Volcanoes shape Earth's surface in dramatic ways, from smooth lava flows to explosive ash clouds. Understanding the landforms and materials they produce helps you connect eruption behavior to the landscapes left behind.
Two things control most of what you see at a volcano: the composition of the lava and the style of eruption. Silica-rich magmas tend to build steep structures and erupt explosively, while low-silica lavas spread out gently. These differences explain why volcanic terrains look so different from one place to the next.
Volcanic Landforms
Main volcanic landforms
Lava flows form when molten rock pours out of a vent or fissure and spreads across the landscape. Not all lava flows look the same, though. The two main types you need to know are:
- Pahoehoe lava cools into smooth, ropy, or billowy surfaces. You'll see this at Kilauea, Hawaii.
- A'a lava has a rough, jagged, clinkery surface because it cools faster and has higher viscosity. Craters of the Moon, Idaho, is a classic example.
Lava domes form when viscous, silica-rich lava piles up around a vent instead of flowing away. The result is a steep-sided, rounded or flat-topped mound. Mount St. Helens grew a lava dome inside its crater after the 1980 eruption.
Pyroclastic flows are fast-moving, ground-hugging mixtures of hot ash, pumice, rock fragments, and gas. They can reach speeds up to 100 km/h and temperatures between 100–800°C. These are among the deadliest volcanic hazards because they move too fast to outrun. The 1991 eruption of Mount Pinatubo, Philippines, produced massive pyroclastic flows.
Lahars are volcanic mudflows made of ash, rock debris, and water. The water can come from melted snow, heavy rain, or a breached crater lake. Lahars travel long distances at high speed and cause enormous destruction. The 1985 eruption of Nevado del Ruiz, Colombia, triggered lahars that buried the town of Armero and killed over 23,000 people.
Volcanic ash and debris
Volcanic ash consists of fine particles (less than 2 mm) of pulverized rock and glass, created when magma or rock is explosively fragmented during an eruption. Wind can carry ash hundreds or even thousands of kilometers. The 2010 Eyjafjallajökull eruption in Iceland shut down European air travel for days because ash damages jet engines.
Beyond aviation, ash causes respiratory problems, contaminates water supplies, and can collapse roofs under its weight.
Two common volcanic rocks form from gas-rich magma cooling quickly:
- Pumice is highly vesicular (full of tiny gas bubbles), light-colored, and low-density. It forms from felsic magma and is famously light enough to float on water. You can find pumice deposits near Mono Lake, California.
- Scoria is also vesicular but denser and darker than pumice. It forms from mafic magma during mildly explosive eruptions and has a rough, cindery texture. Sunset Crater, Arizona, is surrounded by scoria deposits.
Over the long term, pumice and scoria deposits can actually improve soil drainage and fertility. In the short term, though, thick ash and debris smother vegetation and make the land difficult for plants to recolonize.
Factors Influencing Volcanic Landforms
Lava composition and landform morphology
The composition of lava directly controls how it behaves and what it builds.
- Felsic (silica-rich) lavas have high viscosity. They resist flowing, so they pile up into steep-sided lava domes or erupt explosively. Chaitén volcano in Chile erupted felsic lava in 2008, producing a large dome.
- Mafic (silica-poor) lavas have low viscosity. They flow easily and spread out, building gently sloping shield volcanoes through effusive eruptions. Mauna Loa, Hawaii, is a textbook shield volcano.
Viscosity is a fluid's resistance to flow. Three main factors control it:
- Silica content: Higher silica means higher viscosity. This is the biggest factor.
- Temperature: Cooler lava is more viscous. As lava moves away from the vent and loses heat, it thickens.
- Gas content: Dissolved gas can lower viscosity, but trapped gas bubbles in thick lava can increase pressure and lead to explosive eruptions.
The connection between viscosity and shape is straightforward:
- Low-viscosity lavas produce thin, widespread flows and broad shield volcanoes with gentle slopes. Olympus Mons on Mars is an extreme example of a shield volcano.
- High-viscosity lavas produce thick, stubby flows, steep lava domes, and composite volcanoes. Mount Fuji, Japan, is a composite volcano built partly from viscous lava.
Eruption types and resulting landforms
Effusive eruptions involve the relatively gentle outpouring of lava. They're associated with mafic, low-viscosity magmas. Landforms produced by effusive eruptions include:
- Shield volcanoes with broad, gently sloping profiles
- Lava plains and plateaus, like the Columbia River Basalt Province, where repeated eruptions flooded vast areas with lava
- Lava tubes and channels, such as the Thurston Lava Tube in Hawaii, which form when the outer surface of a flow solidifies while lava continues moving inside
Explosive eruptions involve violent fragmentation and ejection of magma and rock. They're driven by felsic, high-viscosity magmas or by magma interacting with water (which flashes to steam). Resulting landforms include:
- Composite volcanoes (stratovolcanoes) built from alternating layers of lava flows, pyroclastic material, and ash. Mount Rainier, Washington, is a classic composite volcano.
- Lava domes formed by extrusion of viscous lava, like Novarupta in Alaska.
- Pyroclastic flow deposits left by surges of hot ash, pumice, and gas. Mount Vesuvius buried Pompeii this way in 79 CE.
- Calderas formed when a volcano's summit collapses after the magma chamber empties. Yellowstone, Wyoming, sits inside a massive caldera from past super-eruptions.