Another Way Physics Can Kill You

We interrupt the hysteria about climate change disasters (droughts and fires out west, multiple destructive hurricanes in the southeast)

Photo from CNN.com

to talk about the Mexico earthquake.  Haven’t heard much about it?  That’s because the 24/7 news media have something closer to home to frighten us with….

So, here is how physics killed over 60 people in Mexico last Thursday:

Adding energy to things makes the molecules move faster and try to move apart.  That is why warmer air is less dense than colder air and rises.  Warmer water floats to the surface.  So does hot rock in the Earth’s mantle.  But with the weight of the Earth’s crust on top of it, the hot rock can’t expand.  Pressure builds up, and the hot magma will escape any way it can.

The way a lot of the magma escapes is through cracks in the Earth’s crust.  The crust primarily a rock called granite, which has less iron in it than the rock in the mantle which is mostly basalt, so the continents float on top of the mantle.  The crust is made up of many large and many more small plates.  The plates are moving around.  There are several hypotheses about exactly how they move, which we will not worry about here.  The important thing is that the rising of hot magma and sinking of cooler magma in convection currents has something to do with it.

The major tectonic plates.  Yellow shows frequent earthquakes, red is volcanoes.

The crustal plates can move towards each other, with either one going under the other like happens on the western coast of the Americas, or simply collide and push up mountains like those plates that are forming the Himalayan mountains.  These are called convergent boundaries.

The plates can slide past each other, like the North American and the Pacific plates are doing along the San Andreas fault in California.

Or, the plates can move away from each other, as they do in the middle of the Atlantic Ocean.  In fact, the Atlantic is growing wider at the rate of 1 to 2 inches every year.

If the Atlantic is growing wider, then something must be shrinking, because the Earth’s diameter isn’t changing.  That something is the Pacific Ocean.  All along its edges the Ocean crust is being pushed (or pulled, depending on which hypothesis you subscribe to) under the continents.  

As the crust sinks it sticks, then the pressure is released in earthquakes as the crust pops free.

This is what happened off the coast of Mexico. The magnitude 8.1 quake, which was felt as far as Mexico City and Guatemala City, was registered off Mexico's southern coast.  The epicenter was in the Pacific Ocean, some 600 miles southeast of the Mexico City.  It was the most powerful quake in Mexico in over 100 years.

 

Photo from CNN.com

Fortunately, building codes in Mexico require buildings to move with the quakes instead of trying to stay rigid, failing, and falling.  This minimized damages and injuries.