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BHS Earth and Space Science » Density is the key to understanding volcanism. » Remember mantle convection from two weeks ago. » Hotter, less dense rock within the mantle rises and sometimes finds its way through the lithosphere to the surface. Rut roh. » To do this, it must either melt or break through the crust, which is anywhere from 6-50km thick. » Most volcanoes burst onto Earth’s surface along the boundaries of the planet’s shifting tectonic plates. » Unique types of volcano pop up on different spots on the plates. » Volcanoes can be found: ˃ Where two oceanic plates converge (lots of examples in Indonesia) ˃ Where two oceanic plates diverge (Iceland) ˃ Where a hot blob of magma within the mantle rises to the surface (Hawaiian islands) ˃ Where an oceanic plate subducts under a continental plate (Mt. St. Helens, WA) » Volcanoes can have several shapes: ˃ ˃ ˃ ˃ ˃ Shield Cinder cone Composite (AKA stratovolcano) Lava dome Caldera » The shapes are determined by the magma, as we will see in this week’s lab. Talk about anticipation! » So you live next to a volcano. How silly do you feel? Depends on the type of magma within. » Volcanoes either erupt quietly or explosively. » For a million extra credit points, which one sounds worse?* » *Extra credit will not be awarded. » Shield volcanoes usually erupt quietly. » Named because they are shaped like a warrior’s shield (really). » Form from continual flows of highly fluid (lowviscosity) basaltic lava that cool in thin sheets. » Can be 5-6km in diameter and 500-700m high. » Simplest type of volcano. » Erupt explosively. » Built from particles and blobs of congealed lava ejected from a single vent. » Fragments solidify and fall as cinders around the vent to form a circular or oval cone. » No more than 300m high. » Alternate between explosive and gentle eruptions. » Gentle lava flows build up the volcano into an impressive mountain. » Near the peak, pyroclastic material can form a steep-sided cone. » Can be over 2km high! » Mt Fuji – 16 eruptions since 781. » Form around vents that erupt lava high in silica. » This viscous (sticky) lava doesn’t flow far, so forms a steep-sided, rounded dome. » The dome’s vents may be blocked up by hardened lava. » Pressure of the underlying magma may build, causing an explosion. » An explosive eruption can empty a magma chamber beneath a volcano. » This may cause the roof of the chamber to collapse under its own weight. Stupid gravity. » Leaves a huge crater that may fill in and become a lake, but may erupt explosively again. » Lil YouTube clip