Hard and soft rocks
We tend to think of all rocks being hard but when we begin to compare them we find that some rocks are softer than others. When we look closely at rocks, we also find that they are made in different ways and this affects their hardness. Some rocks are made from crystals which interlock strongly like the pieces in a jigsaw puzzle. You can see this in a piece of granite. In this rock large pink cubes of feldspar and small black flakes of mica are locked together in a white quartz. In rocks, such as basalt the interlocking crystals are very small. Granite and basalt are made from hot rock inside the Earth. They are called igneous rocks.

Some rocks are made from the broken down bits of other rocks. Sandstone, for example, is made from sand as its name suggests. The grains of sand are held together by a natural cement that forms when the grains are pressed together to form a rock. Mudstone is made in a similar way but from tiny pieces of mud. Mudstone is very soft. In fact if you grind it up with a little water you can make the original mud from which the rock formed. Rocks, which are formed by particles settling down together, are called sedimentary rocks.

Sometimes it gets so hot in the Earth's crust that the rocks in it change. The heat may be caused by hot rock passing up to a volcano or by slabs in the crust pushing together to form mountains. The heat bakes the rocks and makes them change. If mudstone is heated it changes to slate. This rock cannot be easily ground up to make mud. It breaks into large, thin, brittle sheets instead. A rock like slate, that has changed from another rock through heating, is called a metamorphic rock. These rocks are hard like igneous rocks. They are not soft like some sedimentary rocks.

Are granite and basalt the only igneous rocks?
No. There are a number of different igneous rocks. There are even a number of different kinds of granite. One unusual igneous rock is obsidian. It is a black, glass-like rock. When it is broken, it forms very sharp edges and has been used for cutting tools in the past. Rhyolite is a rock that has similar minerals to granite but forms much closer to the surface. It may be found plugging the pipe of an extinct volcano.

Where was sandstone made?
Some sandstones were made in the desert and some were laid down by rivers. Sandstones made from desert sand tend to be red. This colour is due to an iron mineral called hematite. It holds the grains together but is a weak glue. You can rub off grains with your fingers. If you look at the grains with a magnifying glass you can see that they are round. They have been made this shape by the desert winds blowing them together. Sandstone formed from river sand has a much stronger glue. The grains have jagged edges because they are less worn than desert sand. Some sandstones have large grains. They are called coarse sandstones or grit. Some kinds of grit have weak glue and can lose their surface when rubbed.

What kind of rock is limestone?
Limestone is a sedimentary rock. It is made from the shells of sea creatures which collected on the floors of ancient seas. The shells became pressed together and a cement formed to hold them in place. Some limestones are full of fossil shells. The shells are from snails, mussels and crinoids - a kind of starfish with a long stalk to hold it in place on the seabed. One kind of limestone is made from tiny pieces of shell and sand. These rolled on the bottom of the sea and became coated in lime. The pieces stuck together to form a rock that looks as if it is made from fish eggs. This limestone is called oolitic (or egg-like) limestone. It is still forming today in the sea around Florida.

Are fossils found in other kinds of rock?
Yes. They are found in other kinds of sedimentary rock such as mudstone and sandstone. They are also found in rocks that formed from volcanic ash. They are not found in igneous or metamorphic rocks. Any animals or plant bodies falling into lava would be burnt away. When a rock holding fossils is heated and changed into a metamorphic rock, the fossils in it are destroyed.

Is chalk similar to limestone?
Yes, it is. Chalk is made from the shells of tiny sea creatures that floated in the upper waters of ancient seas. When the shells settled on the seabed they became pressed together like the larger shells in limestone.

What kind of rock is marble?
It is a metamorphic rock. It forms when limestone is heated up. The fossil shells are destroyed and the marble develops a sugary appearance.

How are rocks heated?
They are heated in two ways. They may be heated when hot rock moving through the crust passes nearby. They may be heated when mountains form. Mountains can form when two plates in the crust push together. They make the rock fold up. If you lay a tablecloth flat on a table then push two sides inward you get folds in the middle. Mountain building is similar to this but at the base of the mountains the push on the rocks makes heat. This bakes the rocks and changes them into metamorphic rock.