Volcanoes eject molten rock or magma that formed deep within the Mantle. When the magma emerges from the volcano it is now called lava. Collectively, anything solid ejected from a volcano is called tephra.
A volcano can explode violently (like Mt. St. Helens or Mt. Vesuvius) or it can erupt quietly (like Kilauea in Hawaii). The factors that determine whether a volcano will be violent or quiet are: Viscosity Dissolved gases
VISCOSITY Viscosity is a substance’s resistance to flow. Maple syrup is more viscous than water.
Fluids with high viscosity flow slower than fluids with low viscosity. Low viscosity liquids flow faster. Viscosity is affected by temperature. As lava cools down it becomes more viscous and slows down.
The magma that flows from volcanoes contains rock/minerals. If the magma is high in silica, the higher the viscosity and the less the lava flows. The lower the silica level, the lower the viscosity and the more easily the lava flows.
Sooooooo………… High Silicates = High Viscosity Low Silicates = Low Viscosity
DISSOLVED GASES Many different gases can be ejected from a volcano into the atmosphere. The gases provide the force to eject the tephra from the central vent. Some common gases are: water vapor (H 2 O), carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), hydrogen chloride (HCl), hydrogen fluoride (HF)
The more viscous (thicker) the magma, the slower the gases can rise through the central vent and, therefore, the more pressure that builds and the more explosive the eruption.
Soooooooo……. Thicker lava (more viscous) = more pressure building up = more explosive eruption
Pyroclastic Flows Pyroclastic Flows are a special type of volcanic eruption that tends to kill a lot of people. They are fast-moving currents of hot gas and tephra (rock) which travel away from a volcano at speeds of up to 450 miles/hour. A pyroclastic flow can reach temperatures of 1800 ⁰ F. Pyroclastic flows normally hug the ground and travel downhill.
There are four main types of volcanoes: Stratovolcano/composite Cinder cones Shield volcanoes Lava domes
Stratovolcano/Composite Volcanoes Stratovolcanoes are one of the most common types of volcanoes in the world. Stratovolcanoes are tall and conical with many layers of hardened lava, tephra and volcanic ash.
Stratovolcanoes are tall and steep. They have periodic, explosive eruptions. These volcanoes are tall and steep because they erupt viscous (thick) lava that cools and hardens before is spreads very far.
A list of Stratovolcanoes Mammoth Mountain, CA (dormant) Mt. Fuji, Japan Mt. Pinatubo, Phillipines Mt. Etna, Sicily Mt. Shasta, CA (dormant) Mt. Unzen, Japan Mt. Ranier, WA Mt. St. Helens, WA Krakatoa, Indonesia Mt. Vesuvius, Italy
Shield Volcanoes Shield volcanoes have shallow-sloping sides (looking like a shield from the side. These volcanoes form from fluid lava that can travel for long, long distances before hardening. This leaves a large volcano with a small slope.