Seismic and volcanic activity is due to tectonic plate movement. Tectonic plates are pieces of Earth’s surface that fit together like a giant jigsaw puzzle. Earth has five layers, starting with two inner cores, two mantles, and a crust (surface). Some of the layers are solid, but some are liquid or a combination of both. Just below the crust lies a mantle layer of semi-solid and incredibly hot melted rock, iron, and other minerals called magma.
The crust layer is broken into about 20 tectonic plates that float like rafts on the roiling magma below. The slabs continually drift apart, grind against, or slide above or below each other. Plates move at different rates, with some moving as little as only an inch per year and others moving as much as six inches or more (imperceptible to us).
Over millions of years, this movement creates islands, mountain ranges (including underwater), and deep ocean trenches. The continual movement, intense pressure, and extreme heat generate tremendous energy until, eventually, that energy needs to release. The release causes earthquakes, some so small we don’t even notice them, and others that cause incredible damage. Volcanoes frequently form along the edges of tectonic plates, and the plate movements cause them to erupt.
The Ring of Fire follows the boundary between the massive Pacific Plate and several smaller tectonic plates. You can see some of these deep underwater trenches and volcanic activity in this 2.5-minute video from the National Oceanographic Atmospheric Association.
Tectonic Plate Boundaries
The boundaries between the roughly 20 tectonic plates that form Earth’s crust are not all the same, and as a result, their movement dictates the outcome of formations. Some boundaries create deep trenches, whereas others form mountains, faults, or volcanic arcs. Geology.com has a detailed map showing the different types around the world. One plate can have multiple boundary types along its border.
Divergent Boundaries: When plates drift apart causing a divergent boundary, a narrow rift or fissures form. Hot magma wells up in the rifts and seawater quickly cools and solidifies it. Repeated eruptions and cooling create high underwater ridges along each side of the plates. In the Ring of Fire, divergent boundaries exist between the giant Pacific Plate and the smaller Cocos Plate (west of Central America), the Nazca Plate (west of South America), and the Antarctic Plate. Often, hypothermal vents form in the fissures, which release hot minerals and water much like an underwater geyser.
You can see a divergent boundary in action in Iceland. Iceland sits on the Mid-Atlantic Ridge, the boundary between the Eurasian and North Atlantic Plates. (Seeing the boundary is one of Iceland’s top tourist activities.) You’ll also find evidence of volcanic activity such as active volcanoes, lava fields, geysers, and bubbling hot springs.
Convergent Boundaries: These boundaries occur where tectonic plates grind against each other. Mountain ridges form as the plates crumple and push material upward. (The Himalayas are an example of this type of boundary.)
Sometimes, a process called subduction takes place — instead of grinding and pushing material upward, one plate slips under the other, creating a deep ocean trench. The upper plate pushes the lower plate into hot magma, causing it to melt and build up pressure (this animated .gif shows how the process works). Hot magma funnels through the upper plate and forms a string of volcanoes along its edge, which is also known as a volcanic arc. Many islands and mountain ranges owe their formation to subduction. For example, the Andes Mountains are a volcanic arc that developed when the smaller, denser Nazca Plate slid under the larger, more buoyant South American Plate. The action also created the Peru-Chile Trench that runs along the coast.
The Ring of Fire has multiple volcanic arcs running alongside deep ocean trenches, including Alaska’s Aleutian Arc (along the Aleutian Trench), the Japanese Arc, and multiple arcs along the Pacific Plate’s eastern edge, from the Philippines down to New Zealand.
The Cascade Mountains along the United States’ northwestern coast are also part of a volcanic arc, created by subduction at the convergent boundary between the Juan de Fuca and the North American Plates. Some of us remember when Mount St. Helens in Washington erupted in 1980, America’s most deadly volcanic eruption to date. The explosion caused hundreds of millions of dollars in damage, killed 57 people, and distributed volcanic ash and dust across thousands of miles.
Transformant Boundaries: These boundaries occur where plates slide past each other. Sometimes where they touch, they get stuck, and pressure builds up. The intense stress eventually causes one to slip or break suddenly, which causes an earthquake. Boundaries are called faults when slips happen frequently.
One of the Ring of Fire’s most active faults is the San Andreas Fault, which runs along the Pacific and North American Plates' boundary. The San Andreas Fault starts at the northern end of the Gulf of California, passes through western California, and then continues out into the Pacific near San Francisco.
Hot Spots Inside the Ring of Fire
Most volcanic activity and earthquakes take place along the tectonic plate boundaries in the Ring of Fire. However, there are some exceptions. The Hawaiian Islands are one example of volcanic activity not related to a plate boundary. Instead, Hawaii’s volcanoes are due to a hot spot in the middle of the Pacific Plate. There is still some debate over how and why hot spots form, but they're essentially holes in tectonic plates where magma squirts through and creates volcanoes.
As the plate moves, the erupting magma from the hole leaves behind a string of progressively newer volcanoes. The Hawaiian Islands are part of a much larger chain of underwater mountains and islands called the Hawaiian-Emperor seamount chain. The oldest islands, many of which are now atolls formed from extinct volcanoes, lie northwest of Hawaii. Kauai is the northernmost and oldest of the five major Hawaiian volcanic islands, and the Big Island is the southernmost and youngest. The islands formed one at a time at the hole as the Pacific Plate drifted northwest. The youngest Hawaiian volcano, Loihi, lies underwater southeast of the Big Island and is currently erupting.