What goes around, comes around.
Rocks are constantly being recycled. To recycle means to take something old and change it into something new. So, some old rocks that have been around for more than four billion years are being changed into different, newer rocks. Of course, that doesn’t happen overnight. It takes millions of years. To better understand how this happens, let’s take a journey through the rock cycle.
We start in the mantle, in a place where the Earth’s crust is quite thin. Red hot magma is being pushed up towards the crust, which breaks open, and we have a volcano. Some of this magma creeps into the cracks of the volcano, cools and forms rocks such as coarse grained granite (right) while, the rest is forced out of the top. When the magma spews out of the volcano, it is called lava. The lava cools and forms igneous rocks such as basalt (left). (Ignis is Latin for ‘fire’)
Some of the igneous rock rolls very slowly down the mountains formed by the volcanoes, helped down by rainwater and eventually ends up in the ocean. As they roll, bits and pieces of the igneous rocks are broken down and form sediments. Layer after layer of sediments are pressed down and cemented together forming sedimentary rocks. This is Jurassic sandstone, a sedimentary rock on its way to the sea, from Utah, in the USA. Look at the layering which indicates a geological event.
Some of the sedimentary rocks on the very bottom get hot because of the pressure. This heat and pressure changes the rock, interacting with water and minerals to form metamorphic rock. When the metamorphic rock is buried deeper, it gets hotter and melts. Once again, it becomes magma and may eventually be pushed up and out of a volcano.
Then, guess what. The whole process starts all over again. Have a look at this animation