The Earth processes operating today—everything from local erosion to plate tectonics—are the same as those operating since they first arose in Earth’s 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.

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 Grand Canyon offers a large cross-section of Earth history—a window into nearly two billion years of North America’s formation. Some 3900 meters (13,000 feet) of lava and sediment are exposed there, and though these rocks date back over a huge span of Earth’s history, the processes that made them are still at work today. Schists still form from other rocks put under heat and pressure; sediments still become limestones and sandstones, and lavas still cool to form basalts. Rocks born from all of these processes are visible in a number of places throughout the Southwestern US.