Igneous Rocks

The word igneous comes from the Latin word for fire. These rocks start as melted rock and then cool down into solid stone. Basically, they're the planet's lava leftovers.

And here's the cool part: if you live in eastern Idaho, you're standing on them. The Snake River Plain is covered in igneous rock from ancient lava flows and the Yellowstone hotspot.

Same stuff, different address:

• Magma is melted rock underground.
• Lava is melted rock that has reached the surface.

Magma forms deep in the Earth where it's hot enough, roughly 700 °C to 1,300 °C, to melt rock. Since it's lighter than the solid rock around it, magma rises, kind of like a blob in a lava lamp.

Where the rock cools decides what it looks like:

• Intrusive (plutonic) rocks cool slowly, inside the Earth. Slow cooling gives crystals time to grow big. You can see them with your bare eyes. Granite is the classic example. Check out a kitchen countertop.
• Extrusive (volcanic) rocks cool fast, on the surface. Fast cooling means tiny crystals, or none at all. Basalt is the classic example, and it's the dark rock all over the Snake River Plain.

Easy way to remember it: slow = big crystals, fast = small crystals. Like rock candy. Give it time and the crystals get huge.

Texture is the size, shape, and arrangement of the crystals. It tells you the rock's backstory.

• Coarse-grained: big crystals. Cooled slowly underground. (Granite)
• Fine-grained: tiny crystals. Cooled quickly at the surface. (Basalt)
• Glassy: no crystals at all. Cooled so fast the atoms couldn't line up. Obsidian is natural volcanic glass, and Native Americans chipped it into razor-sharp arrowheads.
• Vesicular: full of holes from gas bubbles, like a sponge. Pumice has so many holes it can float on water. A floating rock!
• Porphyritic: big crystals mixed in a fine background. That means it started cooling slowly underground, then got blasted up and finished fast.

Igneous rocks are also sorted by what they're made of:

• Felsic: light-colored, lots of silica. Granite and rhyolite. Felsic magma is thick and sticky, which can cause explosive eruptions. (Yellowstone, we're looking at you.)
• Mafic: dark-colored, rich in iron and magnesium. Basalt and gabbro. Mafic lava is runny and flows easily.
• Intermediate: in between. Andesite and diorite.

• Craters of the Moon National Monument: fresh-looking basalt lava fields, lava tubes, and cinder cones. NASA astronauts trained there in 1969 before going to the Moon.
• The Snake River Plain: basalt layers stacked hundreds of feet deep.
• The Idaho Batholith: a giant chunk of granite in central Idaho, including the Sawtooth Mountains.

Granite countertops and gravestones. Basalt for road gravel. Pumice in exfoliating scrubs and pumice stones for rough feet. Obsidian blades are so sharp that some surgeons have used them.