Geology: The Crust and Upper Mantle

Continuing on with the structure of the Earth, just because I feel like it (and I like rocks).

The Earth’s crust is a patchwork of distinct plates which lie on top of the mantle and move slowly relative to each other. Different plates move in different directions and at somewhat different speeds; however, the usual comparison is that the plates move at roughly the same speed that your fingernails grow.

The plates underneath the ocean are significantly different than the ones underlying the continents. Ocean plates are relatively thin—about seven kilometers thick—and quite young, geologically speaking. The oldest rocks in the oldest ocean plate are only about 200 million years old. The youngest were made two seconds ago; new rock is constantly being added to several of the ocean plates.

The plates underlying the continents are much thicker. The average thickness is about 35-40 kilometers, but some parts of some plates are as much as 70 kilometers thick. Continental plates are usually much older than ocean plates. For example, some rocks in the Canadian shield and in Australia are more than 4000 million years old (i.e. four billion).

Ocean plates are mostly made of basalt, a black rock with a high iron and magnesium content. Continental plates are much more varied, with many different types of rocks and minerals. On average, however, continental plates are less dense than ocean plates. Loosely speaking, this is why continents are (mostly) above sea level: ocean plates are dense and “ride low” on top of the mantle, while continents are lighter and “ride high”.

There’s some weirdness about the dividing line between the crust and the mantle. The line is determined by the Mohorovicic discontinuity, named after a Yugoslavian scientist and generally shortened to “the Moho” because no one wants to type “Mohorovicic” repeatedly. The Moho is a layer where the composition of the rocks changes significantly from the basalt of ocean plates and the more varied continental plates, to a relatively uniform rock called peridotite. This change in composition is quite noticeable in seismological readings, so the Moho was officially taken as the crust-mantle boundary.

However, there’s a different boundary you might also care about. As I noted last time, most of the mantle has the consistency of thick peanut butter: solid but still able to flow very slowly. However, the topmost section of the mantle is cool enough that it doesn’t flow. It’s hard, like peanut brittle.

So the crust is mostly unflowing solid rock, and the top part of the mantle is also unflowing solid rock. This has led geologists to define the lithosphere as the crust plus the part of the mantle that doesn’t flow. Below that is the asthenosphere which is the part of the mantle where stuff starts to flow (and below that is the mesosphere which is a stiffer part of the mantle but it’s still a bit flow-y).

Confused? I certainly was when I was first taught this stuff. But if you think crust/hard mantle/soft mantle, you get the idea. The difference between the crust and mantle is what the rock is made of. The difference between the hard mantle and soft mantle isn’t the composition—they’re made of the same stuff—but the texture.

So those are the basic layers of the Earth. Some of these are divided into sub-layers, but let’s not complicate things. Instead, if I decide to keep going, I’ll move on to plate tectonics, the key to modern geology.

Geology: The Core and Mantle

All right, let’s do this for real: describing the various layers inside the Earth. Remember that these layers are dictated by the combined effects of pressure and heat.

At the Earth’s very center is a solid inner core. The core is very very hot—above 5000 degrees Celsius, about the same temperature as the surface of the sun. That’s so hot that the major elements in the core would be gases on Earth’s surface. So why is the inner core solid? Because the pressure is so high that atoms are squeezed in on each other and can’t flow around as they do in a liquid or gas. The core’s atoms are basically locked into a tight crystal structure even though they’ve got so much heat, they’re jiggling like mad and would jiggle free of the structure if they could.

The inner core is mostly made of iron. That’s because iron is the heaviest element produced during the normal burning of a sun, and our solar system (including Earth) consists of the leftovers from old stars that burned themselves out long ago in this region of the galaxy. These leftovers also included small amounts of elements heavier than iron; such elements can’t be created by a sun’s normal fusion, but they can be created by supernovas smashing smaller atoms together. Earth contains elements heavier than iron, all the way up to uranium, and all such elements were created by supernovas going off in this vicinity long before Earth was born.

