Download Volcanoes PPT - Van Buren Public Schools

VOLCANOES
Volcanoes & Plate Boundaries
• Intraplate Volcanism
– Activity that occurs within a
tectonic plate away from
plate boundaries.
• Most intraplate volcanism
occurs where a mass of
hotter than normal mantle
material called a mantle
plume rises toward the
surface.
• The activity forms localized
volcanic regions called hot
spots.
• An example is the Hawaiian
Islands
Magma vs. Lava
• Magma
– Molten rock beneath the Earth’s surface.
• Forms when solid rock partially melts in the crust and
upper mantle.
• Complex mixture that contains partly melted mineral
crystals, dissolved gases, and water.
• Formation of magma depends on heat, pressure, and
water content.
• Lava
– Molten material that reaches the Earth’s surface.
– There are two types of lava:
• 1. Basaltic
• 2. Rhyolitic
Magma vs. Lava
• Basaltic Lava
– Has low amounts of silica
– Causes lava to be very fluid (runny)
• Temperature creates 3 types of Basaltic lava:
– Pahoehoe
• Hotter, fast-moving (very runny)
• Forms smooth lava flows.
• Resembles braids in ropes
– Aa
• Cooler, slower-moving lava
• Forms rough, jagged blocks with sharp, spiny projections.
Basaltic Lava Examples
Aa Lava Flow
Magma vs. Lava
• Pillow Lava (typical of submarine mid-ocean
ridge basalts extruded into water and quickly
chilled)
Magma vs. Lava
• Rhyolitic Lava
– Rich in silica
– Causes lava to be very thick
• Traps varied amounts of dissolved gasses under
pressure in molten material.
• Produces violent explosive eruptions.
– Mount St. Helens in Washington State.
Magma vs. Lava
• 3rd type of lava is
Andesitic Lava
– Produces blocky lava
• Smooth-sided, angular
fragments
– Has a higher viscosity
• Intermediate between
basaltic and rhyolitic lavas.
– Typically found at
composite volcanoes
Factors Affecting Eruptions
• Factors that determine the violence of an
eruption
– Composition of the magma
– Temperature of the magma
– Dissolved gases in the magma
• Viscosity
– Viscosity is the measure of a material's resistance
to flow.
Factors Affecting Eruptions
• Viscosity
– Factors affecting viscosity
• Temperature (hotter magmas are less
viscous)
• Composition (silica content)
–High silica—high viscosity
(e.g., rhyolitic lava)
–Low silica—more fluid (e.g., basaltic lava)
Factors Affecting Eruptions
• Dissolved gases
– Mainly water vapor and carbon dioxide
– Gases expand near the surface
– A vent is an opening in the surface of Earth through
which molten rock and gases are released.
– Provide the force to extrude lava
– Violence of an eruption is related to how easily gases
escape from magma
– Gases escape easily from fluid magma.
– Viscous magma produces a more violent
eruption.
Volcanic Material
• Pyroclastic Materials
– Pyroclastic materials is the name given to
particles produced in volcanic eruptions.
– The fragments ejected during eruptions
range in size from very fine dust and
volcanic ash (less than 2 millimeters) to
pieces that weigh several tons.
Volcanic Material
• Pyroclastic Materials
– Types of pyroclastic material
• Ash and dust—fine, glassy fragments
• Pumice—frothy, air-filled lava
• Lapilli—walnut-sized particles
• Cinders—pea-sized particles
• Particles larger than lapilli
– Blocks—hardened lava
– Bombs—ejected as hot lava
Volcano Hazards
• Lava flows
– Destroys everything in their
path.
• Volcanic Ash
– Can bury widespread areas
under thick deposits.
– Clog car and airplane
engines.
– Unsafe to breath
– Can affect climate globally
depending on size of
eruption.
Volcano Hazards
• Pyroclastic Flows
– Scorching mixture of glowing
volcanic particles & gases.
– Sweeps down the sides of a
volcano like an avalanche.
• Lahars
– Water-soaked volcanic rock
and ash slide rapidly down
hill.
– Caused by rain or melting of
Ice and snow on slopes of
volcano.