Plate tectonics

Plate tectonics is the scientific theory that explains how Earth's outer shell, called the lithosphere, is made up of large pieces called tectonic plates. These plates have been moving very slowly for about 3 to 4 billion years. The idea of plate tectonics grew from an earlier concept called continental drift, which scientists started talking about in the early 1900s. By the 1960s, when evidence of seafloor spreading was found, most geoscientists accepted this theory.

Earth's lithosphere, which includes the crust and the upper part of the mantle, is broken into seven or eight major plates and many smaller ones. Where these plates meet, they can move in different ways, leading to different types of boundaries. The movement of these plates is usually very slow, from zero to about 10 centimeters each year. This movement causes many important events on Earth, such as earthquakes, volcanic eruptions, the forming of mountains, and the creation of ocean trenches.

These plates are made up of both oceanic and continental parts, and they move over a softer layer called the asthenosphere. One important process happens when plates move toward each other, causing one to slide under the other in a process called subduction. This helps balance the Earth's surface by creating new oceanic crust in other places through seafloor spreading. While Earth is the only planet we know where plate tectonics happens now, there are signs that other planets and moons, like Jupiter's moon Europa, have had similar activity in the past.

Key principles

The outer layers of Earth are split into the lithosphere and asthenosphere. The lithosphere is the cooler, harder layer on top, while the asthenosphere is hotter and flows more easily underneath.

The big idea of plate tectonics is that the lithosphere is made up of several large pieces called tectonic plates. These plates float on the asthenosphere and move slowly. They can move as slowly as our fingernails grow or as fast as our hair grows. Where these plates meet, we often see earthquakes, mountains, volcanoes, and deep ocean trenches. The area around the Pacific Ocean, called the Ring of Fire, has many active volcanoes.

Types of plate boundaries

Main article: List of tectonic plate interactions

There are three main types of places where the large pieces of Earth's surface, called plates, meet. These places are called plate boundaries, and they change the land in different ways.

• Divergent boundaries: Here, plates move apart. When this happens in the ocean, new ocean floor forms and makes the ocean wider. This can create small volcanoes and gentle earthquakes. When this happens on land, it can eventually create new oceans by pulling continents apart.
• Convergent boundaries: Here, plates move toward each other. One plate can slide under another, creating deep ocean trenches and sometimes volcanoes. When continents crash into each other, big mountain ranges can form.
• Transform boundaries: Here, plates slide past each other without creating or destroying any plate. This can cause strong earthquakes. An example is the San Andreas Fault in California.

Driving forces of plate motion

Tectonic plates move because of the difference in density between the oceanic lithosphere and the weaker asthenosphere beneath it. Heat from the mantle provides the energy needed for this movement through convection. A major force moving plates is the dense oceanic lithosphere sinking into the mantle at subduction zones. When new crust forms at mid-ocean ridges, it starts less dense but becomes heavier as it cools and thickens over time. This weight allows it to sink into the mantle, which is a primary force driving plate movement. The weak asthenosphere lets the plates move easily toward subduction zones.

Driving forces related to mantle dynamics

Main article: Mantle convection

For much of the early 20th century, scientists thought large convection currents in the upper mantle moved tectonic plates. This idea was first suggested by Arthur Holmes and others in the 1930s. Imaging of Earth's interior shows variations in density that cause mantle convection. How this convection affects plate motion is still being studied. Two main ideas exist: convection cells directly moving plates, or friction between convection currents and the plates above them. In subduction zones, the sinking of plates can pull other plates along through a process called slab pull.

Plume tectonics

In the 1990s, some scientists proposed that super plumes rising from deep in the mantle drive major convection currents. These ideas originated in the 1930s and were initially seen as alternatives to plate tectonics. They are linked to hot spots and mantle plumes that leave marks on the geological record. This theory is sometimes used to explain how supercontinents broke apart. It also has support among scientists studying the theory of Earth expansion.

Surge tectonics

Another theory from the 1980s and 1990s suggests mantle flow happens in channels just below Earth's crust, providing friction that moves the plates. Recent computer models show that plate geometry depends on feedback between mantle convection and the strength of the lithosphere.

Driving forces related to gravity

Gravity plays a role in plate motion, especially through slab pull at subduction zones. Another idea is gravitational sliding away from spreading ridges. As new oceanic crust forms at ridges, it cools and thickens, becoming denser and sinking into the mantle. This creates a slope that helps push plates away from ridges, known as ridge push. Slab pull is considered the strongest force moving plates, where the weight of sinking plates pulls others along. However, some plates like the North American plate move without being subducted, which remains a puzzle.

