Rocks that soak up water
If you hold up a piece of granite and look at it through a magnifying glass you will not see any holes. You will see crystals of different shapes and sizes all stuck tightly together. If you put the rock on a table and poured a teaspoon of water onto it, the water will flow all over its rocky surface. It may flow down the sides of the rock and onto the table. But it will not go inside the rock.

When you look at sandstone you see a different picture. Instead of crystals locked together you see sand grains pressed together. The sand grains do not completely touch each other. There are small spaces between them. If you add a teaspoon of water to the top of a sandstone rock, the water disappears inside. Each of the gaps you see on the surface is connected to thousands of other spaces inside the rock. When the water flows into the gaps on the surface, it quickly occupies the spaces inside.

Water has an unusual property. When most liquids turn into a solid, they get slightly smaller or contract. Water is different. It expands when it turns into ice. If water inside the sandstone is frozen it expands and pushes on the grains in the rocks. In some places the strength of the push may be strong enough to break the cement that is holding the particles together. When this happens, pieces of the rock fall away. Water and ice can break down huge cliffs of sandstone over a long time.

Can water never break down granite?
Yes, it can. When a piece of granite is first exposed to the air it may have a smooth surface but in time it may get a rough surface. This can happen by particles in the wind chipping the granite. It can also happen by the rock breaking up a little as it is heated by the Sun and cooled by winter weather. When hollows appear in the granite surface, water can collect there. When the water freezes, the expanding ice pushes on the rock around it and snaps pieces off.

How does heating and cooling make a rock break up?
When a rock gets warm, it expands a little. When it cools down, the rock gets smaller or contracts. These changes make parts of the rock push and pull on each other. These pushes and pulls can make pieces of the rock break off. Sometimes the rock breaks in a certain way. A rock called dolerite, which is similar to basalt, makes flakes. As the flakes leave the rock they give the rock rings of ridges and make it look like the skin around an onion.

Can rock hold water without freezing?
Yes. When it rains in sandstone country, for example, the water just seeps into the soil and then into the sandstone. The water moves down through the layer of sandstone until it meets a layer of rock that will not let it pass. The water may then move along the layer of the sandstone and eventually break out of the ground again. The place where the water comes out is called a spring. Then sandstone behaves like a huge water tank storing water under the ground.

What is an oasis?
It is a pool of water that forms in a desert. The water in the pool perhaps fell as rain on mountains a thousand kilometres away. A huge layer of sandstone, inside the mountain and under the desert, allows the water to move slowly underground. If the layer has been folded by movements in the Earth's crust, it may rise to the surface in the desert and release water into a pool. Sometimes the sandstone may have a huge crack in it called a fault. The water can rise up the crack and form a pool too.

Where does the water come from in a well?
It comes from the surrounding rock that is storing water. The level of the water in the well is at the same level as the water stored in the rock. When water is taken out of the well, more seeps in from the surrounding rock. If a large amount of water is pumped out of the well, the level of water in the well falls. This happens because there is less water left in the rock to fill up the well.

How are caves made in limestone country?
Under the soil in limestone country is a huge layer of limestone. In some places there are vertical cracks in the layer. In other places there are horizontal cracks. When it rains, the water seeps through the soil into the limestone and runs down and along the cracks. In time, the water makes the cracks wider. The vertical cracks become shafts and the horizontal cracks become galleries. Water may continue to flow through the shafts and galleries and make them larger still. These large spaces in the rock are called caves. As the water cuts its way down through the rock it may leave dry caves above, which people can visit at any time. Lower down, the caves may have pools and streams and may fill with water in wet weather.