Layer cakes of rock
The upper part of the earth’s rocky crust is like a layer cake. It is
made up of many layers of different kinds of rock, one on top of the
other. These layers were built up slowly, one after the other. Here is
how this might have happened in one place.
At first, there was only the top of the bare rock crust. Volcanoes
erupted, pouring out ash and lava that hardened into a layer of black
rock called basalt.
Earthquakes caused this part of the land to sink. Ocean water flowed in
to form a smaller body of water called a sea. Rivers dumped tons of sand
into the sea for thousands of years. The sand sank down and formed a
thick layer on top of the basalt.
Snails, clams, and other little shelled animals moved in from the ocean
to live in the sea. When they died, their bodies sank down onto the
sand. Their soft parts rotted away and only the shells were left. For
millions of years, these shells piled up until they formed a thick
layer.
The weight of the shells and the water squeezed the sand together until
it became a layer of the rock called sandstone. Squeezed together by
their own weight and the weight of the water, the shells were crushed
into powder. Minerals in the water glued the powder together. After
millions of years, the powdered shells were a layer of the kind of rock
called limestone. The layer of limestone was on top of the layer of
sandstone, which was on top of the layer of basalt.
This kind of building up of layers of rock is still going on. The layer
of mud that now lies at the bottom of a sea will some day be a layer of
rock!
