A Volcano’s Anatomy How Volcanoes Work Created when a fissure in the earth turns up towards the surface. The magma moves up the hollow pillar created and out a vent. Lava build-up from eruptions is what makes volcanoes grow. The shape and the size of volcanoes are largely dependent on the type of magma within them.
Not All Magma Is The Same CompositionSilica Content ViscosityGas ContentTendency to Form Pyroclasts Volcanic Landform Basaltic~50%Least1-2%LeastShield volcanoes, basalt plateaus, cinder cones Andesitic~60%Medium3-4%IntermediateComposite cones Rhyolitic~70%Greatest4-6%GreatestPyroclastic flows, volcanic domes Ejected rock fragments
Types of Volcanoes Type of Volcano Description Shield Very shallow slope, typically created from basaltic lava. Often, these come out of the ocean to create islands. (Ex: Hawaii and Iceland) Cinder Cone Formed from ejected lava fragments, steeply sloped. Often the result of a single, long eruption. Usually only erupt once, as the magma pipe often solidifies. Composite Cone Large, made up of alternating layers of lava and ejected volcanic material. Usually have gas-rich and andesitic magma. Have the most explosive eruptions, but the lava only travels a short distance. Composite cones can also produce very dangerous “pyroclastic flows” and “lahars”. The flows are masses of hot gas, glowing ash, and rock fragments rolling down a slope at upwards of 200km/h. Lahars occur when volcanic material mixes with water and creates a dangerous mudflow.
Viscosity “A substance’s resistance to flow.” Ex: Elmer’s Glue is more viscous than water. Magma from sudden eruptions is typically much less viscous than slowly leaked water, therefor making it flow faster. Temperature: The hotter something is, the less viscous it is. As lava cools, it hardens and slows down. The silica content of magma is also directly related to its viscosity. More silica = more viscous.
Dissolved Gases Explosive eruptions can trigger gases trapped in magma to eject molten rock from openings in the surface. Usually water vapor and CO 2. As magma gets closer to the surface, the pressure gets reduced, allowing gas to suddenly be released. Because thinner lava allows these gases to escape more easily, those eruptions are usually much quieter. Thicker lava traps the gases, creating larger and larger bubbles that increase in pressure until they explosively “pop.”
Gases Pressure can trap dissolved gases in molten rock. Gas doesn’t make up much of a magma’s weight. 1 – 6%, max The composition of volcanic gases is important because they have contributed greatly to the gases that make up the atmosphere. The vast majority of volcanic gas’s composition is water vapor.
Pyroclastic Material Pyroclastic material: the fragments and particles ejected during eruptions When basaltic lava leaves a volcano (is “extruded”), gas can shoot lava pretty high into the sky. Some of this material may land near a vent and make cone-shaped structure. This is pyroclastic material. Smaller than 64mm: “lapilli”. Larger: “blocks” when they are hardened. “Bombs” when they are still glowing hot.
Calderas – A large depression in a volcano. They usually form from the collapse of a composite or shield volcano. Necks/Pipes – Pipes are the tube that connects a magma chamber to its vents. If the tube remains standing even after the hill has eroded, it is a neck. Lava Plateaus – Low-viscosity lava that pours out of fissures on a volcano’s side can create plateaus, as opposed to hill shapes.