4.2 Volcanic processes and types of eruptions

Volcanic Materials

Magma and Lava

Magma is molten rock beneath Earth's surface that contains dissolved gases and sometimes crystals. When magma erupts onto the surface, it's called lava. As lava cools, it solidifies into igneous rocks like basalt or obsidian.

The composition of magma controls how it behaves during an eruption. The key factor is silica content:

• Felsic magma has high silica content, making it thick and viscous. It traps gases easily, which builds pressure and leads to explosive eruptions.
• Mafic magma has low silica content, making it runny and fluid. Gases escape more easily, so eruptions tend to be gentler and effusive.

Pyroclastic Material and Volcanic Gases

Pyroclastic material refers to fragments of rock ejected during explosive eruptions. These fragments are classified by size:

• Ash: particles smaller than 2 mm
• Lapilli: fragments 2–64 mm
• Bombs and blocks: larger than 64 mm (bombs are molten when ejected; blocks are solid)

Pyroclastic density currents (often called pyroclastic flows) are fast-moving, ground-hugging avalanches of hot ash, pumice, rock fragments, and gas. They're among the deadliest volcanic hazards because of their speed and extreme temperatures.

Volcanic eruptions also release large volumes of volcanic gases, primarily water vapor ($H_2O$), carbon dioxide ($CO_2$), and sulfur dioxide ($SO_2$). These gases can pollute the air and affect climate on a global scale. Sulfur dioxide injected into the stratosphere reflects sunlight and can cause temporary global cooling. When volcanic gases mix with atmospheric moisture, they can also produce acid rain.

Volcanic Landforms

Stratovolcanoes and Shield Volcanoes

Stratovolcanoes (also called composite volcanoes) are steep-sided, cone-shaped mountains built from alternating layers of lava, ash, and pyroclastic material. They form at subduction zones, where one tectonic plate dives beneath another, and they tend to produce explosive eruptions. Mount St. Helens and Mount Fuji are classic examples. These volcanoes can reach several thousand meters in height.

Shield volcanoes have a broad, gently sloping profile built almost entirely from repeated flows of fluid mafic lava. They're commonly associated with hotspots and produce effusive eruptions. Mauna Loa in Hawai'i is the largest active shield volcano on Earth, standing over 9 km tall when measured from its base on the ocean floor to its summit. Even Olympus Mons on Mars is a shield volcano, showing that this landform isn't unique to Earth.

Calderas

A caldera is a large, roughly circular depression that forms when a volcano's summit collapses after a massive eruption empties or partially empties the magma chamber below. Calderas can be several kilometers across and hundreds of meters deep.

Yellowstone and Crater Lake (Oregon) are well-known examples. Crater Lake formed about 7,700 years ago when Mount Mazama collapsed after a huge eruption, and the depression gradually filled with rain and snowmelt. Over time, calderas may also fill with new volcanic material if eruptions resume.

Eruption Types

Effusive and Explosive Eruptions

The two broad categories of eruptions are effusive and explosive, and the difference comes down to magma composition and gas content.

Effusive eruptions involve the relatively gentle outpouring of fluid lava with little explosive activity. They're typically fed by low-viscosity mafic magma, which allows dissolved gases to escape without building dangerous pressure. Shield volcanoes like Kīlauea in Hawai'i and many Icelandic volcanoes erupt this way. The lava flows can travel long distances and cover large areas, but they rarely kill people because they move slowly enough to evacuate.

Explosive eruptions happen when viscous, gas-rich felsic magma reaches the surface. Because the thick magma traps dissolved gases, pressure builds until the magma fragments violently. This produces:

• Tall eruption columns that can reach the stratosphere
• Pyroclastic density currents
• Widespread ash fall

Mount Pinatubo (1991) and Krakatoa (1883) are dramatic examples of explosive eruptions. Pinatubo's eruption injected so much $SO_2$ into the stratosphere that global temperatures dropped by about 0.5°C for the following year.

Volcanic Hazards

Lahars and Pyroclastic Hazards

Lahars are volcanic mudflows or debris flows. They form when volcanic material mixes with water, often from the rapid melting of summit snow and ice during an eruption. Lahars can also be triggered by heavy rainfall on loose volcanic deposits long after an eruption ends.

These flows travel at high speeds along river valleys and can reach distances far from the volcano itself. The 1985 eruption of Nevado del Ruiz in Colombia generated lahars that buried the town of Armero, killing over 23,000 people. It remains one of the deadliest volcanic disasters in modern history.

Pyroclastic density currents can exceed speeds of 100 km/hr and reach temperatures of several hundred degrees Celsius, destroying everything in their path. The 79 CE eruption of Vesuvius buried Pompeii under pyroclastic material. Even ash fall alone causes serious problems: the 2010 Eyjafjallajökull eruption in Iceland disrupted European air travel for weeks, and heavy ash accumulation can collapse roofs, contaminate water supplies, and cause respiratory illness.

Volcanic Gases and Lava Flows

Volcanic gases are a quieter but still dangerous hazard. High concentrations of $SO_2$ cause respiratory problems and contribute to acid rain. During Kīlauea's 2018 eruption, volcanic smog ("vog") created serious air quality issues across Hawai'i.

Carbon dioxide is particularly insidious because it's colorless and odorless, and being denser than air, it can accumulate in low-lying areas. In 1986, a massive release of $CO_2$ from Lake Nyos in Cameroon flowed downhill and asphyxiated over 1,700 people and thousands of livestock in nearby villages.

Lava flows destroy property and infrastructure in their path, but the speed and extent of damage depend on the lava's composition and the volume erupted. During the 1973 eruption of Eldfell in Iceland, residents used seawater to cool and slow advancing lava flows, successfully saving the town's harbor. Similar diversion efforts have been attempted on Mount Etna in Sicily (1992). While lava flows cause enormous property damage, they rarely cause deaths because most flows move slowly enough for evacuation.