Earth was created when leftover star junk started to clump together in a molten mass. At that point, a good percentage of the heaviest elements sank to the middle of the mass. The most abundant of these elements was iron. Other elements sank too—the core probably contains a significant amount of nickel—and it’s not like all the heavy elements sank, because there’s a significant amount of iron and heavy elements in other layers of the Earth. But the core is predominantly iron with some other stuff thrown in.

Surrounding the solid inner core is the liquid outer core. It has a similar composition to the inner core; the major difference is that the pressure is lower (because the outer core isn’t as deep down) so the pressure isn’t high enough to squeeze everything into a solid. Apart from that, the outer core and inner core aren’t very different.

Above the outer core is the mantle. One major difference between the mantle and the core is that the mantle doesn’t have nearly as much iron (since most of the iron sank down to the core when the Earth was molten). The major elements of the mantle are oxygen, silicon and magnesium, which together make up almost 90% of the mantle by weight.

The mantle is solid, but mobile. I like to picture it as very very thick peanut butter: solid, but it has some give. Over long periods of time, the contents of the mantle can and do move. Some very hot bits down near the core slowly rise, in the same way that hot air rises. Cooler bits that start near the top of the mantle slowly sink…so you get very slow circulation and convection currents.

We’ll leave it there for now. Next time, I’ll talk about the crust which sits on top of the mantle, including how the crust and the mantle interact with each other.

Geology: Pressure and Heat

Okay, let’s talk about the different layers in the Earth’s structure.

But first, let’s talk about why layers happen at all.

As you go deeper into the Earth, the pressure increases. Why? Think about it. Here on the surface, what’s weighing down on our heads? About 100 km of air, most of which is in the lowest few kilometers—the air thins out pretty quickly the higher you go. The result is an air pressure of about 1.03 kilograms per square centimeter (14.7 pounds per square inch) at sea level.

Now if we go down, say, a kilometer under our feet, what’s weighing down on our heads? A kilometer of rock. That’s a heck of a lot heavier than air, so any rock down that deep is under a lot of pressure. Go down another kilometer, and now there’s two kilometers of rock weighing down on our heads: roughly double the pressure (if the rock maintains the same density).

So the pressure goes up the farther you go down, i.e. the closer you get to the center of the Earth. At certain levels, this leads to phase shifts, which are changes to the physical properties of matter.

One well-known example of a phase shift is the formation of diamonds. Diamonds are made of carbon atoms locked into a particularly tight framework of chemical bonds. You can only make such a framework by crushing the atoms close together. Normally, carbon atoms don’t like to be really near each other—the nucleus of each atom is surrounded by negatively-charged electrons, so when two atoms start getting close, the electrons on one atom repel the electrons on the other. Under the pressures we’re used to, the repulsion force is stronger than the pressure trying to push the atoms together. The atoms never get close enough to form the framework that a diamond needs.

As you go down into the Earth, however, the pressure increases. Eventually, it’s high enough to squish carbon atoms close enough together, despite the repulsion force trying to keep them apart. At that point, suddenly the atoms can link together in the required framework. The bonds established are strong enough to hold the diamond together even if you reduce the pressure again…which is why diamonds don’t explode if some geological process sends them rising to the surface. (Note however that diamonds aren’t completely stable. They do explode if they take certain types of damage.)

Another type of phase shift involves going from solid to liquid or vice versa. In this case, the cause is loss or gain of heat, but the effect is somewhat similar to the creation of diamonds.

The atoms in a solid have a fixed framework. At any temperature other than absolute zero, the atoms jiggle a bit but they pretty much stay in their position within the framework. However, if you keep adding more heat, the atoms jiggle more and more until they’re finally jumping around too much to stay in position. At that point, the framework breaks down and the solid becomes a liquid.

The temperature of the Earth increases as you go downward, just like the pressure. Why? Because the center of the Earth contains a lot of heat left over from the planet’s creation. The Earth came into existence when the remains from burnt-out stars started to cluster together due to gravity. Chunks of matter clotted together by random chance until they had enough gravity to draw in other nearby chunks. The new chunks added more mass to the whole, which increased the gravity, which dragged in more chunks, etc.

