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Volcanism and Extrusive Rocks
How do Volcanoes Interact
with Earth's Systems?
How do Volcanoes Interact
with Earth's Systems?
• Atmosphere- Earth's first
atmosphere was created from
gases released by volcanoes.
• Hydrosphere- Earth's water was
likely produced by condensation
of volcanic water vapor early in
Earth’s history.
• Biosphere- Volcanoes have both
positive and negative effects on
the biosphere:
Living With Volcanoes
• Tourism– Iceland, Hawaii, Mt. St. Helens
• Fertile Soils– Hawaii, South America
• Geothermal Energy- This natural steam
provides a clean energy resource (Iceland).
Living With Volcanoes
• Direct fatalities: In 1815, 92,000 people were
killed by an eruption in Indonesia.
• Climate: Very large eruptions can result in
measurable global cooling (1815 was "the year
with no summer") resulting in crop failures and
What is Lava?
Lava- Magma that has reached Earth’s surface. It can flow
form from a volcano vent, the side of a volcano, or directly
from the ground.
How Does Lava Behave?
Volcanic eruptions can be:
Slow and predictable in
nature (Hawaii).
Explosive and unpredictable
Lava behavior (slow vs explosive)
depends on…
• 1) Amount of dissolved gases (water vapor, carbon
dioxide, sulfur dioxide, hydrogen sulfide). More gases =
more explosive.
• 2) Viscosity (fluid’s resistance to flow), which depends on
two factors:
• a) Silica (SiO2) content. Higher silica (more felsic)
magmas are more viscous than low silica magmas. More
viscous = more explosive.
• b) Lava temperature. Cooler magmas are more viscous
(and trap gases better) than hot magmas. Cool magmas =
more explosive. Which is usually cooler, mafic or felsic
Comparing Magmas
MAFIC- Gasses escape
FELSIC- Gases escape
Explosive Volcanoes:
Pyroclastic Flows (Felsic Magmas)
• Pyroclast: Solid/solidified matter
blown away from an explosive
volcano during an eruption.
• Pyroclastic flow: Fast moving
mixture of pyroclasts and gas (solids
float on gas cushion).
• Temperatures can reach 1000 C
• Speeds can reach 700 km/hr (450
• The most deadly aspect of volcanic
Volcanic Rock Compositions
Can be:
Volcanic Rock Textures
Crystals < 1 mm (extrusive).
Glassy (no mineral crystals) due to
extremely rapid cooling.
Porphyritic rocks
Large crystals (phenocrysts) surrounded
by a fine matrix (groundmass).
Indicates two-stage cooling history of
Common in andesites.
Vesicular rocks
Rocks that contain trapped gas bubbles.
Light to medium colored and porous
(rhyolite or andesite) and partly glassy.
Formed from very viscous lava in which
gases could not escape. May float on
Dark colored and porous (basalt).
Volcanic particles blasted apart by
explosive eruptions.
Dust and ash (<2 mm)
Cinders/Lapilli (2-64 mm)
Blocks (or bombs, if they are
streamlined during flight) (>64 mm)
Volcanic tuff
Contains fine-grained pyroclasts
(fragments of dust and ash). The hot
pyroclastic fragments often weld
together (welded tuff).
Volcanic breccia
Contains coarser-grained pyroclasts.
Parts of a Volcano
• Volcano-Topographically
elevated body of
extrusive rock. Forms as
lava and/or pyroclastic
debris pile up over time.
• Vent – Any opening
through which lava erupts
• Crater - A basin-like
depression over the vent
at the summit of the
A Volcanic Crater
What is a Caldera?
Caldera - A volcanic depression (> 1 km) much larger than
the original crater (ex. Crater Lake in Oregon).
Caldera Formation
A calderas will form if the summit blows
off, or…
The summit collapses into a vacated
magma chamber.
• What factors make a magma flow easily?
• What factors make a magma explosive?
The 3 Types of Volcanoes
Shield Volcanoes- Formed by lava flows (mafic
Cinder Cones- Formed by pyroclasts (felsic
Composite Volcanoes (aka stratovolcanoes)Formed by both flows and pyroclasts
(intermediate andesites).
Shield Volcanoes (Hawaii)
• Large (km-sized) and 5-10
degree slope.
• Composed of solidified lava
• Slopes are broad and gentle.
• Formed by low viscosity lava
(usually basalt) that spreads out
• Lava tube: Tunnel formed by
molten lava flowing through
lava flows that have already
Shield Volcanoes (Hawaii)
• Two types of lava flow
• Hot magma
• Cooler magma
•  aa.
Cinder Cones (Flagstaff)
• Small volcanoes (usually
meters across) with slopes =
• A pile of loose pyroclasts
ejected from a central vent.
• Slopes are steep.
• Formed from relatively viscous
• Erode easily due to their
unconsolidated nature.
Composite Volcanoes (Mt.St. Helens)
• Moderate size and 5-30 degree
• Alternating layers of pyroclastic
debris and solidified lava flows.
• Moderately to steeply sloping.
• Composed primarily of intermediate
composition volcanic rocks (i.e.,
• Lava flows result from the low
viscosity lavas (hot).
• Pyroclastic flows result from higher
viscosity lavas (cooler).
• Most common type of volcano at
convergent plate boundaries.
What is a “lahar”?
• It is a mixture of pyroclasts and fluids (H2O
emitted by volcanic steam).
• This causes a lahar, or deadly mud flow.
• Common in explosive volcanoes (cinder
cones and composite/stratovolcanoes).
Ground Eruptions: Flood Basalts
• Very low viscosity mafic magma can seep out of the ground,
forming horizontal layers of basalt of extremely large areas.
• These flood eruptions can create extensive lava plateaus (e.g.,
Columbia River Plateau and the Rio Grande Rift in New
• In Siberia, extensive flood basalts coincide with Earth's largest
mass extinction (the Permian extinction, 251 mya).
The Plates and Plate Boundaries
There are 12 main lithosphere plates.
Much geology happens at the boundaries
between these plates!
Three Types of Plate Boundaries:
1) Transform Boundaries
2) Divergent Boundaries
3) Convergent Boundaries
Tying Volcanism to Plate Tectonics
Divergent Boundaries
MOR = Mafic = Basalt
Convergent Boundaries
Felic to Intermediate =
Rhyolites + Andesites
Locations of Volcanoes
Most volcanism occurs at plate boundaries, including:
Divergent (mid ocean ridges)- Basalt
Convergent (Subduction zones)- Mostly andesite.
The interiors of plates are mostly devoid of volcanoes.
Most (80%) of the Pacific Rim population lives near volcanoes
Eruptions at Mid Ocean Ridges
• Basaltic (no time to evolve to felsic).
• Pillow structures indicate the lava flowed
and solidified under water.