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.

See Chapter 1: Geologic History for more about the tectonic processes that led to the formation of North America as we know it today.

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. The Yellowstone hot spot, currently located in Yellowstone National Park in northwestern Wyoming, is a mantle plume that has melted the crust (and induced volcanic eruptions) as the North American plate has passed over it. The trail of volcanic rock from these eruptions crosses southern Idaho, forming the Snake River Plain and ending at Yellowstone National Park. Major explosive caldera eruptions have occurred on a cycle of around 600,000 years— this recent geological history of volcanism has led the Yellowstone area to be classified as a supervolcano.