  • Periodotite contains iron and is black or dark.
  • Pegmatite is a coarse-grained granite with large crystals of quartz, feldspar and mica.

Here’s a few pictures…

Feldspar can be either sodium, potassium, or calcium aluminium silicate
Quartz is pure crystalline silicon dioxide

Mica is particularly interesting.

Mined from the earth in thin sheets, this mineral is extremely finely ground for use in cosmetics such as eyeshadow, mineral makeup, powder, lipstick, and sometimes nail polish.

The word mica comes from the Latin word “micare,” meaning to shine or glitter.

There are almost 50 different varieties of mica. They has a general formula

AB2-3(X, Si)4O10(O, F, OH)2

where A can be either potassium, sodium or calcium, B can be either aluminum, lithium, iron, zinc, chromium, vanadium, titanium, manganese and/or magnesium and X is usually aluminium, all the other symbols having their usual meanings.

Have you noticed? Often, transition metals are present. This is what gives the crystals their colours. Finally, here’s some polished malachite. It contains copper, so this gives it its green colour. It’s copper carbonate mostly and is formed by reactions between other minerals and water. It would be more correct to say that malachite is therefore metamorphic – see next post.

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