Transcript Ch 9 Volcanoesx

Chapter 9: Volcanoes
Magma Basics
• Magma: liquid rock below ground
– Primary mineral: Silica
• Liquid magma is less dense than solid
rock. When melted it will rise within
the earth.
• Most abundant elements in earth's crust
and mantle are oxygen and silicon.
Si + O2  SiO2
The three major types of magma are:
Basaltic Magma -- 50% silica
Andesitic Magma -- 60% silica
Granitic Magma -- 70% silica
• Magma contains dissolved gases.
• Erupting magma lets some/all of the gasses
escape, it…
– becomes lava when it reaches the surface.
~OR~
– explodes as pyroclastic material (gas = 0.5% to 6%)
– basaltic magma = low in dissolved gas
– andesitic & granitic magmas = higher gas content
Pyroclastic Flows
• high-density mixtures: hot,
dry rock fragments & gases.
• high speeds.
• destroys nearly everything
in its path.
– rock fragments ash →
boulders
– ground speeds < 80 km/hr
– Temps: 200°C - 700°C
• Water (H2O) = highest % gas dissolved in magma
• carbon dioxide (CO2) is second.
• Magma temps range: 700°C - 1350°C
– highest silica content = lowest temps.
– lowest in silica = highest temps.
Viscosity
• …the resistance to flow exhibited by a fluid
• high temp magmas = low viscosity (runny)
• higher silica = higher viscosity (thick)
• Basaltic magma: ↑ temp, ↓silica, ↓ gas
– erupts on surface: fluid lava flows, little explosive
activity.
• Andesitic magma: ↔ temp, ↔ silica,↑ gas
– More viscous than basaltic magma, more likely to
explode on surface (harder for gas bubbles to
escape from viscous magma).
• Granitic magma: ↓ temp, ↑ silica, ↑ gas
content.
– So viscous it gets stuck before reaching surface.
Where are Volcanoes found?
• Divergent & subduction plate boundaries.
• Hot Spots
Subduction formations
• Form on overriding
continental plate.
• Ocean boundaries
form volcanic arcs
above overriding plate.
Divergent formations
• Thinner crust materials let magma rise (less
dense)
• Found at mid-ocean ridges.
• Ex: Iceland
Hot spot formations
• Hot plume of magma below crust associated
with mantle, not plate.
• Spot remains while plate moves over it.
• Ex: Hawaiian islands, Yellowstone
Lava basics
• Name change when it hits the surface
• Basaltic lava types:
– pahoehoe: rope-like
– aa: sharp, jagged
– Pillow lava: formed under
water.
• aa rapid lava flow
– rapid heat loss
– increased viscosity
– high discharge rates & steep
slopes
• pahoehoe lava flows slowly
– well-developed surface skin
forms reducing heat loss
– lower discharge rates & gentle
slopes
Volcanic Land Forms
• Shield volcano: layers of lava built up over
time
• Cinder cone: material thrown high, gathered
around vent forming cone shape, found in
clusters, small size
• Composite (stratovolcanoes): layered from
repeated explosive eruptions.
Calderas
• Large crater shaped basin formed when top of
volcano collapses.
• Valles Caldera in NM
Lahar
• hot or cold mixture of water & rock fragments
flows down slopes of a volcano &/or river
valleys.
– “wet concrete” carrying rock debris: clay - 10 m
diameter boulders
Volcanism in the Solar System
• Moon: maria formed from basaltic lava flows.
• Mars: shield volcanoes and Olympus Mons,
the largest known in the system, 27 km high.
(size indicates no moving plates on planet)
• Venus: shield volcanoes, lava flows, calderas
• Io: Jupiter’s 3rd largest moon highly volcanic.
Active vs Dormant vs Extinct
• Active volcano: at least one eruption in past
10,000 years.
• Dormant volcano: no eruptions in recent
history.
• Extinct volcano: no eruptions in past 10,000
years.