No, rocks don’t last forever but some have been around for a very long time!
The Earth is believed to be 4.54 billion years old but the rocks that form the crust are generally much younger, for example, rocks that formed at the time the dinosaurs were wiped out are only 66 million years old. In this article, you will find out why only a tiny fraction of rocks are extremely old. It helps if you know about the three types of rock (igneous, sedimentary and metamorphic).
The Rock Cycle
The simple answer to why rocks don’t last forever is the rock cycle. In the rock cycle, one kind of rock changes into a different kind of rock – rocks are constantly being formed, uplifted to the surface and eroded away. Plate tectonics is the force that keeps the rock cycle going, helped also by the weather!
To understand the rock cycle, we need to start somewhere so let’s start with metamorphic rocks. When any of the three types of rock gets pushed down into the Earth’s crust, heat and pressure changes it from what it was originally. Rocks formed in this way are called metamorphic rocks.
If plate tectonics pushes any rocks far enough down into the Earth, they melt and become magma. Magma is a liquid so the original rock no longer exists. The magma rises through cracks in the crust and can solidify. When the magma is solid, it is called an igneous rock.
Sometimes rocks of all three types are slowly pushed towards the surface of the Earth by plate tectonics, forming mountain ranges. The rocks of the mountains are worn down by the weather creating smaller fragments. These fragments of rock are washed to lakes and the sea by streams and rivers where they end up on the sea or lake bed as layers of sediments. After thousands or even millions of years, these sediments turn into new rock.
Round and Round…
So the rocks go round and round the rock cycle, changing from one type of rock into the other types of rock over periods of millions of years.
The oldest rocks of the Earth are about 4 billion years old. They found in very stable areas of our planet that are called ‘cratons’ that have not been affected by plate tectonics for all those billions of years. They are never completely destroyed, they are changed from one type into another. Geologists have found fragments of rocks that are even older, almost as old as the Earth itself – look up zircon crystals on the Internet to find out more!