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 information about rifting.

Extending from the eastern edge of modern-day Lake Superior to Kansas, a rift valley began to split North America apart around 1.1 billion years ago. Intense volcanism along the rift produced 7.6-kilometer-thick (4.7-mile-thick) igneous deposits. This rift zone continued to spread for about 20 million years, after which it began to sink and eventually become filled with sediment.

Today, the ice sheets of Greenland and Antarctica make up some 95% of all the current glacial ice on Earth. The study of these modern glaciers and their influences on the environment, such as through the formation of U-shaped valleys, is key to interpreting glacial deposits of the past, which are thought to have formed under the same processes as those operating today.