Gravitational sliding can also happen away from large mantle domes, an idea from older theories. This can work on scales from island arcs to entire ocean basins.

Driving forces related to Earth rotation

Alfred Wegener originally suggested tidal and centrifugal forces from the Moon and Sun might move continents, but later changed his view to convection currents. In modern plate tectonics, some scientists still consider tidal forces from the Moon as a possible driver. In the 1970s, some evidence suggested a westward drift of Earth's lithosphere due to tidal forces. This idea has been supported and debated since then. Other forces like the Coriolis effect are considered too small to drive plate motion.

Role of water

Water is thought to play an important role in plate tectonics on Earth.

Relative significance of each driving force mechanism

The movement of a plate depends on all the forces acting on it. Plates attached to subducting plates, like the Pacific plate, tend to move faster. Slab pull from sinking plates is often seen as the main driver, but recent studies suggest mantle convection upwelling might also be important. The exact forces moving plates are still being researched in geophysics and tectonophysics.

History of the theory

The idea of plate tectonics changed science and our understanding of Earth. It all started about 50 years ago when scientists began to debate and discuss how Earth’s surface moves.

Around the early 1900s, scientists tried to explain how continents fit together. In 1912, a scientist named Alfred Wegener introduced the idea of continental drift. He thought continents were once joined together in one big landmass called Pangaea, and later split apart and moved to their current positions. Wegener wrote a book in 1915 called The Origin of Continents and Oceans to explain his ideas.

At first, many scientists did not agree with Wegener because they did not know how the continents could move. Debates continued for years between those who supported the idea and those who did not. Over time, new discoveries provided more evidence. Scientists found that the ocean floors had patterns in their magnetic fields and that earthquakes happened in specific zones. These discoveries helped support the idea that Earth’s surface is made of moving pieces called plates.

By the 1960s, the theory of plate tectonics was formed. It explained many things about Earth, like how mountains form and why earthquakes happen. This theory became widely accepted and changed the way scientists study our planet.

Implications for life

According to a hypothesis, plate tectonics might be important for planets to support complex life because it helps control the carbon cycle.

The idea of moving continents also helps scientists explain why some plants and animals on different continents look similar but have different ancestors.

Plate reconstruction

Main article: Plate reconstruction

Reconstruction helps us learn about how the Earth's big pieces of land, called plates, looked in the past. This helps us understand how old landmasses, called supercontinents, were shaped and where they were located.

Active plate edges can be found where earthquakes happen. We can also find old plate edges inside current plates by looking for clues like special rocks that tell us about oceans that no longer exist. Scientists study how plates moved in the past using many kinds of evidence. They look at how well continents fit together, like puzzle pieces, and use patterns in rocks that record Earth's magnetic field. They also study hotspots—places deep in the Earth that stay in one spot over time—and the positions of certain rocks and fossils. All these clues help us piece together how plates moved and how continents formed and broke apart over millions of years.

Modern plates

Main article: List of tectonic plates

The Earth's surface is made up of large pieces called plates. There are usually seven or eight big plates, such as the African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian. The Indo-Australian plate is sometimes split into the Indian and Australian plates.

Besides these big plates, there are many smaller ones, including the Arabian, Caribbean, Juan de Fuca, Cocos, Nazca, Philippine Sea, Scotia, and Somali. Scientists use special tools and satellites to study how these plates move.

Other celestial bodies

Plate tectonics, the movement of Earth’s outer layers, might also happen on other planets, depending on their size and other factors. Earth is special because it has a lot of water, which helps its crust move.

Venus, a planet similar in size to Earth, does not show signs of current plate tectonics. Some think it may have had them in the very distant past, but now its surface looks very different. Mars, which is smaller, also does not have active plate tectonics today. Scientists are still studying whether plate tectonics might have played a role in shaping Mars long ago.

Some scientists also wonder if plate tectonics could happen on larger planets beyond our solar system, called super-Earths. Whether this is possible depends on many factors, including if the planet has water. Studying plate tectonics helps scientists search for signs of life on other planets.

See also: Geology of Venus, Geology of Mars

Main article: terrestrial planets, more massive planets than Earth, eutectic, lithosphere, impact craters, dating method, shear zones, Martian Crustal Dichotomy, mantle, Tharsis, Northern Lowlands, Valles Marineris, Mars Global Surveyor, super-Earths, search for extraterrestrial intelligence, extraterrestrial life