Volcanoes - School District 27J

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Transcript Volcanoes - School District 27J

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