We can account for many of the differences in the texture of igneous rocks by considering the environment in which they cooled. But the color of these intriguing rocks varies as well. Something else must be going on.
The road cut in the picture below is a famous geological stop on the east side of the Jemez Mountains between San Ildefonso Pueblo and Los Alamos. Below is a sheet of very dark lava. This lava is buried in many feet of light-colored, nearly white pumice, a kind of volcanic ash. There is a striking contrast of color as well as texture here.
The road cut in the picture below is a famous geological stop on the east side of the Jemez Mountains between San Ildefonso Pueblo and Los Alamos. Below is a sheet of very dark lava. This lava is buried in many feet of light-colored, nearly white pumice, a kind of volcanic ash. There is a striking contrast of color as well as texture here.
Igneous rocks are almost entirely composed of silicate minerals. We noted earlier that the two most abundant chemical elements in the Earth's crust are oxygen and silicon. Silicate minerals are chemical combinations of oxygen and silicon with a handful of other metallic elements.
Silicate minerals rich in the metallic elements iron and magnesium are typically dark in color. Geologists, with their knack for making up hybridized words - preferably from obscure languages - use a term compounded out of magnesia and ferrum to describe these dark minerals: mafic. Apparently everyone got tired of writing "ferromagnesian" in their papers.
This term soon applied to the dark-colored volcanic rocks as well.
Silicate minerals lacking iron and magnesium are light in color. These minerals coincidentally contain a higher proportion of silica - silicon and oxygen combined - than the mafic minerals. Volcanic rocks composed of these minerals are called silicic.
So the color of volcanic rocks is a reflection of their mineral content. In the road cut you can see, in literal black and white, the contrast between mafic and silicic volcanic rocks.
Silicate minerals rich in the metallic elements iron and magnesium are typically dark in color. Geologists, with their knack for making up hybridized words - preferably from obscure languages - use a term compounded out of magnesia and ferrum to describe these dark minerals: mafic. Apparently everyone got tired of writing "ferromagnesian" in their papers.
This term soon applied to the dark-colored volcanic rocks as well.
Silicate minerals lacking iron and magnesium are light in color. These minerals coincidentally contain a higher proportion of silica - silicon and oxygen combined - than the mafic minerals. Volcanic rocks composed of these minerals are called silicic.
So the color of volcanic rocks is a reflection of their mineral content. In the road cut you can see, in literal black and white, the contrast between mafic and silicic volcanic rocks.
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