Volcanoes

Watch footage of Kilauea in Hawaii and you will see glowing rivers of lava with photographers walking alongside them. Watch footage of Mount St. Helens in 1980 and you will see the top 400 meters of a mountain disappear in seconds. Both are volcanoes. The difference comes down to the magma, and two things about the magma control everything.

The first is silica, the silicon-and-oxygen compound that makes up most rock. Silica makes magma thick. Basaltic magma is low in silica, so it has a low viscosity and flows easily, like warm syrup. Rhyolitic or granitic magma is high in silica, so it is stiff, sticky, and barely flows at all. The second ingredient is dissolved gas, mostly water vapor and carbon dioxide. It is held in the magma under pressure the way carbonation is held in a sealed bottle of soda.

Put the two together and the result follows. In runny, low-silica magma, gas bubbles rise and escape gently. The eruption is effusive, which means lava pours out and builds land. In thick, high-silica magma the gas cannot escape. Pressure builds until the magma shatters all at once, blasting out ash, rock fragments, and gas in an explosive eruption. It works like a shaken bottle of soda. The more easily the liquid pours, the quieter the opening.

Introduction to Volcanoes

Volcanoes are not scattered randomly. They form in exactly three settings, and two of them you already know from the plate boundaries chapter.

Most of the famous, dangerous volcanoes sit above subduction zones. As the sinking slab heats up, water baked out of it lowers the melting point of the mantle above. That makes magma, which rises through the plate on top. The magma picks up silica on the way up, so subduction volcanoes tend to be explosive. Line them up and you get the volcanic arcs around the Pacific, including the Andes, the Cascades, Japan, Indonesia, and the Aleutians. Together they are called the Ring of Fire.

The most productive volcanoes, though, are ones almost no one ever sees. The mid-ocean ridge system erupts more lava than every other volcano on Earth combined, quietly, underwater, as runny basalt filling the gap between separating plates. Iceland is that process poking above the waves.

The third setting has nothing to do with plate boundaries. These are hot spots, and they get their own section below. As for New York, there are no active volcanoes here and no magma anywhere below us. But ancient volcanic rocks in the Hudson Highlands and the Adirondack region record volcanic arcs from hundreds of millions of years ago. Those arcs existed before the collisions that built the Appalachians.

A volcano's shape tells you what kind of magma built it. Shield volcanoes are broad with gentle slopes, built up flow by flow from runny basaltic lava that spreads for kilometers before it hardens. Mauna Loa in Hawaii is the classic shield volcano, and measured from its base on the seafloor it is the largest mountain on Earth by volume. Composite volcanoes, also called stratovolcanoes, are the steep cones you see on postcards. They form at subduction zones from alternating layers of thick lava and explosive ash. Fuji, Rainier, St. Helens, and Vesuvius are all composite volcanoes. Cinder cones are the smallest. They are steep piles of hardened lava fragments from a single short eruption, and they often grow on the sides of larger volcanoes.

One more landform hides in plain sight. When a huge eruption drains a magma chamber, the ground above can collapse into it. That leaves a broad basin called a caldera. Crater Lake in Oregon fills one. Yellowstone sits in one so large that scientists only recognized it from aerial surveys. Visitors stand inside the volcano without ever seeing a mountain.

Hawaii sits in the dead center of the Pacific Plate, thousands of kilometers from the nearest boundary. Its volcanoes exist because of a hot spot. A rising column of unusually hot mantle rock, called a mantle plume, melts through whatever plate happens to be overhead. The plume stays roughly fixed while the plate above it drifts, so instead of one volcano, a hot spot manufactures a conveyor belt of them.

The result is a chain of volcanoes that age in one direction. The island above the hot spot is active. The islands that have drifted past it are extinct, eroding, and sinking, each older than the last. The Hawaiian island chain records the Pacific Plate's motion so faithfully that geologists read the plate's speed and direction straight off the island ages. A bend in the older, submerged part of the chain even records a moment when the plate itself changed direction.

Hot spots work under continents too. The Yellowstone hot spot has burned a track of ancient calderas across Idaho's Snake River Plain as North America drifted southwest over it. The track ends at the caldera the park sits in today.

You control the magma. Set the silica content and the trapped gas, and watch the volcano's shape and eruption style respond. Try to build a Hawaiian shield, then a Cascade composite, then push both sliders to the top.

Rank volcanic hazards by how they look in movies and lava wins. Rank them by what actually harms people and lava comes last. Lava flows move slowly enough to walk away from, and they destroy property rather than lives. Volcanic ash is far more serious. It collapses roofs under its weight, chokes engines and lungs, and grounds aircraft across whole continents. Lahars are volcanic mudflows of ash and meltwater. They race down river valleys long after an eruption ends, and they are the reason communities below Mount Rainier run evacuation drills. The deadliest hazard is the pyroclastic flow, an avalanche of searing gas, ash, and rock that pours down a volcano's side faster than a car on a highway. One destroyed Pompeii. Another killed nearly 30,000 people at Mount Pelee in 1902 in a matter of minutes.

Here is the good news, and it is a real difference from earthquakes. Volcanoes almost always give warning signs first. Rising magma shows up as swarms of small shallow earthquakes, swelling of the ground surface, and changes in the gases leaking from the summit. Monitoring those signals works. The forecast before Mount Pinatubo's huge 1991 eruption moved tens of thousands of people out of the way in time. Earthquakes cannot be predicted. Volcanoes, watched carefully, usually can be.

Scientists watch volcanoes largely by listening for small earthquakes beneath them. Below is that idea running on live data. It shows every earthquake recorded in the past seven days beneath three famous volcanic regions, counted straight from the USGS feed. Hawaii normally has dozens of tiny quakes in a week, and that steady background is healthy. What monitors watch for is change, such as a sudden swarm, or quakes moving toward the surface.

• Mineral Composition of Igneous Rocks (page 14). This chapter in chart form. The left side of the chart is felsic: high silica, light-colored, lower density, granitic, the explosive end. The right side is mafic: low silica, dark, denser, basaltic, the runny end. Practice placing basalt, granite, rhyolite, and gabbro, and connect each to the eruption style its magma would produce.
• Locations of Selected Hot Spots (page 12) and Global Tectonic Activity (page 13). Find the hot spots, including Hawaii and Yellowstone, and notice which volcanic regions sit on convergent boundaries versus divergent ones.
• Model of Earth's Interior Structure (page 11). Where the melting starts, and why the mantle beneath a hot spot can rise.

Download the current tables from NYSED (2026 Revised Edition, used from the 2026-27 school year): Reference Tables for Earth and Space Sciences.

• Silica and gas control eruptions. Low-silica basaltic magma flows quietly; high-silica magma traps gas and explodes.
• Volcanoes form in three settings: subduction zones (explosive arcs), divergent boundaries (quiet basalt, mostly underwater), and hot spots (mid-plate chains).
• Shape records magma: broad shields from runny lava, steep composites from lava-and-ash layers, small cinder cones from single eruptions, calderas from collapse.
• A hot spot stays fixed while the plate drifts, so island chains like Hawaii age in the direction of plate motion and record the plate's speed.
• Ash, lahars, and pyroclastic flows are the real killers; lava mostly destroys property.
• Unlike earthquakes, eruptions usually give warnings: quake swarms, ground swelling, and gas changes make monitoring and evacuation possible.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.