Their Campsite, Our Core

Picture a campsite-one of the standard-issue kinds replicated in national forests and parks around the country. In a lot of ways, it's the quintessential American image: the kind of image you'd find on a postage stamp or in an ad for a new made-in-the-USA truck or maybe even in your favorite Brady Bunch episode. In so many ways, these sites represent our collective values: After all, these campgrounds exist because of good national policy, good government-real bedrock stuff.

At the particular site I have in mind, there's a little gravel pull-off for a vehicle and a bare pad just large enough for a four-person tent. Off to the side, a picnic table cozies up to a well-used fire ring lined with blackened stones. Maybe you've slept at sites like this. Perhaps you and your kids have roasted marshmallows in that fire ring, feeling the chipped paint of the picnic table on bare legs as you stretched toasting sticks into the embers, liking the way the soles of your shoes heated up on the rocks.

Even if you haven't actually done this, I bet you can still imagine it. Why? Because it is a deeply American scene: "Go West, young man" meets wholesome family togetherness, complete with hot dogs and Jiffy Pop.

Now imagine the force required to cleave that land-to strike a fissure between that picnic table and fire ring. One moment, the two objects sit side by side, just a few feet separating them. The next moment, the fire ring and everything to the east of it drops twenty feet. Your toes are no longer touching warm stones. They're dangling off a cliff.

This scenario sounds like the sort of thing that can only happen in Hollywood. But sixty years ago, that's precisely what happened at the Cabin Creek Campground, just outside Yellowstone National Park. On August 17, 1959, a magnitude 7.5 earthquake ruptured with an epicenter just a few miles from there. The sheer force of that rupture caused the land to split and half of the site to plummet in what geologists call a scarp, or a sharp line of cliffs caused by seismic activity. As terrifying as that would have been for anyone there, it was the least of the damage to the area: Just across the river, the nation's biggest recorded rock slide-over 73 million metric tons of debris-roared down a canyon wall, burying nineteen people. They were never recovered. At least nine other victims died of their injuries. Today, a massive lake rests where their campsites once stood. Nearby, you can find geysers and fissures and sinkholes where there were none-all because of this single earthquake.

You can be forgiven for thinking that the ground beneath your feet is solid. For most of us, it certainly appears as much. When I was seven, I ran away with my motherÕs formal silverware, wrongly thinking there was a utensil shortage in China. Because I knew that the shortest distance between two points was a straight line, I also took my dadÕs shovel, figuring I could dig my way there in a couple of days. I lasted about an hour before I hit the dense clay that makes up so much of the Mississippi River corridor. To my scrawny arms, it was impenetrable. I assumed everything below it was equally unyielding. And so I returned home, defeated, before anyone had even noticed that the silver and I were missing.

If basic earth science had been covered in my first grade curriculum, I'd like to think I wouldn't have bothered with the shovel. Maybe you remember that iconic drawing of the planet cut into a cross section that's often used in geology textbooks. I always think it looks like a peach: thin outer skin, flesh that's not quite solid or liquid, a tidy pit. That outer layer, insofar as the planet is concerned, is called the crust. This is a misleading term. It's actually made up of a bunch of jagged pieces known as plates. The current best guess is that, right now, there are fifteen big ones and a handful of smaller ones floating around. The thicker and less dense ones are known as continental plates. The heavier ones are oceanic. We'll talk more about each in the next chapter.

What's important to know now is that these plates, which are forever in motion themselves, also bear the scars of billions of years of upheaval and trauma on this planet. For instance, beginning near Lake Superior, there's a 1000-mile forked crack down the center of this country known as the Midcontinent Rift. One tine of this crack snakes down to Oklahoma. The other works its way clear to Alabama. This rift exists because, about a billion years ago, the continent began to break apart. Scientists aren't sure why the rift began to form, though they think it might have been because of volcanic activity below it. Even more puzzling to them is why we're still in one piece (a similar rift threading through East Africa appears to be succeeding in ripping that continent in two).

Other marks are no less monumental. The Appalachian Mountains, the oldest on the planet, are actually scabs from a head-on collision of two plates. They are composed largely of rock that once made up the seafloor-and they were once taller than the Himalayas. This kind of dramatic shifting happens on our planet all the time. While I was writing this chapter, a new island, about a half mile wide, popped up just south of Fiji, thanks to an active volcano there. Another one appeared off the coast of Japan in 2013. Meanwhile, a new plate appears to be forming below the Indian Ocean, perhaps birthed by the 2012 Sumatran earthquakes.

While all this activity is visible on the crust, most of it is actually caused by what lies just underneath: the mantle, which is a combination of solid and liquid rock. The mantle makes up most of our planet's volume, and there's a huge variation in its temperature from top to bottom. Up near the crust, it's a cool 1800¡F. As you plunge deeper, it reaches a temperature of almost 7000¡F. (Silver, incidentally melts at 1763¡F, which is just one reason why my chosen path to China was a bad one-at least where the longevity of my mother's salad forks was concerned.)

This difference in temperature between the outer and inner mantle creates a dynamic heat exchange as hotter rock and magma rise to the surface and cooler rock falls back down. Want to see this in practice? Think of a lava lamp. Or, if that's too groovy for you, dump a can of minestrone into a pot and watch it boil on the stove. There's a certain rhythm to the rotation of carrots and macaroni as they are pushed to the surface and then back down again to the bottom of the pot. It's mesmerizing-at least until you remember that we're floating on top of a very similar process.

Soup eaters or not, seismologists love the mantle. It's where everything happens in one big, dynamic mess. Parts of the mantle are solid. Some of it is plastic or even viscous. Its movement is responsible for the drifting and colliding of our plates. Earthquakes occur there.

Below the mantle lies the core, which is probably the least interesting layer for any book about seismicity. First discovered in 1936, it's also the least understood. What is known is that the core is made up mostly of nickel and iron and is divided into two parts-the outer, which is molten, and the inner, which is solid but only because it is under immense pressure. It is also the very hottest part of our planet-scientists think it's temperature is probably between 9,000 and 13,000¡F (by comparison, the sun's chromosphere-the deepest layer we can currently observe-ranges between 6,700 and 11,000¡F).

To reach the middle of the earth's core, you'd have to travel down about 3400 miles. Then you'd have to go back up about another 3400 miles if you wanted to pop out on the other side of the planet. (It's about 6500 miles from Davenport, Iowa, to Beijing overland, which is yet one more reason my silverware reallocation plan was a lousy one.)

If you've been counting, I've already used the word "about" eight times in this chapter alone. It's a word you hear even more frequently among scientists who study the inner workings of our planet. In the past decade or so, they've come up with some sophisticated tools to help them visualize what's going on below us, including 3-D imaging techniques. Nevertheless, saying this instrumentation gives us any kind of definitive knowledge about what goes on inside the earth is a lot like saying you've mastered the inner workings of a human body because you've seen an X-ray or CAT scan. You might have a decent idea of what's in there, but you're missing a lot of nuance.

Where geology is concerned, these gaps are not for lack of trying.

In 1958, a group of American scientists attempted to drill into the place where the crust and mantle meet, which also, incidentally, is the place where most of our seismic activity occurs. Geologists call this boundary the Mohorovicic discontinuity, so named for Andrija Mohorovicic, the Croatian seismologist who discovered it in the early 1900s. (Pro tip: If you want to look like you know what you're doing at a seismology conference, refer to this space as "the Moho.") Like the crust, the Moho is far from uniform. On average, it tends to sit about five miles...