How rocks transform from one type to another over millions of years in an endless cycle. From our 6th-grade earth-science deep-dive year — a full 10-question lesson, exactly what your child would see.
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How rocks transform from one type to another over millions of years in an endless cycle. The granite of a mountain can become the sand of a beach, harden into sandstone, get squeezed into a new metamorphic rock, melt into magma, and cool back into granite — and there is no “final” form. Earth has been recycling its own crust this way for roughly 4 billion years, powered by heat from within and the Sun and gravity from above.
You already know how to identify minerals by their properties, and you know the three great rock families: igneous rock that cools from molten material, sedimentary rock built from pressed-together layers of sediment, and metamorphic rock changed by heat and pressure. It would be easy to think of those families as fixed labels — that a granite is granite forever. They are not.
The rock cycle is the never-ending set of processes that transform rock from one family into another. Given enough time, any rock can become any other. A sedimentary sandstone can be buried, heated, and squeezed into a metamorphic quartzite; that quartzite can melt into magma and cool into igneous granite; that granite can be worn down into sand and pressed back into sandstone. The cycle has no starting point and no ending point.
Two great engines drive it. Earth’s internal heat — left over from the planet’s formation and the decay of radioactive elements deep in the core and mantle — melts rock and lifts it upward. The Sun and gravity drive weather, water, and erosion at the surface, tearing rock back down. Heat from below builds; the Sun and gravity from above break apart. Between them, the rock you stand on is always on its way to becoming something else.
In the minerals lesson you learned to identify minerals by hardness, luster, streak, and cleavage. In the three-rock-families lesson you learned how igneous, sedimentary, and metamorphic rocks form. This lesson connects them with one big idea: the families are not permanent. The minerals locked inside a rock and the family it belongs to are temporary — the rock cycle keeps converting them into one another, again and again, across deep time.