When most people think of rocks, they probably think of them as fairly consistent over time. While it’s true that the composition of rocks doesn’t change all at once, they do transform over time thanks to natural forces. The process of rocks gradually changing forms is called the rock cycle. The rock cycle has been taking place since the beginning of time, like the water cycle and other natural cycles that define the world around us. Thanks to the rock cycle, rocks are always undergoing change into different forms. However, these changes often take place in a time frame of centuries or longer, so they are difficult to observe directly.
- What is the Rock Cycle?
- The Rock Cycle Explained for Kids
- Rocks and the Rock Cycle
- Interactive Diagram of the Rock Cycle
- Rock Cycle Activity for Students
There are three major types of rock, and each one represents a different stage in the rock cycle. Understanding the different rock types and the nature of their formation gives you a good idea of how the rock cycle works. Igneous rock is also known informally as “lava rock,” formed through the gradual solidification of lava or magma from a volcano. This type of rock cools and hardens after a volcanic eruption, becoming exposed on Earth’s surface. Once it reaches the surface, it is subject to other forces, such as erosion from wind and rain. Gradually, these forces act to further change the properties of the rock, though it might take eons.
- Identifying Igneous Rocks
- Ten Common Igneous Rocks
- The Rock Cycle and the Rock Types
- All About Igneous Rocks (PDF)
- The Cycles of the Earth
Sedimentary rocks are rocks that are gradually built up from smaller deposits of material, hence the name, referring to sediment. Sedimentary rocks tend to have a stratified appearance that allows you to see the gradual accumulation of layers. One of the most common sedimentary rocks is sandstone, built up of sand as it is slowly deposited on a beach. Sandstone can contain relatively large amounts of quartz, feldspar, and other elements that are very common in Earth’s crust. No matter how simple a piece of sedimentary rock might look, it is a truly diverse collection of materials that might have traveled thousands of miles in the ocean or in rivers.
- Detrital Sedimentary Rocks Defined
- Identifying Sedimentary Rocks
- Types of Rocks Compared for Young People
- Rock Cycle and Rock Types at Windows to the Universe
- Rock Cycle Experiment for Classrooms
Igneous rocks and sedimentary rocks are subject to a number of pressures, but no other kind of rock faces the intense pressures of the metamorphic rock. Metamorphic rocks are formed by some of the most powerful forces at play beneath Earth’s crust. As higher rock layers are moved in tectonic action, metamorphic rock can be formed below them. Metamorphic rock can also be created by the movement of ultra-hot molten magma beneath the surface of the Earth, before it erupts through a volcano. Powerful distorting forces and friction can give metamorphic rock a variety of forms, colors, and characteristics. Slate and marble are both metamorphic rocks.
- Detailed Discussion of Metamorphic Rocks and Geology
- Metamorphic Rock Entry at Encyclopedia Britannica
- Metamorphic Rocks and the Rock Cycle (PDF)
- The Tectonic Rock Cycle
- Rock Cycle Lesson from the Royal Society of Chemistry
How can we understand all of these different forces in terms of a single rock cycle? It is actually easier than it might seem. Although we can look at any point in the cycle as its “starting point,” it is often simplest to think of igneous rock first. Igneous rock is formed by volcanic action, but once it lies on the surface, it becomes subject to the forces that create sedimentary rocks – mainly erosion by wind and rain that can reshape it into a sedimentary form. Sedimentary rocks, in turn, might be subject to intense heat and become igneous rocks or might be transformed by intense pressure into metamorphic rocks. At each point in the cycle, transformation is possible!
- What is the Rock Cycle?
- The Rock Cycle and Associated Terms (PDF)
- James Hutton and the Discovery of Rock Cycling
- Geological Fundamentals and the Rock Cycle
- “Rocking the Rock Cycle” Activity for Teachers (PDF)
No matter what kind of rock you find, it has within it the chemical and energetic potential to be made into a completely different kind of rock. The rock cycle isn’t a one-way process but is instead something that can affect different rocks at different times in multiple ways. Over the course of millennia, rocks can undergo many different “lives.” Under the right conditions, with the appropriate passage of time, one type of rock can become any other type. That’s the long and the short of what we call the rock cycle – a cycle that will go on for as long as Earth exists.