Introduction to Igneous Rocks Intrusive Igneous Rocks Igneous Intrusions/Plutons
Volcanoes and Lava Flows Igneous Rocks Quiz Credits
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!
geology students enjoying a solidified basalt flow, showing classic pahoehoe surface texture, Pisgah Volcano, Mojave Desert