Understanding where hydrothermal vents occur requires a closer look at the Earth’s structure and the forces at work deep within the planet.

The deeper you go inside the Earth, the hotter it gets. Scientists have calculated that the Earth’s inner core — a solid sphere composed primarily of iron — is about 5,500° C (10,000°F). That’s about the same temperature as the surface of the sun. The solid inner core is about 2,400 kilometers (1,500 mi) in diameter. It is surrounded by a liquid outer core about 2,225 kilometers (1,380 mi) thick.

Bordering the liquid outer core is the mantle, which is composed of hot, molten rock called magma. The churning of the magma generates pressure on the Earth’s surface layer, or crust. The crust is very thin and brittle compared to the other layers. It ranges in thickness from only about 3 kilometers (2 mi) in some areas of the ocean floor to some 120 kilometers (75 mi) deep under mountains on the continents.

According to the theory of plate tectonics, the Earth’s crust is made up of about a dozen plates on which the continents and the oceans rest. These plates are continually shifting because the surface beneath them — the hot, magma-filled mantle — is moving slowly like a conveyor belt, driven by the heat in the Earth’s core. The plates currently move about a centimeter (0.5 in) to 15 centimeters (6 in) per year in different directions.

The Earth’s tectonic plates can move apart, collide, or slide past each other. The Mid-Ocean Ridge system — the Earth’s underwater mountain range — marks where the plates are moving apart. As the plates part, the seafloor cracks. Cold seawater seeps deep down into these cracks, becomes super-heated by magma, and then gushes back out into the ocean, forming hydrothermal vents.

As the plates move farther apart, magma from the Earth’s interior percolates up to fill the gap, sometimes causing earthquakes and the eruption of undersea volcanoes. This process, called seafloor spreading, is how new seafloor is formed.

The Earth’s size is constant, so as the crust expands through seafloor spreading in one area, crust must be swallowed up elsewhere. Crust is destroyed when the edge of one tectonic plate is forced underneath another. This dynamic process is called subduction. It results in earthquakes, volcanoes, and the formation of deep ocean trenches.