Imagine the early Earth dragging in more and more asteroids from the cosmic neighborhood. Each time a new asteroid collides with the growing planet, it adds mass and a lot of heat. The result was a stage when Earth was completely molten.

Eventually Earth had sucked in all the nearby matter, so it stopped getting a regular bombardment of random stuff. At that point, the surface started to cool, losing its heat to outer space…but the interior of the planet cooled much more slowly, because most of what the Earth is made of doesn’t conduct heat very well. The heat is trapped inside and only leaks out very slowly. Earth still contains a healthy proportion of the heat it acquired from its early components slamming together.

There’s one other source of heat inside Earth: radioactive decay. Radioactive minerals only make up a tiny percentage of the planet’s mass, but they’re constantly pumping out heat as they decay, just like a nuclear reactor. This actually makes a significant contribution to the Earth’s internal temperature.

So why does the inner Earth have layers? Because pressure and heat cause phase shifts that change the nature of how matter behaves. Matter deep down has different properties than matter near the surface, even when atoms of the various elements are present in the same proportions. You have the same stuff, but it acts differently.

And sometimes the stuff doesn’t stay the same. But we’ll talk about that next time.

[Picture of earth structure by Kelvinsong [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D, from Wikimedia Commons]

Geology: Earth is Not Round

So I’ve decided to write a few posts about geology because basically I love rocks. Last time I talked about why we think we know what’s inside the Earth, even though we can’t see what’s down there. In fact, we’ve never come close to direct observation—the world’s deepest drill hole only goes about 12.3 km deep (7.5 miles). Earth itself is 6353 to 6384 km in radius depending on where you measure, so percentage-wise, we’ve still barely scratched the surface.

Aside: The world is roughly spherical, but with numerous irregularities.

First, of course, are all the mountains and valleys, and even the bulge of tides as they travel across the oceans.

Second, the poles are flattened, or rather the rest of the planet bulges because of centrifugal force. The poles are stationary relative to the Earth’s axis, while the equator is spinning quite quickly. This means that there’s a “force” that pushes back against gravity, and reaches its maximum at the equator. (Yes, I know that centrifugal force is fictitious, but you know the comparative effect is real.) So the Earth balloons wider at the equator than the poles, just like a person’s clothes billow outward if the person starts spinning. This effect was predicted by Newton long before anyone had good enough measurements to see that it was true.

Third, there are leftover effects from the most recent Ice Age. 20,000 years ago, a big patch of the planet had several kilometers of ice weighing it down. The effect was similar to what happens when you press your thumb into bread dough: it made a dent. But much of that ice is gone now, so it’s like you’ve removed your thumb…and very very slowly, the surface is rising back to where it was before the ice started pressing down. This is called isostatic rebound; it’s still happening today and it’s easy enough to measure. Since some spots were under more ice than others, the speed of the rebound varies from place to place, and that actually makes a practical difference to our world. For example, the speed of water flow in the Great Lakes depends on height differentials between the lakes, and those heights are gradually changing because of rebound. In future, it may become necessary to build locks between Lake Huron and Lake St. Clair because of changing height differences.

Finally, the Earth’s surface has other dips and bulges caused by irregularities in the planet’s density. Simply by the luck of the draw, some places have denser rocks than other places. This affects the local force of gravity—dense rocks mean a stronger pull toward the center of the Earth, which means the whole region sinks a little, in comparison to places where the rocks are less dense and gravity less strong. The result is a shape called the geoid which is an idealized version of the Earth’s “true” shape if you ignore mountains, tides, etc.

When I started to write this post, I intended to start describing the layers under our feet. However, my “aside” about the shape of the Earth has made the post long enough already, so I’ll stop here. But next time, I promise I’ll talk about Earth’s underlying structures.

[Picture of the geoid by http://en.wikipedia.org/wiki/User:Citynoise [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D, via Wikimedia Commons. Notice, by the way, that this is called the EGM96 geoid. As with most scientific models, the “official” geoid gets updated from time to time as better data becomes available.]

Geology

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 (https://creativecommons.org/licenses/by-sa/3.0)%5D, from Wikimedia Commons]