Igneous Rocks Tour

Introduction to Igneous Rocks    Intrusive Igneous Rocks    Igneous Intrusions/Plutons

Volcanoes and Lava Flows    Igneous Rocks Quiz    Credits



Volcanic or extrusive igneous rocks form when molten elements erupt from Earth's interior through a volcanic vent or fissure and cool rapidly at the surface. Such rapid cooling of lava, molten elements at Earth's surface, generally does not allow mineral crystals to grow large enough to be seen with the unaided eye, so extrusive rocks are usually fine-grained or aphanitic in their texture.    Shown below are the three basic types of volcanic rocks.

Rhyolite is a felsic, volcanic rock that usually forms from hot ash and larger pyroclastic particles ejected during a violent volcanic eruption.   If hot enough, the ash and other fragments weld together forming volcanic tuff (mostly ash-sized particles)  or volcanic breccia (combination of small and large particles).   Rhyolite has a high component of silicon dioxide, so it is felsic in composition.

hand samples of rhyolite tuff and breccia

rhyolite outcrop showing pyroclastic nature of welded ash and larger particles

rhyolite outcrops of Bandelier National Monument were carved into cliff dwellings by native Americans

Nancy Darigo climbs on the face of an exposure of rhyolite in Colorado


Andesite is an intermediate-composition volcanic rock associated with both explosive and fluid eruptions from stratovolcanoes like Mt. St. Helens (1980) or Mt Pinatubo (1991).   Andesite is often porphyritic in texture, reflecting the initial slow cooling of intermediate magma that began underground, forming large mineral crystals, and then rapid cooling that resulted from the eruption of the partially cooled magma/lava, which formed tiny mineral crystals.

hand sample of porphyritic andesite

another example of porphyritic andesite

hand sample of andesite, with characteristic gray coloration

Diane holding a chunk of andesite from Mt. Shasta, northern California


Basalt is a mafic-composition volcanic rock usually related to fluid eruptions of very hot lava.   Since basalt is composed of dark-colored mineral crystals, it is black in color unless it has experienced a lot of chemical weathering, which can turn basalt orange to red in color due to oxidation of iron-rich minerals.   The Hawaiian Islands and much of the upper oceanic crust are composed of basalt, by far the most abundant volcanic rock on Earth!

hand sample of basalt

geology students enjoying a solidified basalt flow, showing classic pahoehoe surface texture, Pisgah Volcano, Mojave Desert

a basaltic lava flow in the Mojave Desert; Jeff Brenner provides scale

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