Rocks vary in several ways. Some are soft and crumbly – others are immensely hard and resistant. Some allow water to pass through them – others are totally impermeable. Some have crystals – others don’t, and some form layers or strata – others don’t.
In these last two differences we can see the clues to a rock’s origin, and begin to understand in which of the three basic groups a rock should be placed.
The earth is constantly changing. Mountains are thrust up by various means, then are worn down again. The worn rock material is carried away by wind, water or ice, eventually reaching the seabed. So that while the land areas are gradually being eroded and made lower, the seabed is being covered with sediment and made higher.
This sediment will be of stone, sand or silt mixed up with calcium carbonate – the remains of sea creatures. In simple terms the type of sediment depends on the depth of the sea and the distance from shore.
Imagine a river flowing into the sea carrying a mixture of sediment. As soon as the water reaches the sea the current ceases to flow and the sediment begins to be deposited. Heavier material, like shingle and stones, will be deposited first, therefore near to the coast. Further out the sand particles will be deposited, and the silt will be carried further out still before it settles down as sediment.
In the deepest ocean areas, where no sediments reach, the sea bed will be covered in the calcium remains of marine creatures.
This process of sedimentation can continue for millions of years, the layers accumulating and being compressed over time into rock. In this way sand can become sandstone, silt turns into shale or clay and the calcium deposits will become chalk or limestone. The clue to the origin is to be found in the fossils of marine creatures which can be found in these types of rock.
Being formed originally as sediment, these are called sedimentary rocks and the layers of sediment become the strata of rock we sometimes find, often folded and bent to appear at different angles.
Volcanic activity is constant in certain parts of the world, mainly due to the moving tectonic plates and the processes which occur at their boundaries. There are many materials which can be thrown or poured from volcanoes, but the most clearly defined is molten rock, or lava.
Lava begins life as underground magma, simply lava containing various gases. When a lava volcano erupts the gases are released and the lava flows down, gradually cooling as it goes. Sometimes magma does not reach the earth’s surface, but cools below ground when the volcanic activity ceases. This cooling will be slow, giving time for large crystals to grow, whereas lava which has reached the surface will cool relatively quickly, allowing time for only small crystals to grow.
All rocks formed by cooling lava or magma or called igneous rocks, from the Latin word for fire. Those which cool slowly are mainly granite, and those cooling quickly are basalt.
There are times when the tremendous heat from volcanic activity can affect surrounding rocks. This will initially soften the rocks as they heat, then as they cool down, crystals will form and the rock will take on a quite different and much harder character.
This complete metamorphosis from one rock to another leads to the name metamorphic rock – the third rock group, including slate made from clay, quartzite made from sandstone, gneiss made from granite, and many more.
In reality there are other much more subtle distinctions between rocks, but all are considered to form one of the three main groups: sedimentary, igneous and metamorphic.