Transcript Volcanoes - Helena High School

What is a volcano?
Melted rock from subduction
Zones and mantle plumes
called Magma, rises to the
surface through an opening
(becoming Lava).
Igneous rocks “Born of Fire”
form from cooled magma.
Distribution of Volcanoes
Pacific Plate forms the “Ring of Fire”
Cascade Range Volcanoes
Where do Volcanoes Occur?
Divergent boundaries: rift valleys and midocean ridges.
Convergent Oceanic-Continental
Convergent Oceanic-Oceanic.
Intraplate Volcanism: occurring in areas within the
plate due to hot spots found under the continents.
Ex. Yellowstone and Hawaii.
Types of Magma
Mafic
• Non Violent Eruption
• Low Amounts of Silicates
(SiO2)
• Less Viscous (warm honey)
• Oceanic crust origin.
• Ex. Hawaii Hot Spot
• Less dissolved gas
Felsic
• Violent Eruption
• High Amounts of Silicates
(SiO2)
• Viscous (cold honey)
• Continental crust origin
• Ex. Mt. St. Helens
• More dissolved gas
Types of Magma
• Basaltic magmas (MAFIC)– Divergent Boundaries and
Convergent oceanic-oceanic
• Andesitic magmas (INTERMEDIATE) Convergent
oceanic-continental
• Rhyolitic magmas (FELSIC) Varies in origin.
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Volcano Characteristics
Classifying Volcanoes
• An active volcano is one that is currently erupting or
has erupted recently (in geological terms).
• A dormant volcano is one that has not erupted lately
but is considered likely to do so in the future.
• An extinct volcano is one that has not erupted for a
very long time and is considered unlikely to do so in the
future.
Volcano Types
•Shield volcanoes - very large
•Composite (or Stratovolcano) – mid sized
•Cinder cones - small
Composite volcanoes (or
stratovolcanoes)
•FELSIC- Erupt violently
ejecting igneous rocks called
pyroclastic material.
•Alternating violent eruptions
and non violent eruptions.
Cinder cones
• Tend to be smaller
than other types of
volcanoes.
• Made up of cinders.
Cinder Cone
Shield volcanoes
• MAFIC - Non violent
eruptions
• have broad bases and
gently sloping sides.
Example: Mt. Loa, Hawaii
Lava Flows
Lava flows on land:
1. Pahoehoe lava less viscous (runny)
lava that cools as smooth, ropelike
surfaces.
2. Aa lava more viscous (thick) lava that
cools as rough, jagged surfaces.
Lava that cools underwater:
Pillow lava has a rounded, “pillow-like”
form with a hard crust.
Pahoehoe (Ropy) Lava Flow
Slow-Moving Aa Flow
Mount Kilauea
Pyroclastic Materials
(ejected into the air).
• Ash -the smallest pieces with a
diameter or less than 2 mm.
• Lapilli -Intermediate sized pieces
with a diameter from 2 – 64 mm.
• Bombs -the largest sized pieces
with a diameter greater than 64 mm.
Pyroclastic Flow
The eruption of Mount Vesuvius
in A.D. 79 produced a pyroclastic
flow (Pyroclastic material and
gas) that buried the Roman city
of Pompeii.
Mt. Vesuvius & Pompeii
Hazards:
1. Lava Flow
2. Lahar (debris
flow)
3. Pyroclastic
Flow (gas & ash)
4. Landslides
5. Lava Bombs
6. Acid Rain
Volcanoes & the Atmosphere
Volcanic eruptions are
thought to be
responsible for
• global cooling; and
• Mass extinction.
Igneous Rock
Plutonic
Igneous Rock that cools
underground.
Larger crystals due to
slower cooling rate.
Volcanic
Igneous Rock that cools
above ground.
Smaller crystals due to
faster cooling rate.
Types of Igneous Plutons
Batholiths
Sills
Sill