Home >> Science & Nature >> Rocks & Minerals

Rocks that Form from Lava

Some of the oldest rocks found are called igneous, taking their name from the Greek word for fire. Rocks formed from lava also compose 95 percent of the earth's surface. In its liquid form, molten rock possesses less density than rock and thus moves upward from beneath the earth's crust and crystallizes as it cools. From the eroded black beaches of Hawaii to the pumice stones found on store shelves, igneous rocks are commonly found throughout daily living.

Basalt
- The most common igneous rock, basalt takes its name from the basaltic lava it forms from. The high levels of iron and magnesium give basaltic lava its dark color and density, according to the Utah State Office of Education. As an extrusive igneous rock, basalt forms from a lava flow that has reached the earth's surface. The air causes rapid cooling, which sometimes creates small crystals.
Granite
- Coarse grains of granite are visible to the eye
  
  An intrusive igneous rock, granite hardens beneath the earth's surface from magma or underground molten rock and contains large crystals. High levels of silicon and oxygen make up granitic magma. In addition, the minerals quartz, feldspar, mica and hornblende compose granite. Granite's ability to hold thousands of pounds of pressure makes it a prime construction material, particularly for bridges and buildings. In addition, it works well for monuments and engraving since it weathers slowly.
Gabbro
- An intrusive igneous rock like granite, gabbro encompasses a plethora of minerals like hornblende, feldspar, mica and magnetite. Chemically equal to basalt, according to the Cascades Volcano Observatory, gabbro shares that rock's dark complexion, generally ranging from black to dark gray to greenish gray. Due to its slow cooling process, gabbro also features coarse visible grains.
Pumice
- Pumice makes for a good abrasive cleaner
  
  An extrusive igneous rock, pumice forms from granitic lava. Silicon and oxygen compose this light-colored lava with less density than basaltic lava. While pumice cools quickly, a serious of gaseous eruptions create the pockmarked surface that characterize this igneous rock. So lightweight, this rock can float on water.
Obsidian
- Another extrusive igneous rock forming from granitic lava, obsidian cools very rapidly. Called volcanic glass, its smooth, glasslike surface appears black to the eye due to its density and lack of crystals. However, thin slivers of obsidian, like Apache's tears from Arizona, actually look nearly transparent, writes the Utah Sate Office of Education. In the ancient world, people often used obsidian to make arrowheads, knives and other superior cutting tools.

Rocks & Minerals