Chapter 1C Main Ideas

1. Volcano anatomy and explosivity

Volcanoes are places where molten rock erupts or flows out onto Earth’s surface through a vent or fissure which are fed through conduits from a magma chamber beneath the volcano. Sometimes a volcano collapses into empty space in the magma chamber beneath, forming a caldera.

Eruptions can be explosive or effusive. The explosivity of an eruption is controlled by the magma type: viscous, more silica rich magma (andesitic and rhyolitic) that traps more gas is more explosive than runny, silica poor magma (basaltic) where the gas easily escapes.

2. Volcano types and other eruption landforms

The three main volcano types are shield volcanoes (built by runny basaltic lava flows), stratovolcanoes (alternating layers of more viscous lava flows and pyroclastic material), and cinder cones (rubbly piles of pyroclastic material). Other eruption landforms include lava domes (built by very sticky rhyolitic lava flows and typically associated with stratovolcanoes) and flood basalts (fissure eruptions of massive volumes of basalt).

3. Eruption products and hazards

Volcanoes produce lava flows, gas, and pyroclastic material (tephra). The features created by a lava flow depend on whether the lava is thin and runny (basaltic with low gas content) or thick and sticky (andesitic or rhyolitic with high gas content). Gases include water vapor, carbon dioxide and sulfur dioxide. Pyroclastic material is classified according to size. Ash is less than 2 mm in diameter, lapilli is between 2 mm and 64 mm, and blocks and bombs are larger than 64 mm.

The hazards associated with these products include threats from lava flows themselves, air quality/toxicity from gases, pyroclastic flows (fast flowing turbulent mixtures of gas and ash), lahars (volcanic mudflows), ashfalls, and climate disruptions.

4. Plate tectonics and volcanism

Volcanism is closely related to plate tectonics. Most volcanoes are associated with convergent plate boundaries (at subduction zones), where flux melting is responsible for creating magma. These volcanoes are dominantly stratovolcanoes. Volcanic activity also occurs at divergent boundaries from decompression melting creating basaltic magmas and pillow lavas and shield volcanoes form. Oceanic hotspots also create shield volcanoes. Continental hotspots can have varied styles of volcanism including major rhyolitic explosive eruptions like Yellowstone and flood basalts like the Columbia River Basalts. Continental rifting also has a wide range of volcanism including shield volcanoes, stratovolcanoes and flood basalts. The variations in these two areas are a result of magma moving through the thick continental lithosphere leading to variable changes in magma composition. 

5. Monitoring eruptions

Geologists monitor specific aspects of volcanic activity to help give warnings for when a volcano is about to erupt. These include changes in the shape of the landscape (ground deformation) as magma moves into the magma chamber, increased seismic activity located under the volcano, changes in the composition or amount of gas being emitted from the volcano.

6. Assessing volcanic risk

Geologists use historical records and evidence from the rock record to make volcanic hazard maps which tell the public what volcanic hazards are likely to affect an area. This information can be used to help make evacuation plans and inform emergency response plans.