This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Let's go back to basics for a bit. I've had a challenge thrown in my teeth. Southern Geologist didn't intend it as a challenge, I'm sure, when he* said, "The big picture/history drags people in much more easily than discussing rock types." But I'm a contrary sort of person. And something went ping. I see no reason why we can't have our rock cake** and eat it, too. Besides, understanding the basic rock types is essential for geology. Most of you probably know them already, but what if a physicist or a biologist or one of those other hammer-deprived science types stumbles in here? We don't want them going, "Ig-meta-whowha?" and running away, now, do we?
Besides, I have purty pictures and a snarky sense of humor. That will hopefully be enough to entertain those of you who can recite the three basic rock types in your sleep.
Let us begin in fire, because that is the way the world began***.
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Igneous
Igneous comes from Latin, and means "of fire." An igneous rock is a fire rock. See how easy geology terms are?
If the rock started out melted, it's probably igneous. Just like this beautiful basalt from near Mount St. Helens, which might either belong to her or another volcanic event in the area. It's hard to tell, exactly, because this rock's been river rafting, and I haven't got the facilities in this apartment to delve into its chemical composition in order to determine its precise origin. We know it's igneous, and it's from near Mount St. Helens. That's all that matters for the moment.
Igneous rocks come in more than just your basic basalt flavor. There's a huge variety of magmas, and depending on how and where they cool, you can end up with anything from black to pink or white, fine-grained to chock full of glittering crystals. There's underground (intrusive or plutonic) varieties, and above-ground, extrusive (lava) types. Sometimes it's calm and erupts in lovely streams. Sometimes, it blows a mountain apart.
The ash to the left is pulverized rock from Mount St. Helens. It's a pyroclastic material: "fire-broken." You can see how the finer stuff sailed along a fair distance before falling out of the sky, while the coarser fragments landed sooner. The ash layers from Mount St. Helens' various eruptions make excellent dating tools, used by geologists, archaeologists, and other -ists in the area. Jewelers make lovely gemstones from it.
Mount St. Helens has erupted all sorts of stuff: basalt, andesite, dacite. So when you hold different hand samples from her, you're holding a diverse eruptive history. She's one of the feistier of our Pacific Northwest volcanoes, and we'll be getting to know her quite well soon. I'm doing you up a series for her 32nd anniversary. For now, we'll just use her as our first example as to why hand samples are a gateway to some fairly dramatic landscapes, and as a poster volcano for our igneous rocks.
I should reiterate the point that not all igneous rocks come from big strapping stratovolcanoes. Volcanoes exist in bewildering variety, some of which you wouldn't consider a volcano at all, not in the sense of a pointy mountain with fire coming out of it. And a rock doesn't have to erupt to be igneous. All it needs is a history as magma. If it decides to chill out underground instead of bursting out all over the surface, it still counts.
Igneous rocks are everywhere, forming massive portions of our continents and our ocean floors. They make up some of our most dramatic landscapes. And they're the only rocks outside of meteorites that are likely to get your attention with a bang. Pretty hot stuff, amirite?
We'll be on about sediments next. In the meantime, if you want to explore the wonders of igneous rocks in a bit more detail, I commend you into Andrew Alden's capable hands.
* Southern Geologist describes him/herself as a "fellow," anyway.
** Rock cake is an actual thing in Britain.
*** Elements forged in the Big Bang and the hearts of stars, people. That's firey.