Why Earth is the only planet with plate tectonics

Terrestrially, plate tectonics are a vital part of Earth’s evolution.

This map of Earth shows, in black, the more than 300,000 earthquake epicenters identified from 1964-present. The earthquake locations clearly trace out a number of “lines” on the map, which correspond to a number of boundaries between tectonic plates here on our planet.

The crust and upper mantle form the lithosphere: fragmented into a series of plates.

At the boundary between two plates on Earth, they can either diverge, where new crust is produced as the plates pull apart, converge, where crust is destroyed as one plate is pushed beneath another, “transform” where they slide horizontally past one another, or at boundary zones where interactions are unclear. These are responsible for and related to surface features such as mountain-building, earthquakes, volcanoes, and more.

These plates collide, spread apart, uplift, and subduct, creating diverse surface features.

The Hawaiian islands, like most island arcs that form on Earth, initially arose as a mantle plume delivered material up to Earth’s surface by rising through the crust. Over time, the lava builds up to poke above Earth’s oceanic surface, and then, as the plate slides over so that the forming, growing mountain is no longer over the same hot-spot, a new island begins to form. Once a mountain has moved off of its hotspot, it can only erode, not grow any further.

From mountain formation to volcanic island chains to oceanic spreading, plate tectonics affect Earth globally.

Lake Baikal, as viewed from space aboard NASA’s OrbView-2 satellite. Lake Baikal is the 7th largest lake in the world by surface area, but holds more fresh water than any other lake by quite a wide margin. It is the deepest continental rift valley, formed when plates spread apart, known on Earth.

Continental drift creates and breaks apart supercontinents many times throughout history.

This animation shows the breakup of supercontinent Gondwanaland, which itself was a large subsection of Pangaea at one point, into the smaller continents of South America, Antarctica, Africa, Australia, as well as components of other continents that are recognizable, such as Arabia and India.

But is Earth unique? No other known planet possesses plate tectonics.

This cutaway view of the four terrestrial planets plus Earth’s moon shows the relative sizes of the cores, mantles, and crusts of these five worlds. Note that Mercury has a core that’s 85% of its interior by radius; Venus’s core/mantle boundary is highly uncertain; and that Mercury itself is the only such world we know of without a crust. Yet only Earth exhibits plate tectonics; the other three rocky planets all possess only single plates to the best of our knowledge.

Mars is a single-plate planet, enabling Olympus Mons to form.

This computer-generated view of Olympus Mons shows the volcano’s size, its caldera, and its long, sloping sides that make it the largest planetary volcano presently known. Because Mars lacks plate tectonics, the magma chamber beneath Olympus Mons, when it erupts, keeps on growing this one volcano over and over. It’s been the Solar System’s largest for billions of years, and continues to grow over geologic timescales.

With an unmoving uniplate and a hotspot beneath it, Olympus Mons is the largest planetary volcano.

Mars Orbiter Laser Altimeter (MOLA) colorized topographic map of the western hemisphere of Mars, showing the Tharsis and Valles Marineris regions. The impact basin Argyre is at lower right, with the lowland Chryse Planitia to the right (east) of the Tharsis region. Olympus Mons, near the upper-left, is the largest and tallest of the four major tall planetary volcanoes shown here on Mars.

Mercury lost most of its mantle early on, having cooled to form a solid, one-plate planet.

When it comes to the large, non-gaseous worlds of the Solar System, Mercury has by far the largest metallic core relative to its size. However, it’s Earth that’s the densest of all these worlds, with no other major body comparing in density, owing to the added factor of gravitational compression. Unlike Venus, Earth, and Mars, Mercury has no separate crustal layer to speak of.

But Venus, almost the size and mass of Earth with comparable internal heat, appears single-plated today.

Earth, in visible light at right, and Venus, as seen in infrared at left, have nearly identical radii, with Venus being approximately ~90–95% the physical size of Earth. Despite producing similar amounts of internal heat, Earth exhibits plate tectonic activity while Venus only has one single, non-moving plate. Both worlds, however, are volcanically active.

Although it’s still volcanically active, Venus’s surface deforms, but doesn’t flow.

These two images of the same region of the surface of Venus, taken by the Magellan spacecraft in 1990 and 1992, show evidence of a changing landscape: consistent with a volcanic eruption resurfacing and adding material to part of the imaged landscape depicted here. The resurfacing, or covering-over of previous craters, is extremely strong evidence for such a phenomenon.

Perhaps Earth owes its tectonic uniqueness to global oceans, with hints found elsewhere.

This artist’s rendition shows observed surface features on Europa mapped onto the theoretical subsurface structure of Jupiter’s second Galilean satellite. Numerous features that show evidence for plate tectonics are visible on the surface, although they’re ice plates, not rock plates on Europa.

Europa, Jupiter’s ice-covered moon, exhibits evidence for ice-plate tectonics.

This conceptual illustration of the subduction process (where one plate is forced under another) shows how a cold, brittle, outer portion of Europa’s 20-30 kilometer-thick (roughly 10-20 mile) ice shell moved into the warmer shell interior and was ultimately subsumed. A low-relief subsumption band was created at the surface in the overriding plate, alongside which cryolavas may have erupted. The Europa Clipper mission aims to research this moon of Jupiter further.

Subduction and subsurface liquid upwelling occur there, with similar activity possible on Pluto.

Liquid water may be the key.

The geological features and scientific data observed and taken by New Horizons indicate a subsurface ocean beneath Pluto’s surface, encircling the entire planet. There may be plate-like behavior as various regions of Pluto’s icy crust collide, and possibly uplift and subduct: something that it may have in common with many worlds with large surface and subsurface quantities of water.

Internal heat plus water’s lubricating effects, combined, likely enable Earth’s flowing, sliding plates.

The Earth’s crust is thinnest over the ocean and thickest over mountains and plateaus, as the principle of buoyancy dictates and as gravitational experiments confirm. It is likely the combined effects of Earth’s internal heat as well as the large amounts of liquid water on and within Earth’s surface that allow the lithosphere, the uppermost portion of the mantle plus the crust, to fragment into plates that slide over one another, collide, and spread apart.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.