Billions of years ago, the Earth’s surface was an ocean of molten rock. As the molten magma progressively cooled, it solidified to form a continuous rocky crust. The denser minerals sank towards the Earth’s core, while the less dense minerals rose to the surface.
“This is how the Earth’s surface plates were formed,” explained Catherine Rychert, a geophysicist on the Woods Hole Oceanographic Institute in Massachusetts. “Plate refers to the crust, and underneath that is part of the mantle… Beneath that is the weaker material.”
This weaker material is warmer and more mobile. The difference in strength between these layers allows the Earth’s surface plates to maneuver by colliding, moving apart, or rubbing against one another. Fissures, mountains, volcanoes and earthquakes form in these zones.
But what number of such plates exist on the Earth’s surface? This number ranges from a dozen to almost 100, depending in your perspective.
Most geologists agree that there are 12 to 14 “primordial” plates covering a lot of the Earth’s surface, as Saskia Goes, a geophysicist at Imperial College London, points out. Each of those plates has an area of no less than 20 million square kilometers, the most important of that are the North American, African, Eurasian, Indo-Australian, South American, Antarctic and Pacific plates. The largest is the Pacific Plate, covering roughly 103.3 million square kilometers, closely followed by the North American Plate, which covers 75.9 million square kilometers.
In addition to those seven large plates, there are five barely smaller ones: the Philippine Sea, the Coconut Sea, the Nazca Sea, the Arabian Sea and the Juan de Fuca Sea. Some geologists consider the Anatolian Plate (a part of the larger Eurasian Plate) and the East African Plate (a part of the African Plate) to be separate entities because they move at different speeds relative to the predominant plates. This represents a difference within the variety of motherboards, from 12 to 14.
The situation becomes more complicated after we consider plate boundaries, where plate tectonics causes plates to interrupt up into smaller fragments called microplates. These microplates are lower than 1 million square kilometers in size, and there are estimated to be about 57 of them on Earth. However, they are usually not often depicted on world maps, reflecting the uncertainty about their formation.
“The number of microplates will change as different scientists define them and learn more about the location of deformations at the boundaries of the plates,” Goes noted.
As geologists decipher this complex puzzle, Earth’s shifting plates are giving rise to intriguing scenarios. The fastest moving is the Pacific Plate, moving northwest at a rate of seven to 10 centimeters per yr, driven by the gravitational forces of the encompassing subduction zones, generally known as the Ring of Fire, where plates are being pulled into the Earth. Rychert suggests that this constant movement may even end in submerged continents, as sometimes continents can sink, causing pieces to fall into the mantle.
Given these dynamic forces, what our planet’s plate-covered surface will appear to be in just a few billion years stays a mystery.
