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metamorphic rocks

OLogy Series
geology
card
246

metamorphic rocks

OLogy Series
geology

Deep inside the Earth, intense heat and pressure can change rocks into entirely new metamorphic rocks. In fact, the conditions are so extreme, they can change the rock's texture, color, and the shape of its crystals. Even the rock’s minerals are broken down and new ones are formed. This can happen to any rock—igneous, sedimentary, even other metamorphic rocks. It's all part of the Earth's rock cycle.

Gem Under the Mountains
How did a gray speckled rock turn into the beautiful Gore Mountain Garnet, with minerals that have grown to be the size of a football? Imagine the heat of an erupting volcano or the weight of a mountain. In other words, it takes extreme heat and tremendous pressure. The Gore Mountain Garnet was once a gabbro, a dark igneous rock with tiny crystals. Over time, the Earth's crust slowly moved, and the gabbro was squeezed, crushed, and pushed deep underground. This movement of the crust formed the Adirondack Mountains in northern New York, where the gabbro was buried under Gore Mountain. The intense heat and pressure broke down the minerals in the gabbro to form black hornblende and red garnet. As this was happening, fluids seeped into the rock, which helped the garnet grow especially large. The gabbro's color, texture, and minerals completely changed to form the Gore Mountain Garnet.

The word "metamorphic" comes from the Greek word morphe, which means "form," and meta, meaning:

rock

change

minerals

Are you right?

Correct!

Metamorphic means "to change form"-which is exactly what metamorphic rocks do. The heat and pressure change everything: their texture, color, grain size, and the shapes and even the types of their crystals.

Metamorphic rocks are likely to be found:

at the bottom of rivers

on the beach

deep under the Earth's mountains

Are you right?

Correct!

Metamorphic rocks are likely to be found deep under the Earth's mountains. That is where rocks are crushed, folded, and squeezed by the movement of the Earth's tectonic plates.

Rondi Davies, Earth scientist

Metamorphic rocks are the most beautiful rocks you can imagine. They have swirling layers and large, beautiful minerals in all colors of the rainbow!

Metamorphic rocks can only form from sedimentary and igneous rocks.

Fact
or
Fiction
?

Fiction

Metamorphic rocks can also form from other metamorphic rocks. In the rock cycle, every kind of rock can be turned into another kind of rock.

A metamorphic rock has gone through so much heat and pressure, it is usually weaker than the original rock it formed from.

Fact
or
Fiction
?

Fiction

The heat and pressure actually make the rock stronger. When limestone becomes the metamorphic rock marble, it becomes much more durable.

Definition: rock that changes from heat and pressure
Where it forms: usually deep within the Earth's crust where it is hot and pressures are high
Common examples: marble, slate, schist
Cool fact: The mantle is made entirely of metamorphic rock.

Image credits: courtesy of AMNH; Rondi Davies: courtesy of AMNH.