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.]