Big Idea 4: Physical and chemical principles are unchanging and drive both gradual and rapid changes in the Earth system

The Earth processes operating today—everything from local erosion to plate tectonics—are the same as those operating since they first arose in Earth history, and these processes are obedient to the laws of chemistry and physics. While the processes that constantly change the planet are essentially fixed, their rates are not. Tipping points are reached that can result in rapid changes cascading through Earth systems. For example:

During the Precambrian, the evolution of photosynthetic organisms led to significant changes in the planet’s atmosphere. Prior to this event, there was little free oxygen in the atmosphere, but with photosynthesis producing oxygen as a waste product, the very existence of these organisms flooded the seas and atmosphere with free oxygen, changing the planet forever. But life’s evolution represents just one of the processes working upon Earth systems.

The New Madrid Seismic Zone has a long history of earthquake activity, with major quakes coming every few centuries, though it is has been generally calm for the last 200 years (which, from a geologic perspective, is a quite short amount of time). During the quakes of 1811 and 1812, the Mississippi River was rerouted, and, for a brief period, had at least one waterfall. The seismic waves from these events caused church bells to ring as far away as Boston, Massachusetts. Tectonic processes have been at work in the same way for billions of years, opening and closing oceans and building up and tearing down landscapes.

See Chapter 10: Earth Hazards for more information about earthquakes in the South Central.