March 26. Granite versus basalt

Today, we take a break from the chronology of earth history to talk a little about rocks types: igneous, derived from molten magma, sedimentary, deposited as tiny broken fragments of other rocks, as sand, silt, or mud, or precipitated chemically, like limestone, and metamorphic rocks, which are the changed forms derived from anything else that’s been subjected to high temperatures or pressures or both.

Let’s expand a little on igneous rocks. The word is from Latin, ignis, meaning fire, and it’s the same root that gives us words like ignition. These rocks come from fire, from volcanoes, but there’s more to it than that.


When magma – molten rock – flows out on the surface of the earth as lava, it cools quickly. “Quickly” is relative, of course – in geological terms that could mean a million years, but when we talk about cooling of rocks, I’d day that generally means anything from hours to years, maybe even hundreds of years. When magma cools quickly, the stuff, the minerals, that are crystallizing from the magma don’t have much time to grow, so they are usually pretty small. Sometimes molten rock can cool so quickly that it solidifies into a glass – obsidian – where the crystals are so tiny that they are invisible. In fact, in natural glass, obsidian, it isn’t really crystalline at all. The stuff just solidified, but didn’t crystallize. Give it a little more time, say months or more, and tiny crystals can grow, like salt forming around a pan of evaporating salty water. They might still be microscopic, but they’re there, as crystals. Give it a long, long time – say, a few hundred thousand years – and the crystals have time to grow to be pretty big, visible to the naked eye, or maybe even a few inches long. How can it take a hundred thousand years for molten rock to solidify? Well, generally, it would have to be pretty well insulated to retain heat that long, so we’re talking about magma that is NOT erupted onto the surface of the earth, where it cools quickly, but magma that’s down inside the earth – maybe miles down, where it cools very very slowly.

Small crystals, quick cooling. Big crystals, long time cooling. The size of the crystals is part of the texture of the rock, one of the most important things in giving it a name and figuring out its story.

granite

The other important thing about igneous rocks is their composition. You can mix up the chemicals in hundreds of different ways, depending on the abundance of the elements in the mix. But only a relatively few common minerals will form from whatever elements are present. So, if you have a lot of silicon in the melt, you’re probably going to get a lot of quartz, silicon dioxide, in the solidified rock. Quartz is the most common mineral in the earth’s crust, but there are rocks that have none. They’d often be rocks that solidified from magma with more iron and magnesium, but don’t visualize a simple one-to-one relationship between silica and iron. There’s way too much variety for that.

But, as a first pass, we can think of two common igneous rock types – granite and basalt. Granite is silica rich, and it’s made of a lot of quartz and feldspar, another silica rich mineral that includes aluminum and other things. Basalt is silica poor, and contains more iron-rich minerals than granite. Granite is coarse-grained, telling us it formed down inside the earth and cooled over a long period of time. Basalt is fine-grained, telling us it cooled relatively quickly.

basalt

But you can have the same composition as granite that cools quickly, such as in lava flows. That rock, fine-grained but silica rich like granite, is called rhyolite. And likewise, you can have magma of basaltic composition that cools over a long time within the earth. We call that rock gabbro.

The two other terms I’m sure I’ll use in these presentations are volcanic – you know what that means – and plutonic, which means the rock formed inside the earth, in the realm of Pluto, so it took a long time to cool. Both granite and gabbro are plutonic, usually.

I think that’s enough jargon for today! Just remember that molten rock has various compositions, and cools to either fine-grained or coarse-grained rocks depending on how long it took to solidify.

—Richard I. Gibson

Granite photo by Friman, via Wikipedia, under GNU free documentation license.

Basalt photo by USGS (public domain)