Explanation:
A volcano is a feature in Earth’s crust where molten rock is squeezed out onto the Earth’s surface. This molten rock is called magma when it is beneath the surface and lava when it erupts, or flows out, from a volcano. Along with lava, volcanoes also release gases, ash, and solid rock.
Volcanoes come in many different shapes and sizes but are most commonly cone-shaped hills or mountains. They are found throughout the world, forming ridges deep below the sea surface and mountains that are thousands of meters high. About 1,900 volcanoes on Earth are considered active, meaning they show some level of occasional activity and are likely to erupt again. Many others are dormant volcanoes, showing no current signs of exploding but likely to become active at some point in the future. Others are considered extinct.
Volcanoes are incredibly powerful agents of change. Eruptions can create new landforms, but can also destroy everything in their path. About 350 million people (or about one out of every 20 people in the world) live within the “danger range” of an active volcano. Volcanologists closely monitor volcanoes so they can better predict impending eruptions and prepare nearby populations for potential volcanic hazards that could endanger their safety.
Plate Tectonics
Most volcanoes form at the boundaries of Earth’s tectonic plates. These plates are huge slabs of the Earth’s crust and upper mantle, which fit together like pieces of a puzzle. These plates are not fixed, but are constantly moving at a very slow rate. They move only a few centimeters per year. Sometimes, the plates collide with one another or move apart. Volcanoes are most common in these geologically active boundaries.
The two types of plate boundaries that are most likely to produce volcanic activity are divergent plate boundaries and convergent plate boundaries.
Divergent Plate Boundaries
At a divergent boundary, tectonic plates move apart from one another. They never really separate because magma continuously moves up from the mantle into this boundary, building new plate material on both sides of the plate boundary.
The Atlantic Ocean is home to a divergent plate boundary, a place called the Mid-Atlantic Ridge. Here, the North American and Eurasian tectonic plates are moving in opposite directions. Along the Mid-Atlantic Ridge, hot magma swells upward and becomes part of the North American and Eurasian plates. The upward movement and eventual cooling of this buoyant magma creates high ridges on the ocean floor. These ridges are interconnected, forming a continuous volcanic mountain range nearly 60,000 kilometers (37,000 miles)—the longest in the world.
Another divergent plate boundary is the East Pacific Rise, which separates the massive Pacific plate from the Nazca, Cocos, and North American plates.
Vents and fractures (also called fissures) in these mid-ocean ridges allow magma and gases to escape into the ocean. This submarine volcanic activity accounts for roughly 75% of the average annual volume of magma that reaches the Earth’s crust. Most submarine volcanoes are found on ridges thousands of meters below the ocean surface.
Some ocean ridges reach the ocean surface and create landforms. The island of Iceland is a part of the Mid-Atlantic Ridge. The diverging Eurasian and North American plates caused the eruptions of Eyjafjallajökull (in 2010) and Bardarbunga (in 2014). These eruptions were preceded by significant rifting and cracking on the ground surface, which are also emblematic of diverging plate movement.
Of course, divergent plate boundaries also exist on land. The East African Rift is an example of a single tectonic plate being ripped in two. Along the Horn of Africa, the African plate is tearing itself into what is sometimes called the Nubian plate (to the west, including most of the current African plate) and the Somali plate (to the east, including the Horn of Africa and the western Indian Ocean). Along this divergent plate boundary are volcanoes such as Mount Nyiragongo, in the Democratic Republic of Congo, and Mount Kilimanjaro, in Kenya.
Convergent Plate Boundaries
At a convergent plate boundary, tectonic plates move toward one another and collide. Oftentimes, this collision forces the denser plate edge to subduct, or sink beneath the plate edge that is less dense. These subduction zones can create deep trenches. As the denser plate edge moves downward, the pressure and temperature surrounding it increases, which causes changes to the plate that melt the mantle above, and the melted rock rises through the plate, sometimes reaching its surface as part of a volcano. Over millions of years, the rising magma can create a series of volcanoesThe majority of volcanic arcs can be found in the Ring of Fire, a horseshoe-shaped string of about 425 volcanoes that edges the Pacific Ocean. If you were to drain the water out of the Pacific Ocean, you would see a series of deep canyons (trenches) running
