Photo: Surf crashes onto the  Giant's Causeway rocks.

Photograph by Jim Richardson

Rocks are so common that most of us take them for granted—cursing when we hit them with the garden hoe or taking advantage of them to drive in tent pegs on summer camping trips.

But what exactly is a rock?

To geologists, a rock is a natural substance composed of solid crystals of different minerals that have been fused together into a solid lump. The minerals may or may not have been formed at the same time. What matters is that natural processes glued them all together.

There are three basic types of rock: igneous, sedimentary, and metamorphic.

Extremely common in the Earth's crust, igneous rocks are volcanic and form from molten material. They include not only lava spewed from volcanoes, but also rocks like granite, which are formed by magma that solidifies far underground.

Typically, granite makes up large parts of all the continents. The seafloor is formed of a dark lava called basalt, the most common volcanic rock. Basalt is also found in volcanic lava flows, such as those in Hawaii, Iceland, and large parts of the U.S. Northwest.

Granite rocks can be very old. Some granite, in Australia, is believed to be more than four billion years old, although when rocks get that old, they've been altered enough by geological forces that it's hard to classify them.

Sedimentary rocks are formed from eroded fragments of other rocks or even from the remains of plants or animals. The fragments accumulate in low-lying areas—lakes, oceans, and deserts—and then are compressed back into rock by the weight of overlying materials. Sandstone is formed from sand, mudstone from mud, and limestone from seashells, diatoms, or bonelike minerals precipitating out of calcium-rich water.

Fossils are most frequently found in sedimentary rock, which comes in layers, called strata.

Metamorphic rocks are sedimentary or igneous rocks that have been transformed by pressure, heat, or the intrusion of fluids. The heat may come from nearby magma or hot water intruding via hot springs. It can also come from subduction, when tectonic forces draw rocks deep beneath the Earth's surface.

Marble is metamorphosed limestone, quartzite is metamorphosed sandstone, and gneiss, another common metamorphic rock, sometimes begins as granite.

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