Metamorphic Rock Examples

Most people probably don't think of rocks undergoing changes or growing, but one of the three main categories of rocks explains that phenomenon. Called metamorphic rocks, this type of rock occurs when existing rocks are subjected to extreme heat, intense pressure, or in some cases, both. Metamorphic rocks are those that started out as either sedimentary, igneous, or as other, older metamorphic rocks, then underwent a chemical or physical change due to the conditions around them.

How hot does it have to get? Usually, metamorphic rock arises from temperature conditions of higher and 200 degrees Celsius. In order for pressure to affect rock, it usually requires at least 1,500 bars of pressure.

Metamorphic rock is usually formed from being deep below the surface of the Earth where temperatures are at their hottest, or by being a part of a tectonic event like an earthquake whose heat and friction cause the rock to change. Magma can also have an impact on metamorphic rock due to its high temperature.

The Earth's crust contains a lot of metamorphic rock, and these rocks are actually very important to geologists who study the Earth's layers. They're categorized by texture, chemical composition, and by the minerals that are present, and they can give geologists a clear picture of the different cataclysmic events that led to their formation. Due to erosion and a process called uplift, metamorphic rock can be exposed and allow scientists to determine how the rocks were formed and what was happening on the Earth at that time.

1. Quartzite

Quartzite is formed when grains of quartz sand melt together under high heat and extreme pressure. While most quartzite is white or gray, if the sand contained iron oxides, then the quartzite formed can be a nice shade of soft pink or rose. The resulting rock is very hard and very uniform in its texture.

2. Marble

Marble is a metamorphic rock that is formed when limestone or dolomite is exposed to the right heat and pressure conditions. While limestone is mostly calcium based, dolomite is rich in magnesium. Marble has a uniform texture when it forms, and is sought after as a building material for its strength, its beautiful striations and colors, and its sheen when polished. Interestingly, marble will produce a fizz when it comes in contact with acid, such as household vinegar.

3. Shale

Shale is the parent rock, or protolith, that can produce several different types of metamorphic rock, depending on the heat and pressure conditions it is exposed to. These types include slate, schist, phyllite, or gneiss, and of these four, slate is the least metamorphosed form. That means slate has a smooth texture and a dark gray, uniform color; it tends to split rather easily on a parallel grain to produce fairly uniform pieces, making it well-suited for building materials since it can be easily cut to fit.