What Are Volcanoes? How Are Volcanoes Formed? Types of Volcanoes Explained

Erupting with fiery rage, reshaping landscapes, and revealing secrets from deep within our planet, volcanoes offer a glimpse into our planet’s geological history. But what are volcanoes? And how do volcanoes form? What causes volcanic eruptions? And why are some volcanoes gentle while others explode with devastating force?

This post is all about the science of volcanoes. We’ll discover the fascinating science behind how volcanoes work, explore the different types of volcanoes, and the powerful impact they have in shaping our world.

What Are Volcanoes?

A volcano is an opening in Earth’s crust where molten rock, called magma, makes its way to the surface. It acts like a vent—or a natural chimney—providing an escape route for the release of various materials, including lava, gases, steam, and small rock fragments. Beneath the surface, magma is stored in magma chambers, pushed upward by pressure and buoyancy until it finds a weak spot in the Earth's crust to break through, triggering the eruption.

Over time, the materials that pour out during eruptions build up and shape the volcano itself. A volcano isn’t just a mountain—it’s a dynamic system that connects Earth’s fiery interior to the surface through a network of vents and molten reservoirs.

How Are Volcanoes Formed?

Volcano formation starts with magma, which forms deep beneath the Earth's surface, in the lower crust and upper mantle. Since magma is less dense than the surrounding rock, it begins to rise to the crust, helped also by the pressure from gas within it. As the magma ascends, it can gather in linked storage areas beneath the surface, called magma chambers.

But the magma doesn't remain there. The increasing pressure in the magma chamber triggers the eruption, with the magma traveling through a central pipe system, breaking through weak points in the crust and escaping through a surface vent. Once it reaches the surface, magma becomes lava, initiating the volcano formation process.

But that is just the beginning. It takes a lot more than just one eruption to build the physical structure of the volcano. Lava flows, ash, solid rocks, and gases that are expelled during the subsequent eruptions start to accumulate around the vent. It is the slow and continuous buildup of this material over long periods of time that creates the structure of the volcano.

Tectonic Plates and Volcanoes

Volcanoes often form at the boundaries between tectonic plates, influencing the formation of volcanoes.

Divergent Boundaries

When tectonic plates move away from each other, magma rises to fill the gap, giving birth to volcanic mountain ranges on the sea floor, like the Mid-Atlantic Ridge.

Convergent Boundaries

When a continental plate collides with an oceanic plate, the denser, oceanic plate is pushed beneath the lighter, continental plate. As the oceanic plate sinks into the mantle, it melts and joins the magma, which can rise to the surface during volcanic eruptions. This process is responsible for creating many volcanic systems along subduction zones, such as those in the "Ring of Fire,” around the Pacific Ocean.

Hot Spots

Some volcanoes even form over hot spots, away from tectonic plates boundaries. Located deep within the Earth's mantle, hot spots are extremely hot areas capable of melting the tectonic plate above them, creating magma that erupts onto the top of that plate. This process leads to the creation of volcanic chains, like the Hawaiian Islands.

Submarine volcanoes

Submarine volcanoes are located beneath the ocean's surface. The high pressure from the water at those depths prevents explosive eruptions and leads to gentle lava flows that slowly build seafloor mountains. If these grow tall enough, they can break the surface and become volcanic islands.

Types of Volcanoes

Volcanoes may all erupt, but they don’t all look or behave the same! There are several main types of volcanoes, each with its own style, shape, and personality.

Shield Volcanoes

Shield Volcanoes are the gentle giants. These broad, dome-shaped volcanoes can be wider than 100 kilometers and their slope is only around 2° to 10°. They're built by runny, low-viscosity lava (usually basaltic lava) that flows easily and spreads far. Eruptions are typically calm and steady, not explosive. Famous shield volcanoes include Mauna Loa and Kilauea in Hawaii, and they are also found in the Galapagos Islands.

Composite Volcanoes

Mount Fuji in Japan

Composite Volcanoes are also known as Stratovolcanoes. Typically, several thousand meters high and up to 10 km wide, with steep slopes of up to 35°, these towering volcanoes are made from alternating layers of lava, ash, and volcanic debris. Because of its high viscosity, this magma doesn't flow easily and gas can get trapped, leading to pressure buildup, and ultimately to powerful, explosive eruptions. These volcanoes are usually found at subduction zones, especially around the “Ring of Fire” in the Pacific Ocean. Examples include Mount St. Helens in Washington State in the US, Mount Fuji in Japan, and Mount Vesuvius in Italy.

Cinder Cones

Cinder Cones (or Pyroclastic Cones) are the small volcanoes, only about a few hundred meters wide, with only a few of them higher than 200 meters. With slopes steeper than 20°, these conical volcanoes are built from volcanic material like ash, cinders and volcanic rocks that are ejected during gas-charged early stages of eruptions. Their eruptions can be explosive, blowing lava into the air, where it cools and solidifies, falling back as cinders. These eruptions are usually short-lived, and the cones often form in just weeks or months, but they are also easily eroded. You’ll find them around the world, including Eve Cone in Canada and Paricutin in Mexico.

Lava Domes

While not technically a type of volcano, Lava Domes are worth mentioning. They are mounds of highly viscous lava that flows slowly from a vent and piles up near the surface, as it is too thick to go any further. Examples of lava domes include Novarupta Dome in Alaska and the Tower of Pelée in Martinique.

What’s your favorite volcano or eruption story?