So the good news is that two weeks ago, I finished the first draft of Project Tech-Bro. I’ve set that aside to marinate, i.e. to clear my head and get a little distance from the novel. In a few weeks, I’ll go back and start Draft 2.

In the meantime, I’ve been getting a ton of working done on Project Moon (see here for a completely uninformative mention of these projects).

The bad news is that with all the writing that I’m charging through, I haven’t had time to write blog entries. I haven’t even had time to think of blog topic ideas. But recently I chatted with someone who had no idea of even the most basic principles of geology. And since I’m a fan of geology (as perhaps revealed in All Those Explosions Were Someone Else’s Fault), I’ve decided to write some posts about the basics of geology.

We’ll start with the picture at the top of this post. It’s a cartoon of what the Earth is like inside. (One of my first geology profs insisted on using the word “cartoon” for such pictures to emphasize that they’re huge oversimplifications. Real geology is messy, messy, messy; the Earth has been around for 4.6 billion years, and in that time, it’s developed all kinds of anomalies and glitches.)

So how do we know what the inside of the Earth looks like? A lot comes from measurements taken around the world after an earthquake occurs. Earthquakes cause four different types of vibrations, which then travel outward as waves. Two of these waves go through the planet, while the other two mostly stay on the surface. Each of the four waves has different properties, including the speed with which they travel and what they will or won’t go through.

For example, so-called secondary waves (S-waves) can’t pass through liquid, but primary waves (P-waves) can. So let’s say there’s an earthquake somewhere. Monitoring stations all over the world detect the quake’s vibrations as they travel outward. Some stations pick up both the P-waves and S-waves, while others only pick up the P-waves. This indicates that the S-waves must have hit a liquid layer inside the earth and couldn’t keep going.

By taking measurements from many earthquakes at many monitoring stations, scientists gradually built up a picture of the layers that make up the inside of the planet. That’s what you see in the picture above.

Next time, I’ll talk about what these layers are and why we might care.

[Picture of earth structure by Kelvinsong [CC BY-SA 3.0 (, from Wikimedia Commons]

One thought on “Geology

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