Being fully cognizant of the futility of the exercise, I think it is nonetheless instructive to occasionally deconstruct a creationist theory, if for no other reason than to serve as an example for people who don't have an extensive scientific background about how science really works. I'll take as my example for today from a comment on my recent post about how earthquakes and the Hawaiian Islands should be a thorn in every creationist's side (or maybe a spear would be a more apt metaphor).
C. David Parsons disputes the Hawaiian islands chain as evidence for an old earth in part on the grounds that "The gravitational tug of the moon is ... responsible for earthquakes" and "the gravitational attraction of the moon is the mechanism that facilitates the expansion and forced invasion of pressure solids through a geodic crack in the earth's crust." Does this claim stand up to scrutiny? Well, no, it doesn't.
Unfortunately for Parsons, we have an exceptionally good scientific understanding of gravity. Newton's theory is now over four hundred years old. During that time it has only been revised once, and that was over 100 years ago. If these theories were wrong, space flight (and GPS) would not be possible.
It is an elementary exercise to work out the magnitude of the moon's gravitational influence on the earth using Newton's formula:
F = G x m1 x m2 / r^2
where F is the force between two bodies, m1 and m2 and the masses of the bodies, r is the distance between them, and G is the gravitational constant, one of the fundamental constants of physics.
Let us work out the relative magnitudes of the gravitational influences of the earth and the moon on an object at the earth's surface. We could simply calculate the actual forces, which is not too hard, but it turns out that it's simpler to calculate the ratio directly because G and the mass of the test object cancel each other out and can be safely ignored. (If you don't believe me you can work this out for yourself. It's an elementary exercise in algebra.) The upshot is that although the moon is very heavy (about 7.22x10^22 kg) it is also very far away (about 3.84x10^8 meters) and the gravitational influence decreases with the square of the distance. So at the surface of the earth, the moon's gravitational influence is tiny -- only about 3 millionths as strong as the gravity of the earth itself.
Compare this to the gravitational influence of the sun, which is a lot further away (1.46x10^11 meters) but also a lot heavier (about 2x10^30 kg). The sun's gravitational influence at the surface of the earth is about 190 times greater than the moon's. Standing on the deck of the world's largest supertanker with a fully loaded mass of about half a million tons, the gravitational influence of the ship is about the same as the gravitational influence of the moon.
And yet, the moon clearly does have manifest influences at the surface of the earth, most notably, the tides. If the moon can push zillions of tons of seawater around, isn't it plausible that it could also move zillions of tons of magma around too? Well, no, it isn't. To see why you have to understand how the tides actually work. It is tempting to think that the moon causes the tides by pulling water towards itself via the force of gravity. But this theory has a major problem: if this were how tides worked, there should be one high tide every day (when the moon was overhead pulling the water towards it) and one low tide each day (when the moon was on the opposite side of the earth pulling the water away). But in fact there are two cycles of high and low tides each day. How can this be?
The answer is that the moon does not cause tides by "pulling" on the water. It casues tides because of tidal forces (imagine that). Tidal forces are somewhat complicated to explain, but the easiest (though not quite correct) way to explain them is that the earth and the moon make up a two-body system that rotate around a common center of gravity. Because the moon's mass is a significant fraction of the earth's mass, this common center of gravity is not at the center of the earth, but lies about 3300 miles from the earth's center. As the earth rotates about this offset center of gravity, centrifugal forces "fling" the water away.
The important point is that the tidal force is not the force of gravity, but the difference in the force of gravity on the two sides of the earth. And as small as the raw gravitational influence of the moon on the earth is, the tidal forces that it generates are even smaller. This is why the actual tides on earth, while they may appear significant to us on a human scale, are actually miniscule relative to the scale of the planet. There is nowhere near enough energy in the moon's tidal influences to account for volcanism. And it's a good thing too, because if there were then that energy would get dissipated in the world's oceans and they would all boil away.
The title of this post is "how to detect bullshit", and the answer is: do the math. Don't take my or anybody else's word for it. Do it yourself. It isn't hard. As an exercise, consider the Biblical claim that Joshua made the sun stand still (which is to say, that he made the earth stop rotating). Calculate how much rotational energy would have to be dissipated to make that happen. Compare that to the total amount of energy in, say, the world's nuclear arsenals. Here's all the information you need:
The formula for the rotational energy of a sphere is 2/5 x m x r^2 x omega^2 where m is the mass of the sphere, r is the radius, and omega is the rotational velocity in radians per second. The mass of the earth is about 5.97x10^24 kg. It rotates 360 degrees (or 2pi radians) in 24 hours. A kiloton of TNT is about 4x10^12 joules.