Metamorphic Rocks for Middle School Science

Feb 20

2 min read

Metamorphic rocks are formed when existing rocks are changed by heat, pressure, or both deep inside the Earth. This process does not melt the rock but changes its structure and minerals over a long time. For example, limestone can turn into marble, and shale can become slate. These rocks are often stronger than the original rocks and can have shiny layers or wavy patterns. Metamorphic rocks are used in buildings, sculptures, and countertops because they are tough and beautiful.

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Metamorphic Rocks

Metamorphic rocks are the densest rocks on Earth. They form deep underground within the Earth's crust, where intense heat and pressure transform sedimentary and igneous rocks into metamorphic rocks. Existing metamorphic rocks can also change under heat and pressure.

The temperature of Earth's crust increases as it gets closer to the mantle. The mantle is heated by the outer core. Rock heated by the high temperatures of the core below the mantle slowly rises to the top of the mantle. While the inner core is a blazing 7000° Celsius, the mantle is only 900° Celsius. Of course, 900° Celsius is hot enough to melt the rocks of the crust that come near the mantle.

Pressure also increases as depth increases. Temperature and pressure are also high where tectonic plates meet. As a result, metamorphic rocks form both deep underground and at tectonic plate boundaries.

The heat and pressure where metamorphic rocks form squeeze minerals closer together, transforming small crystals into larger crystals. Heat and pressure can also change the chemical composition of minerals. For example, the sedimentary rock shale is full of tiny quartz crystals. As the rock metamorphoses, the quartz crystals become mica or garnet.

When the pressure forming metamorphic rock all comes from the same direction, foliated metamorphic rocks form. Because the pressure was all pushing in the same direction, the minerals in foliated metamorphic rocks line up in layers or bands perpendicularly to the direction of the pressure. For example, under direct pressure, granite becomes gneiss, a foliated metamorphic rock with bands of the minerals quartz, feldspar, and mica.

Non-foliated metamorphic rocks form when pressure comes from every direction and is not as intense as the uniform pressure that forms foliated rocks. For example, the sedimentary rock, limestone, becomes the non-foliated metamorphic rock marble.

If metamorphic rock is pushed too close to the mantle, it will melt and become magma. When the magma cools, it will form igneous rock. If metamorphic rock reaches the surface of the Earth, wind, water, plants, and animals will weather the rock into smaller pieces called sediments. Over time, these sediments will collect in layers and become sedimentary rock. Under intense heat and pressure, the igneous and sedimentary rocks will again become metamorphic rock. The rock cycle explains how rocks change form as they move through the environment.

Metamorphic Rocks Videos

I like to use videos to teach new concepts because students can use visual cues to support their learning. Here are some of my favorite videos on metamorphic rocks.