6 Volcanoes and Volcanic Hazards
Learn how magma forms and erupts, how volcanic landforms develop, which hazards eruptions can create, and how monitoring helps communities prepare.
How Volcanoes Form
A volcano is both an opening in Earth's crust through which , gases, and rock fragments reach the surface and the landform built from erupted material. Volcanoes create new land, but eruptions can also threaten people and infrastructure.
forms when rock melts in the mantle or crust. Three processes can promote melting:
: Hot mantle rises, pressure decreases, and rock can melt.
Water addition: Water lowers the melting temperature of rock, a process common above subduction zones.
Heating: Hot material can heat surrounding rock until it melts.
Because is generally buoyant, it can rise through fractures and collect temporarily underground. An eruption may follow if pressure from and gas opens a path to the surface.
Volcanoes are common at divergent plate boundaries, where plates separate and mantle rises, and at convergent boundaries, where a sinking plate releases water that helps melt mantle rock. Volcanoes also form above hot spots within plates. Hawaii is an example.
The key connection is that melting supplies , buoyancy helps it rise, and pressure can drive it toward the surface.
What Controls Eruption Style
An eruption's style depends on composition and temperature, its crystal and gas content, and whether it interacts with water. Silica-rich is generally more viscous, or sticky, than basaltic .
In runny , gas can escape more easily. This favors , which produce flows or fountains. In viscous, gas-rich , pressure can build until it is released explosively. The fragments into , which ranges from fine ash to large blocks. An eruption can shift between styles over time.
Water can intensify explosions in two different ways:
In a phreatic eruption, heated water flashes to steam and blasts apart existing rock; new does not need to reach the surface.
In a phreatomagmatic eruption, interacts directly with external water, fragmenting both and surrounding material.
Together, viscosity, gas escape, and water interaction help explain why some eruptions mainly release flowing while others fragment material explosively.
Landforms Built by Eruptions
Volcanic landforms reflect the materials erupted and how those materials accumulate. Common forms include:
Shield volcanoes: Broad, gently sloping structures built mainly by repeated flows of fluid basaltic . Hawaiian volcanoes are examples.
Stratovolcanoes, also called composite volcanoes: Steep cones formed from alternating flows and deposits of ash and other fragments. Their eruptions can range from quiet emissions to violent explosions.
Cinder cones: Usually small, steep-sided hills made mostly of loose fragments that fall around a vent.
domes: Thick that piles up near a vent instead of flowing far. A collapsing dome can generate dangerous hot currents of gas and fragments.
Calderas: Large depressions formed when the ground collapses after a substantial amount of has been withdrawn from beneath a volcano. They are not simply oversized craters.
Fissures: Elongated cracks that can release along a line and sometimes produce extensive fields.
These categories describe common forms, but a volcanic region can contain several types of vents and landforms.
Volcanic Hazards and Their Reach
The hazards from an eruption depend on its style, the terrain, weather, and where people live. Different hazards can affect areas in different ways:
flows can burn, bury, or sever roads and utilities. Their paths can sometimes be anticipated, but property in their way may be difficult to protect.
Ash fall can reduce visibility, irritate lungs and eyes, contaminate water, damage machinery, burden roofs, and disrupt aviation. Ash can travel far downwind.
are fast, ground-hugging mixtures of hot gas, ash, and rock that can devastate areas in their paths.
Lahars are rapid flows of water mixed with volcanic sediment. They can travel along river valleys, including after an eruption, and threaten communities far from a volcano's slopes.
Volcanic gases, including sulfur dioxide and carbon dioxide, can harm health; some gases can accumulate in low-lying areas.
Landslides and debris avalanches can occur when unstable volcanic slopes collapse during unrest or an eruption. If a large collapse enters water, it may produce a tsunami.
No single hazard defines every volcano. Even a volcano with relatively gentle eruptions can produce dangerous gases or water-driven explosions. Understanding where hazards may travel is important because risks can extend beyond the volcano itself.
Monitoring and Preparedness
Scientists assess volcanic activity by combining multiple kinds of evidence and comparing observations with a volcano's past behavior:
Seismometers detect earthquakes and volcanic tremor as and gas move or rock fractures.
Ground-deformation measurements, including GPS and satellite radar, track swelling, sinking, or shifting.
Gas measurements detect changes in the amount or composition of gases released at vents and other openings.
Thermal and visual observations identify changes in heat, steaming, , or ash emissions.
Geologic studies and hazard maps document past eruptions and show where flows, ash, or lahars may travel.
Changes in these observations can indicate rising unrest, but unrest does not always lead to an eruption. Monitoring also cannot determine the exact time and size of every eruption. It supports assessments and warnings; communities also need hazard maps, alert systems, evacuation plans, and clear public guidance.
Takeaway: Monitoring brings together different clues to assess changing activity, while preparedness helps communities respond to hazards that cannot be predicted precisely.