Earthquakes and volcanoes share a common cause: the slow, powerful movement of Earth’s tectonic plates. These massive slabs of rock grind past each other, collide, and pull apart, generating the forces that shake the ground and push molten rock to the surface. About 90% of the world’s earthquakes and 75% of its volcanoes occur along the Pacific Ring of Fire, a horseshoe-shaped zone where several tectonic plates meet. Understanding why these events cluster together starts with what’s happening deep beneath your feet.
How Tectonic Plates Move
Earth’s outer shell is broken into roughly 15 major plates that float on a layer of hot, slowly flowing rock called the mantle. These plates aren’t sitting still. They creep along at speeds of a few centimeters per year, roughly as fast as your fingernails grow. What drives that movement has been debated for decades, but the current understanding treats the plates and the mantle beneath them as a single connected system rather than one simply pushing the other around.
Heat from Earth’s interior causes mantle rock to rise in some places and sink in others, creating circulation patterns similar to water heating in a pot. The plates themselves are really just the cooled, rigid top layer of this system. Where old, dense plate material sinks back into the mantle at subduction zones, it pulls the rest of the plate behind it. This sinking force generates the strongest pressure gradients in the system and produces some of the fastest plate movements on Earth. At the same time, rising plumes of hot material push plates apart from below. The interplay of these sinking and rising forces keeps every plate in constant motion.
What Causes Earthquakes
Earthquakes happen when rock that has been slowly deforming under stress suddenly snaps and shifts. The process works like stretching a rubber band: as tectonic plates push against or slide past each other, the rock along their boundary bends and stores elastic energy over years or centuries. When the stress finally exceeds the friction holding the rocks in place, the fault ruptures and the rock springs back to its original shape, releasing all that stored energy as seismic waves. Geologists call this elastic rebound.
The type of plate boundary determines the style of earthquake. At convergent boundaries, where plates collide, one plate is forced beneath the other in a process called subduction. The contact zone between these plates, known as a megathrust fault, produces the most powerful earthquakes on record. The 1960 Valdivia earthquake in Chile, the strongest ever recorded at magnitude 9.5, occurred at a subduction zone. The 2004 Sumatra, 2010 Chile, and 2011 Japan megathrust earthquakes all struck along similar boundaries.
At transform boundaries, plates slide horizontally past each other. California’s San Andreas Fault is the most familiar example. Stress builds along the fault over decades until the rocks lurch forward, sometimes displacing the ground by several meters in seconds. At divergent boundaries, where plates pull apart, earthquakes tend to be smaller because the stretching crust fractures more easily and doesn’t store as much energy before releasing it.
What Causes Volcanoes
Volcanoes form wherever molten rock, called magma, finds a path to the surface. But rock deep in the mantle doesn’t just sit around in a melted state waiting to erupt. It needs specific conditions to melt in the first place, and those conditions arise in three main settings.
The first is subduction zones. When an oceanic plate dives beneath another plate, it carries water trapped in its minerals down into the mantle. That water lowers the melting point of the surrounding rock, causing it to partially melt even at temperatures that would normally keep it solid. This process, called flux melting, generates the magma that feeds volcanic arcs like the Andes, the Cascades, and the volcanoes of Japan and Indonesia. The megathrust earthquakes that shake these regions can also alter stress patterns in the overlying crust, potentially influencing where new volcanic systems develop over time.
The second setting is divergent boundaries, where plates pull apart. As rock rises to fill the gap, it experiences lower and lower pressure. Reduced pressure allows the rock to melt without any increase in temperature, a process called decompression melting. Around 75% of all volcanic activity on Earth occurs at mid-ocean ridges through this mechanism. Most of it happens on the ocean floor and goes unnoticed, quietly building new crust in a zone just one to two kilometers wide along the spreading center. Research along the Mohns Ridge in the Arctic has shown that volcanic eruptions can actually occur across the entire width of the rift valley floor, not just along the central axis.
The third setting has nothing to do with plate boundaries at all.
Hotspot Volcanoes
Some volcanoes sit in the middle of tectonic plates, far from any boundary. Hawaii is the classic example. These volcanoes are fed by mantle plumes: columns of unusually hot rock that rise buoyantly from deep within the Earth, similar to the way blobs rise in a lava lamp. When a plume reaches the shallow mantle, the drop in pressure causes it to partially melt, and that magma punches through the plate above.
The plume itself stays roughly stationary while the plate above it keeps drifting. This creates a chain of volcanoes, each one older and more eroded as it moves farther from the hot source. In Hawaii, the Big Island sits directly over the plume and hosts the most active volcanoes. To the northwest, each island in the chain is progressively older, with extinct volcanic remnants eventually sinking beneath the ocean surface as seamounts. Once a volcano is carried away from the plume, its magma supply is cut off and it goes dormant.
The Ring of Fire
The Pacific Ring of Fire is a 40,000-kilometer arc stretching from New Zealand up through Southeast Asia, across to Japan and Alaska, and down the western coasts of North and South America. It marks the edges of several tectonic plates, and the concentration of activity there is staggering: 75% of the world’s volcanoes and 90% of its earthquakes. The Ring includes subduction zones on nearly every side of the Pacific Plate, which is why it produces both enormous megathrust earthquakes and explosive volcanic eruptions. Mount Tambora in Indonesia, which produced the largest volcanic eruption in recorded history in 1815, sits along the Ring of Fire. So does Mount St. Helens in Washington State.
How Scientists Measure These Events
Earthquake size is measured on the moment magnitude scale, which replaced the older Richter scale developed in the 1930s. The Richter scale was originally designed for earthquakes in southern California measured by nearby instruments, and it becomes unreliable for very large or very distant events. Moment magnitude works across all earthquake sizes and distances, making it the standard used by seismologists worldwide. The scale is logarithmic, meaning each whole number represents roughly 32 times more energy released than the number below it.
Volcanic eruptions are ranked using the Volcanic Explosivity Index, or VEI, which runs from 0 to 8. The primary measure is the volume of ash and other material ejected. Like earthquake magnitude, the VEI is logarithmic: each step up represents a tenfold increase in erupted material. The 1980 eruption of Mount St. Helens was a VEI 5, ejecting about 1 cubic kilometer of ash. The 1991 eruption of Mount Pinatubo in the Philippines was a VEI 6, producing roughly 10 cubic kilometers. At the extreme end, the Yellowstone eruption 631,000 years ago was a VEI 8, expelling at least 1,000 cubic kilometers of material.
Earthquakes Caused by Human Activity
Not all earthquakes originate from natural tectonic forces. Human activities, particularly the injection of wastewater deep underground during oil and gas operations, can trigger earthquakes by raising fluid pressure along existing faults. The mechanism is straightforward: faults stay locked because friction holds the two sides together, but injecting fluid into the fault zone reduces that friction, much like turning on an air hockey table makes the puck glide freely.
Oklahoma became a dramatic example of this. The state experienced four magnitude 5 or greater earthquakes between 2011 and 2016, including a magnitude 5.8 event in September 2016, the largest earthquake ever documented from fluid injection. These induced earthquakes can occur 10 or more miles from the injection well and several miles below it. Out of roughly 40,000 wastewater disposal wells tied to oil and gas operations in the United States, only a small fraction have triggered earthquakes large enough to cause public concern. Hydraulic fracturing itself produces smaller events; the largest linked to fracking in Oklahoma was magnitude 3.6.