Supercontinent cycle

The supercontinent cycle describes how Earth's landmasses come together and then break apart over very long periods of time. This process involves the gathering and spreading of Earth's continental crust. Scientists think one full cycle, from when lands join together to when they separate again, takes between 300 to 500 million years.

When continents collide, they form fewer and larger landmasses. But when rifting happens, the land splits into more and smaller pieces. Even though there are different ideas about whether the amount of land is growing, shrinking, or staying the same, everyone agrees that Earth's surface is always changing shape. This slow dance of the continents helps shape the world we live on today.

Theory

The most recent supercontinent, Pangaea, formed about 300 million years ago during the Paleozoic era. Scientists have two main ideas about what happened before that.

One idea is that Earth had a series of supercontinents. These include Vaalbara, Ur, Kenorland, Columbia, Rodinia, and Pannotia. These landmasses came together and then broke apart over millions of years.

Another idea is that there was just one supercontinent, called Protopangea–Paleopangea, that lasted from about 2.7 billion years ago until around 600 million years ago. This idea is based on studies of Earth's magnetic record and rock layers.

The kinds of minerals found inside ancient diamonds suggest that the cycle of supercontinents forming and breaking apart began about 3 billion years ago. Before that time, only certain types of diamonds formed, but after that, different kinds appeared. This change is linked to processes like the movement of tectonic plates and collisions between landmasses.

The supercontinent cycle happens along with shorter cycles called the Wilson Cycle, named after John Tuzo Wilson. These shorter cycles involve the opening and closing of ocean basins. The oldest ocean floor we find today is about 170 million years old, while the oldest parts of continents are about 4 billion years old. This shows that the shorter Wilson cycles are much quicker than the larger patterns of continents coming together and breaking apart.

Effects on sea level

When the continents come together to form a supercontinent, the sea level tends to be lower. This happened when Pangaea and Pannotia formed. When the continents spread apart, the sea level rises. This occurred during the Ordovician and Cretaceous periods.

The age of the ocean floor plays a big role in sea level changes. Younger ocean floor is shallower, which makes sea levels higher. Older ocean floor is deeper, leading to lower sea levels. When continents move apart, the ocean area can change, also affecting sea levels. Higher sea levels can cover more land, while lower levels expose more of the continents.

Relation to global tectonics

When a supercontinent breaks apart, it starts with areas pulling apart, called rifting. After that, the edges of these areas become quiet, called passive margins, while the ocean floors spread out and the oceans get bigger. Finally, these areas start crashing into each other. At first, smaller land pieces and islands bump into the continents, but later, the continents crash right into each other. This happened during the time of the ancient supercontinents, and it is still happening today in the current cycle of supercontinents.

Relation to climate

Main article: Greenhouse and icehouse Earth

The Earth has two main types of global climates: icehouse and greenhouse. An icehouse climate has cold, dry conditions with glaciers and deserts, while a greenhouse climate is warm and humid. These climate patterns are linked to how Earth's continents move together and apart over time.

When continents come together, forming large landmasses, the climate tends to be cooler and drier. This is called an icehouse climate. When continents are spread apart, the climate is usually warmer and more humid, known as a greenhouse climate. The Earth is currently in a short warm phase of an overall cooler climate pattern.

Relation to evolution

The main way that plants, animals, and other living things change over time is through natural selection. When groups of the same kind of living thing are separated from each other, they can change in different ways. This happens more when the land parts of Earth are far apart. When all the land is together, there is less chance for these groups to change differently.

Long ago, when many different kinds of sea animals first appeared, the land parts of Earth were breaking apart. How the land and water are arranged can change how much living things change over time. When the land is arranged from north to south, it creates more chances for different kinds of animals and plants to develop. When the land is arranged from east to west, there is less chance for this to happen.