For more than two centuries, geologists have advanced their science by treating the Earth as a recycling machine. One way of presenting that to students is a concept called the rock cycle, usually boiled down into a diagram. There are hundreds of variations on this diagram, many with errors in them and distracting pictures on them. Try this one instead.
Rock Cycle Diagram
Rocks are broadly classified into three groups: igneous, sedimentary and metamorphic, and the simplest diagram of the “rock cycle” puts these three groups in a circle with arrows pointing from “igneous” to “sedimentary,” from “sedimentary” to “metamorphic,” and from “metamorphic” to “igneous” again. There is some sort of truth there: for the most part, igneous rocks break down at the Earth’s surface to sediment, which in turn becomes sedimentary rocks. And for the most part, the return path from sedimentary rocks back to igneous rocks goes through metamorphic rocks.
But that’s too simple. First, the diagram needs more arrows. Igneous rock can be metamorphosed directly into metamorphic rock, and metamorphic rock can turn directly to sediment. Some diagrams simply draw arrows between each pair, both around the circle and across it. Beware of that! Sedimentary rocks cannot melt directly into magma without being metamorphosed along the way. (The minor exceptions include shock melting from cosmic impacts, melting by lightning strikes to produce fulgurites, and friction melting to produce pseudotachylites.) So a fully symmetrical “rock cycle” that connects all three rock types equally is false.
Second, a rock belonging to any of the three rock types can stay where it is and not move around the cycle at all for a long time. Sedimentary rocks can be recycled through sediment again and again. Metamorphic rocks can go up and down in metamorphic grade as they are buried and exposed, without either melting or breaking down into sediment. Igneous rocks sitting deep in the crust can be remelted by new influxes of magma. In fact those are some of the most interesting stories that rocks can tell.
And third, rocks aren’t the only important parts of the cycle, such as the intermediate materials in the rock cycle already mentioned—magma and sediment. And to fit such a diagram into a circle, some of the arrows have to be longer than the others. But the arrows are just as important as the rocks, and the diagram labels each one with the process that it represents.
The Rock Cycle Isn’t Circular
Notice that all these changes have left out the essence of a cycle, because there is no overall direction to the circle. With time and tectonics, the material of Earth’s surface moves back and forth in no particular pattern. The diagram is no longer a circle, nor is it limited to rocks. Therefore the “rock cycle” is poorly named, but it’s the one we’re all taught.
Notice another thing about this diagram: Each of the five materials of the rock cycle is defined by the one process that makes it. Melting makes magma. Solidification makes igneous rock. Erosion makes sediment. Lithification makes sedimentary rock. Metamorphism makes metamorphic rock. But most of these materials can be destroyed in more than one way. All three rock types can be eroded and metamorphosed. Igneous and metamorphic rocks can also be melted. Magma can only solidify, and sediment can only lithify.
One way to see this diagram is that rocks are way stations in the flow of material between sediments and magma, between burial and upheaval. What we really have is a schematic of the material cycle of plate tectonics. If you understand the conceptual framework of this diagram, you can translate it into the parts and processes of plate tectonics and bring that great theory to life inside your own head.