Igneous rocks (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies.
Intrusive Igneous Rocks:
Intrusive, or plutonic, igneous rock forms when magma is trapped deep inside the Earth. Great globs of molten rock rise toward the surface. Some of the magma may feed volcanoes on the Earth's surface, but most remains trapped below, where it cools very slowly over many thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow, so they grow to a relatively large size. Intrusive rocks have a coarse grained texture.
Extrusive Igneous Rocks:
Extrusive, or volcanic, igneous rock is produced when magma exits and cools above (or very near) the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture. Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture.
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- FAQ
How did Half Dome, the massive rock monument in Yosemite National Park, acquire its unique shape?
At the head of the valley in Yosemite National Park - as if on a pedestal - stands Half Dome. It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8,800 feet (2,682 meters) above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized thousands of feet beneath the Earth's surface. The...
Is glacier ice a type of rock?
Glacier ice, like limestone (for example), is a type of rock. Glacier ice is actually a mono-mineralic rock (a rock made of only one mineral, like limestone which is composed of the mineral calcite). The mineral ice is the crystalline form of water (H2O). Most glacier ice forms through the metamorphism of tens of thousands of individual snowflakes into crystals of glacier ice. Each snow flake is a...
What are metamorphic rocks?
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates...
What are sedimentary rocks?
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They form from deposits that accumulate on the Earth's surface. Sedimentary rocks often have distinctive layering or bedding. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock. Common Sedimentary Rocks: Common sedimentary rocks include sandstone...
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