What is a Volcano?
Volcano - A Volcano is an opening in
the Earth’s surface through which
molten material or volcanic gases are
erupted.
A volcano can either be a classic volcanic cone…..
Mt. St. Helens, pre-1980
Mount Ranier
a crack in the ground, through which lava
erupts……
Or a simple volcanic vent.
How Magma Rises
Where are volcanoes located?
Major Volcanoes Around the World
Volcanic Settings
How and Where Volcanoes Form
Volcanic activity takes place primarily at subduction boundaries,
1. Water in the subducted rock is
released into the asthenosphere.
4. Some of the magma reaches
Earth’s surface, and volcanoes
form on the overriding continental
plate.
Oceanic
lithosphere
Asthenosphere
2. The water lowers the melting temperatures of
materials in the asthenosphere, leading to magma
formation.
Continental
lithosphere
3. The magma
is less dense
than its
surroundings,
so it rises.
OceanContinental
Subduction
Pacific
Northwest
Volcanoes
OceanOcean
Subduction
OceanOcean
Subduction
Diverging Boundary Volcano
Hot Spot Volcano
How and Where Volcanoes Form
Magma and volcanoes also form at
hot spots.
Kauai
Direction of
Plate
Movement
Oahu
Molokai
Lanai
Maui
Hawaii
The Hawaiian Islands formed over a hot spot.
Major Hotspots of the World
Plate Tectonics and Magma Generation
Tectonic Settings and Volcanic Activity
Volcanoes are classified as
either Active, Extinct or
Dormant
Active
Mt. Ranier
Dormat
Vesuvius
Extinct
Magma and Erupted Materials
Parameters of volcanism:
1.Viscosity
2.Chemistry of Magma
3.Gases within the magma
4.Water content of magma
Magma and Erupted Materials
Magma and Erupted Materials
VOCABULARY
viscosity
lava
pahoehoe
aa
pillow lava
pyroclastic material
pyroclastic flow
Viscosity: Viscosity is a
measure of how thick (viscous)
and sticky a liquid is.
Types of Magma
Felsic Magma
Characteristics:
-light in color
-high viscosity: means it is very
thick and flows very slowly.
-Associated with composite
type volcanoes and continential
hotspots
-Found along continental-ocean
subduction boundaries.
Mafic Magma
Characteristics:
-dark in color
-low viscosity: means
it is thinner and flows
much faster
-Associated with
shield volcanoes, rift
zones, and oceanic
hotspots
-Found at divergent
boundaries and
oceanic hotspots
Gases in Magma
Water Vapor
CarbonDioxide
Carbon
Monoxide
SulfurDioxide
HydrogenSulfide
Types of Lava
Pahoehoe Lava
Aa Lava
Aa Lava
Pillow Lava
Types of Lava Fragments
Ash
Ash Layers
Pyroclastic Rock- Tephra
Pyroclastic Rock- Pumice
Cinder
Bombs
Blocks
Blocks
Volcanic Blocks
Pyroclastic Flows
There are three types of volcanic
cones:
Cinder – explosive eruptions, small but
• ______
steep slopes, pyroclastics.
Shield – non-explosive eruptions, fluid
• _______
basaltic lava, gentle broad slopes.
Composite – alternating between lava and
• _________
pyroclastics, explosive and non-explosive
eruptions, steep and tall.
Volcano Types
Volcanoes
Volcanic Landforms
VOCABULARY
shield volcano
cinder cone
composite volcano
lahar
caldera
lava plateau
A volcano’s shape and structure depend on how it
erupts and what materials are released.
Shield volcanoes are formed by basaltic lava that
flows long distances before hardening.
Shield
Volcano
Shield and Composite Volcanoes
Volcanoes
Volcanic Landforms
Cinder cones are formed when molten lava is thrown into the air from a
vent and breaks into drops. These drops harden into cinders that form a
steep cone around the vent.
Cinder Cone
Volcanoes
Volcanic Landforms
Composite volcanoes are formed by layers of
pyroclastic materials and lava that have erupted in
the past.
Composite
Volcano
Composite volcanoes
Debris Avalanche and Eruption of Mount St. Helens,
Washington
Nuee ardente: pyroclastic flow, of searing superheated
gas and incandescent volcanic ash and dust
Mount Pelee, on the Carribean island of Martinique, 1902 eruption. All but 2 of the more than
20,000 people in the town of St. Pierre were killed.
Lava Plateau
Volcanoes
Volcanic Landforms
A caldera is a large crater-shaped basin that forms
when the top of a volcano collapses.
Formation of Crater Lake
EFFUSIVE ERUPTIONS
• Generally at hots spots, spreading centers
• Mantle comes directly to surface
• Hot lava; low viscosity, very mafic, flows
easily, gases escape easily
• Forms shields, flood basalts
EXPLOSIVE ERUPTIONS
• Found at subduction zones
• Magma low temp (800 degrees C), high
viscosity, does not flow easilty, more felsic
mineralogy, gases trapped, hard to predict
explosions
• Forms composite volcanoes, cinder cones,
calderas, aerial bombs, nuee ardente gas
flows, very destructive