What Are The 3 Types of Volcanic Rock?

Volcanic rocks are formed from magma (molten rock) that has cooled and solidified. The characteristics of the resulting rock depend on the composition of the magma and the way it cooled. There are 3 main types of volcanic rock:

Igneous Rocks

Igneous rocks are formed when magma cools slowly underground or lava cools slowly on the earth’s surface. As the magma or lava cools, mineral crystals have time to grow, creating rocks with visible crystals interlocked together like granite. Examples of intrusive igneous rocks that cool underground include granite, gabbro, and diorite. Extrusive igneous rocks that cool on the surface include basalt, andesite, obsidian, and pumice.

Intrusive igneous rocks cool slowly underground and have larger crystals while extrusive igneous rocks cool more rapidly on the earth’s surface and have smaller crystals. Obsidian forms when lava cools so rapidly that crystals don’t have time to grow, creating a volcanic glass. Pumice forms from gas-rich lava and has many air pockets from the gas bubbles.

Sedimentary Rocks

Sedimentary rocks are formed from fragments of other rocks and organic material that are compressed and cemented together over time. Common types include sandstone, shale, limestone and conglomerate. Sandstone is composed of compressed grains of sand while shale is made from silt or clay that has hardened. Limestone is formed mainly from calcium carbonate shells and skeletons of marine organisms. Conglomerate contains rounded pebbles and fragments of rock cemented together.

Volcanic sedimentary rocks such as volcanic ash and tuff are formed from the consolidation of volcanic fragments and ash ejected during volcanic eruptions. These particles settle and accumulate, eventually compacting into rock layers. Tuff is composed of smaller ash particles while volcanic breccia contains larger, angular volcanic fragments consolidated together.

Metamorphic Rocks

Metamorphic rocks are formed when existing rocks are exposed to high heat, high pressure, hot mineral-rich fluids or some combination of these conditions inside the earth. This causes their chemistry and/or mineralogy to change, resulting in new metamorphic rocks. Foliated metamorphic rocks have a banded or layered appearance. Common varieties include gneiss, which has a banded texture, and slate, which has very fine layers. Non-foliated metamorphic rocks do not have a layered or banded texture. Common types include marble, which is recrystallized limestone, and quartzite which was originally sandstone.