Like earthquakes, volcanoes are often located close to plate boundaries. Countries near or on plate boundaries, like Japan, Indonesia, Iceland, and New Zealand, are prone to earthquakes, and are also host to a large number of volcanoes.

Convergent Plate Boundaries

Some volcanoes form at convergent boundaries where two plates collide. When there is a density difference between the colliding plates, usually between oceanic and continental plates, subduction can occur. The leading edge of the plate with higher density will sink below the plate of lower density and pull the plate into the hot asthenosphere.

As the descending plate reaches a depth of about 100-200 km (62-124 mi) below Earth’s surface, huge temperature and pressure increases cause the plate to release a mixture of trapped water and gases. This mixture rises toward the surface and melts the lithosphere above, turning it into molten rock.

This hot magma can collect below the surface and form huge magma chambers. Over time, these magma chambers can cool and solidify, forming batholiths, which can eventually become the backbone of a mountain range like the Sierra Nevada mountain range.

If the molten rock does not collect in underground magma chambers, but instead erupts as lava on Earth’s surface, it forms a volcano. Over time, erupting lava and ash can gradually form large volcanic mountain ranges. When two continental plates collide, they both tend to remain afloat. Because their density is similar and lower than the mantle below, neither plate is fully subducted. Instead, they essentially crash into one another, like two equally sized cars crashing head on. This crash crumples the two plates together, increasing the thickness of the crust at these locations. The Himalayan mountains are the result of the Indian and Eurasian continental plates colliding.

In some regions of the world, two oceanic plates of different densities will collide, resulting in the subduction of the plate with higher density. This subduction results in deep ocean trenches, such as the Mariana Trench east of the Philippines. Challenger Deep, the deepest known point of Earth’s oceans, plunges about 11,000 m (6.8 mi) into the ocean—a depth that could fit Mount Everest and still have 2 km (1.2 mi) to spare! These plate collisions can also form a string of volcanic islands called an island arc. Examples of island arcs include the Mariana Islands, Japan, the Aleutians, and the Lesser Antillies.

Divergent Plate Boundaries

When two tectonic plates move apart, volcanic mountain ranges, like the mid-ocean ridge, can form. As plates diverge, the rock layers are placed under tension forces which cause rock layers to thin. Imagine pulling a piece of clay from both ends. The clay stretches and gradually thins near the middle.

Molten rock below the surface can melt through this thinning layer and erupt to the surface, forming volcanic ridges. The hydrothermal vents first discovered by Dr. Ballard and his team are closely associated with these volcanic ridges.

Hot Spots

If you were to drain the entire Pacific Ocean of its water, you would uncover the long and winding mid-ocean ridge, deep and dark trenches, and towering volcanoes. Volcanoes tall enough to pierce the ocean’s surface are known as volcanic islands. Volcanic islands include such places as Hawai'i and Tonga.

You would also uncover many smaller volcanoes, called seamounts, rising up from the ocean floor. The appearance of many islands and seamounts occurring away from plate boundaries suggests to scientists that they may be caused by hot spots, or localized regions of magma close to the crust’s surface.

At these hot spots, currents or plumes of heat are thought to rise from the mantle, melting rock under the crust and turning it into magma. As magma rises, it melts its way through the solid crust and erupts as lava. Over time, the cycle of eruption, cooling, and solidification can form underwater seamounts. In some cases, the volcanic source can persist for millions of years, gradually forming volcanoes high enough to break through to the ocean’s surface, forming islands like Hawai'i.

Volcanic islands can be easy to navigate around because we can see them. However, there are thousands of undiscovered seamounts scattered around our oceans. Dr. Walter Smith’s ocean floor maps are helping to uncover these obstacles, providing safe passageways for ships and submarines.

Geothermal Activity

Regions where volcanoes are found are often associated with geothermal activity. Geothermal activity occurs as magma rises to the surface and heats rocks in the crust. These rocks may contain groundwater which also gets heated. Heated water expands, causing pressure to build up around the surrounding rock. The pressure pushes the hot water up through cracks, moving it to the surface.

Hot water near the surface is a great source of renewable energy. It can warm the ground and even heat to a boil, as seen in some regions of Iceland and Japan, and in Yellowstone National Park. Geothermal activity can also form hot pools of water, hot lakes, and even hot springs. Sometimes, it can emerge with enough force to produce geysers, or jets of hot water and steam that shoot into the air from a vent in the ground.

Deep underground, minerals within the heated rock can sometimes dissolve into the hot water that is brought to the surface. When this occurs, minerals that once were deep inside Earth can be redistributed around the edges of hot springs as minerals precipitate out from the cooling solution.