Chapter 7 VOLCANOES
Sect. 1 & 2
Volcanism• Mov’t of magma toward or onto the
Earth’s surface magma rises cuz its less dense pushing its way to the surface
• Magma on earth’s surface = Lava• Vent = opening in earth’s surface where
magma flows through• Volcano = structure formed from vent &
material builds around it
Major Volcanic zones
• On divergent & convergent boundaries
1. Subduction Zones
2. Mid-Ocean Ridges
3. Hot Spots
Subduction Zones• A subducted plate is subjected to heat & friction & is forced
downward causing it to remelt & become magma• This magma will be forced to surface and forming volcanic
mountains along continental plate
Subduction Zones• Ring of Fire is a result of plates being subducted
• There can also be collision of 2 oceanic plates forming Island Arcs or volcanic islands (ex. Japan)
Mid-Ocean Ridges• Divergent boundary• Magma rises up through rift• Emerging lava forms new ocean floor
• Iceland is exposed part of ridge & is covered with fissures (cracks)
Hot Spots• Magma comes to the surface from within a plate• Hot spot stays put but plate is in constant
motion forming volcanic islands
Hawaiian Islands
Galapagos Islands
Volcanic Eruptions
Windows to Earth’s Interior
Ability to see firsthand materials from mantle
3 General Types of Lava
1. Mafic (basaltic) low silica content and low in water vapor & CO2 dark color
2. Felsic (granitic) high silica & high in water vapor & CO2
light in color
3. Faults between mafic & felsicrange in composition
Kinds of Eruptions• Type of lava (mafic/felsic) determine
how explosive eruption will bedemo
• Oceanic Volcanoes: produce mafic lava very thin
flow easilylow gas erupt quietly
Kinds of Eruptions• Felsic lava (continental
volcanoes) thick lavalarge amt gases
w/lava boil out of vent explosively solid particles fly out w/ magma
Volcanic Rock Fragments• Pyroclastics: large fragments blown
from the cone of felsic volcanoes.
< 2 mm = volcanic ash
< .25 mm = volcanic dust
< 64 mm = lapilli (lil’ stones)
Three types of Volcanoes• Shield Volcano
covers wide areaquiet eruptionsslow build up around vent
Three types of Volcanoes• Cinder cone
made up of solid fragmentsSteep slopesrarely highvery explosive
Three types of Volcanoes• Composite Cone
Three types of Volcanoes• Composite Cone
formed by alternating explosive & quiet eruptionsdevelop high volcanic mountains
Crater vs. Caldera
• Crater = funnel shaped pit at top of vent; formed when material is blown from vent during explosions
• Caldera = large basin shaped depressions formed either by explosions or cones collapsing
Predicting Volcanoes
• seismographs• study past history of volcanoes• bulging of volcanic slopes• changes in gases given off
Extraterrestrial Volcanism
Many planets & moons in the solar system were and are volcanically
active
Extraterrestrial Volcanism
The MoonCovered by Basaltic flows; no longer active; however, no evidence of plate tectonic activity
Extraterrestrial Volcanism
MarsNumrous volcanoes & features; biggest one is shield volcano called Olympus Mons
Extraterrestrial Volcanism
IOMoon on Jupiter; shows 9 active volcanoes erupting on Io