How Rocks Change Over Time
Understand the rock cycle: rocks slowly change from one type to another over millions of years, igneous rock weathers into sediment, sediment becomes sedimentary rock, heat and pressure create metamorphic rock, and melting starts the cycle again
What a learner can do afterwards
- Describe the rock cycle as a continuous process with no beginning or end
- Trace at least one complete path through the cycle from igneous to sedimentary to metamorphic and back
- Explain that the rock cycle operates over millions of years through weathering, pressure, heat, and melting
1 · Read
Rocks are always changing, just very slowly. Over millions of years, one type of rock can turn into another, and then into another. This never stops, and it has no true beginning and no true end, so scientists call it the rock cycle.
Picture a granite mountain, an igneous rock. Wind and rain slowly break it into tiny grains of sand. The sand piles up in a riverbed and presses together into sandstone, a sedimentary rock. Later, that sandstone gets buried deep underground, where heat and pressure squeeze it into quartzite, a metamorphic rock. If the quartzite ever melts, it becomes magma again, ready to cool into new igneous rock. Limestone takes the same road: heat and pressure turn it into marble.
This whole trip, from igneous rock to sediment to sedimentary rock to metamorphic rock and back to melted rock, usually takes millions of years. You will never see it happen, but it is happening under your feet right now.
Rock never stops changing: it weathers, presses, heats, and melts its way around the rock cycle, forever.
More to read
How to Identify Metamorphic Rocks
Some rocks are changed deep underground, where Earth is very hot and the weight of rock above pushes down hard. When a rock gets squeezed and heated for a very long time, it slowly turns into a new kind of rock called a metamorphic rock. The word metamorphic means changed form.
This change is one step in the rock cycle. For example, a sedimentary rock called shale can be pressed and heated until it becomes slate, a metamorphic rock. Limestone can change into marble the same way. Slate and marble are so strong and pretty that people use them to make floors, roofs, and statues.
You can sometimes spot a metamorphic rock by holding it up to the light. Many metamorphic rocks have a shine or shimmer that igneous and sedimentary rocks do not have. The heat and pressure that made them lined up their tiny crystals in neat layers or bands, and those crystals catch the light.
How to Identify Flint
Flint is a kind of sedimentary rock, which means it formed slowly from tiny bits that settled and were pressed together over a very long time. It is also called chert. Flint is hard, smooth, and often gray, brown, or almost black.
Flint is part of the big story of the rock cycle. Igneous rock on the surface gets worn down by wind, rain, and ice into small pieces of sediment. That sediment settles in layers, and after millions of years the layers get compacted and cemented into sedimentary rocks like flint.
Long ago, people discovered that flint breaks with sharp edges, so they shaped it into knives, arrowheads, and spear tips. Striking flint against steel also makes sparks, so campers still use it to start campfires.
If you want to look for flint, search places where sedimentary rocks lie close to the surface, such as rocky fields, river banks, and gravel beds. In some areas it lies right on the ground, because wind and water have worn away the softer rock around it.
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