Lava to Rock: Clues Written in Stone

An unusual case of science in the kitchen.

After preparing some homemade truffles the other day, I noticed a layering pattern in the texture. The outer layers and inner realm of the truffle had different textures, and it reminded me about rocks cooling in phaneritic or aphanitic patterns in geological rock formations.

In this truffle, we might conclude that the outer layers cooled very quickly and formed a smooth shell. The grains of chocolate are so fine they cannot be seen with the naked eye. As a result, they are aphanitic in texture.

The center of the truffle took longer to cool, so the chocolate hardened in larger grains. This rougher texture is phaneritic.

When rocks cool, they do the same thing. Igneous rock cools faster or slower depending on whether it hardens into rock above ground or below it, and this affects the texture of the rock.

I am thinking in terms of intrinsic and extrinsic rock formations. Igneous rock is either mafic (rich in magnesium and iron) or felsic (rich in feldspar and silicon). Rocks become mafic or felsic according to Bowen’s Reaction Series, which describes mineral formation as a ‘rock melt’ cools.

We can track Bowen’s Reaction Series by starting with magma. There are many kinds of minerals in that molten rock, and they are all somewhere on the reaction series. The first minerals to cool out of the magma are mafic ones, rich in magnesium and iron (Fe). These heavy, fast-moving magmas are not very viscous and so easily slip up to the earth’s surface. Once there, they cool quickly into aphanitic basalts. This kind of rock dominates oceanic crust and the volcanic islands that pepper it around the world.

Felsic minerals are rich in feldspar and silica. This kind of rock dominates continental crust, and is the last sludge of a partial melt in the reaction series to solidify into rock. Because of its highly viscous texture, it moves slowly and often cools underground. It cools more slowly than basalt, so it crystallizes into large-grained, phaneritic granite.

The basalt cools quickly and hardens with a smooth aphanitic texture; the granite cools slowly and gets large chunky grains in a phaneritic texture. Every once in a while, however, the thin, mafic magmas get trapped underground and cool slowly. The result is a phaneritic igneous rock called gabbro. If viscous, felsic magma somehow makes it to the earth’s surface and cools quickly in the open air, you end up with aphanitic rhyolite. Because these conditions are not as common, gabbro and rhyolite are rarer than basalt and granite.

The best part of any geology class is that (1) it always involves awesome field trips and (2) at every rock formation, we get to stop and tell a story based off of these kinds of visual clues. Is the texture phaneritic or aphanitic? If igneous, is it mafic or felsic? These kinds of clues can be read to learn about the geologic past.

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