Metamorphic rock, just its very existence at Earth’s surface, signifies great lengths of geologic time –on the order of millions of years.
Consider this rock, high in the Teton Range of Wyoming.
This is a metamorphic rock called gneiss –in a lot of ways, it’s like granite, because it contains a lot of the same minerals –but gneiss forms because an older rock (in this case, probably a granite) was heated to high enough temperatures that its minerals recrystallized into new minerals. And most metamorphism also involves high pressures, so all the new crystals form in a particular arrangement (as opposed to granite, in which the crystals are randomly arranged) –that’s how the layering (called “foliation”) forms in metamorphic rocks: the recyrstallization of new minerals under pressure.
But the key thing here, is that metamorphic rocks form WITHIN the Earth, at depth –and just like granite, require uplift and erosion to get to the surface. This gneiss formed at depths of 10 km or more and was then uplifted to its present elevation, nearly 4 km above sea level. –which requires time.
click here to see more photos of metamorphic rocks
click here to see a geologic map of Grand Teton National Park, Wyoming.