Nick Norman - Box of Rocks: Find, Understand, Collect

Have you ever picked up an eye-catching stone or rock and wondered how it formed? This book will answer some of the questions you may have about rocks. Youll learn about 12 common rock types found in South Africa, many of which you have probably seen or even collected. They are all easy to find, although some may not occur close to where you live. But before we hunt for rocks, lets find out what rocks are made of.

What is a rock made of?

Rocks are made up of grains of various minerals that are packed tightly together. So you can think of minerals as the building blocks of rocks. There are many different sizes, shapes and colours of minerals. Quartz is a good example of a rock-forming mineral. The crystals that make up a mineral can be very tiny and even microscopic, if they formed quickly, or big if they formed slowly, allowing them enough time to grow.

This rock has some big crystals set in a mass of much finer crystals.

There are three main groups of rock on earth igneous, sedimentary and metamorphic rocks. Each group was formed in a different way from the other groups. Lets find out a bit more about how the different groups of rock form.

Igneous rock is the name given to all rocks that harden from liquid magma. Volcanoes often occur where tectonic plates collide and one of the plates gets pushed under the other, deeper and deeper underground. The weight of all the heavy rock layers above pushes down hard on the lower layer and, at the same time, the rocky plate gets hotter and hotter the deeper it goes. Eventually the rocks start to melt, forming magma.

If a crack opens up in the earths crust above it, the magma will quickly escape to the surface where it is cooler and there is much less pressure. Once it emerges (extrudes) onto the surface of the earth, we refer to magma as lava. As the lava cools, it hardens to form igneous rock. Magma can also squeeze (intrude) into and between the upper layers of rock, so it cools and hardens more slowly beneath the earths surface.

When tectonic plates collide, one plate gets pushed under the other, causing the rocks to melt.

(c) Westend61/Photoshot

Volcanoes often occur where tectonic plates collide.

Rocks that float

Some lava comes to the surface with a huge amount of gas in it. It shoots out of its volcano like a fizzy drink out of a bottle thats been shaken up. And it chills in the air with all that gas trapped inside it, so its light like polystyrene. This lava is called pumice and it floats. If you look carefully you may be lucky enough to find pumice on the beach at Arniston. These are the remains of lava blasted out of Krakatoa in Indonesia, which erupted in 1883. Pumice floated right across the Indian Ocean and washed up on our shores!