VOLCANOES AND IGNEOUS
ROCKS
Objectives
• Identify several different categories of volcanic eruptions.
• Identify the volcanic hazards.
• Describe how temperature, pressure, and water content affect
a rock’s melting point.
• Identify three properties that distinguish one lava from
another.
• Distinguish between and identify volcanic and plutonic
rocks.
• Describe the most common plutonic formations.
Volcanoes and volcanic hazards
• Volcano
– A vent through which
lava, solid rock debris,
volcanic ash, and gasses
erupt from Earth’s crust
to its surface
– Can be explosive or
nonexplosive
1
Volcanoes and volcanic hazards
• Lava
– Molten rock that reaches
Earth’s surface
• Magma
– Molten rock, which may
include fragments of
rock, volcanic glass and
ash, or gas
• Gases
Eruptions, landforms and materials
• Eruption types
– Hawaiian type; fluid eruptions, least explosive
– Shield volcano
• A broad, flat volcano with gently sloping sides, built of successive
lava flows
• Produce flood basalts or basalt plateaus (from fissures instead of
central vents)
Lava flows and shield
volcanoes
Iceland
Shield Volcano-Iceland
2
Mauna Loa, as seen from Kilauea
Mauna Loa from Kilauea
Eruptions, landforms and materials
• Eruption types
– Strombolian
• More explosive than Hawaiian
• Create loose volcanic rock called spatter cones or cinder cones
Eruptions, landforms and materials
• Eruption types
– Vulcanian
• More explosive than
Strombolian and, as a result,
can generate billowing
clouds of ash up to 10 km
• Produce pyroclastic flows
– Hot volcanic fragments
(tephra) that, buoyed by
heat and volcanic gases,
flow very rapidly
– Strato (composite)
volcano; mixture of
lava and pyroclastics
3
Eruptions, landforms and materials
• Eruption types
– Plinian
• Named after Pliny the Elder,
who died during eruption of
Mount Vesuvius
• Most violent eruptions,
generating ash columns the
can exceed 20 kilometers
• Produce steep sided volcanoes,
called stratovolcanoes
– Composed of solidified lava
flows interlayered with
pyroclastic material.
– Steep sides that curve
upward
Mt. Fuji
Mt. St. Helens, 1980
Lateral Blast
4
Eruptions, landforms and materials
Shield volcano
Eruptions, landforms and materials
Stratovolcano
5
Eruptions, landforms and materials
• Viscosity
– The degree to which a
substance resists flow,
• Less viscous liquid is runny
• More viscous liquid is thick
• Volcanic materials
–
–
–
–
–
Pyroclasts
Tephra: General term, all sizes
Ash: smallest size
Agglomerates: welded larger particles
Tuff: welded smaller particles
Eruptions, landforms and materials
• Other volcanic
features
–
–
–
–
–
1
3
Craters
Resurgent dome
Thermal spring
Geysers
Fumaroles
2
4
6
Volcanic hazards
• Primary effects
– Pyroclastic flows
– Volcanic gases
• Secondary effects
– Related to, but not a
direct result of,
volcanic activity
•
•
•
•
Fires
Flooding
Mudslides
Debris avalanche
Volcanic hazards
Lahars
Orting, Washington
Mt. Rainier
7
Mt. Pelee, 1902
Town of St. Pierre, Carribean Island of Martinique 30,000 killed
Volcanic hazards
Volcanic hazards
• Tertiary and beneficial
effects
– Change a landscape
– Affect climate on
regional and global scale
– Renew mineral content
and replenish fertility
– Geothermal energy
– Provide mineral deposits
8
Predicting Eruptions
• Establish a volcano’s history
– Active (eruption in recorded
history)
– Dormant
• Monitor changes and
anomalies
–
–
–
–
Earthquakes
Changes in shape or elevation
Volcanic gases
Changes in ground
temperature
– Composition of water
Predicting Eruptions
How, Why and Where Rocks
Melt
• Heat and pressure inside Earth
– Continental crust: temperature rises 30°C/km, then about
6.7°C/km (geothermal gradient)
– Ocean crust: temperature rises twice as rapid
9
How, Why and Where Rocks
Melt
How, Why and Where
Rocks Melt
• Effect of temperature and pressure on melting
How, Why and Where
Rocks Melt
• Heat and pressure inside
Earth
– Fractional melt
• A mixture of molten and
solid rock
– Fractionation
• Separation of melted
materials from the
remaining solid material
during the course of melting
10
How, Why and Where Rocks
Melt
How, Why and Where Rocks
Melt
• Magma
– Molten rock below
surface
• Lava
– Magma when it reaches
the surface
– Differ in composition,
temperature and
viscosity
How, Why and Where Rocks
Melt
• Lava
– Composition
• 45-75% of magma by weight is silica
• Water vapor and carbon dioxide
– Temperature
• Lavas vary in temperature between 750°C and 1200°C
• Magmas with high H2O contents melt at lower temperatures
– Viscosity
• Lavas vary in their ability to flow
• Influenced by silica content and temperature
11
How, Why and Where Rocks
Melt
• The tectonic setting
– Lava characteristics influenced by location
• Oceanic, divergent margins
– Lithosphere (crust) is thin with a steep geothermal gradient
• Subduction zones
– Aleutian Islands; Andes
– Typically have high water content and melt at lower temperatures
• Hot- spots; Hawaiian Islands
– Lava tends to be hot and basaltic
– Build giant shield volcanoes
– Continental; Yellowstone
• Continental divergent margins are all different
– High silica lava
How, Why and Where Rocks
Melt
Pillow lavas, mid-ocean ridge
Continental hot spot; silica rich
How, Why and Where Rocks Melt
Continental-oceanic
subduction zone: andesite
Oceanic-oceanic subduction
zone: andesite
Oceanic hot spot:
basalt
12
How, Why and Where Rocks
spot; red,
Melt Yellow-hot
volcano
Cooling and Crystallization
• Crystallization
– The process whereby
mineral grains form and
grow in a cooling magma
(or lava)
– Classified as:
• Volcanic
• Plutonic
Rate of Cooling
• Rapid cooling:
Volcanic rocks and
textures
– Volcanic rock
• An igneous rock formed
from lava
• Glassy
• Aphanitic
• Porphyritic
• Pumice
• Vesicular basalt
13
Rate of Cooling
Aphanitic, vesicular
Porphyritic; 2 sizes of crystals
Rate of Cooling
• Slow cooling:
Plutonic rocks and
textures
– Plutonic rock
• An igneous rock
formed underground
from magma
– Phaneritic-a coarse
grained texture
• Can have
exceptionally large
grains (pegmatite)
Chemical composition
• Igneous rocks subdivided into three categories
based on silica content
– Felsic-high
– Intermediate
– Mafic-low
14
Plutons and Plutonism
• Plutons
– Any body of intrusive
igneous rock, regardless
of size or shape
• Batholith
– A large, irregularly shaped
pluton that cuts across the
layering of the rock into
which it intrudes
Plutons and Plutonism
• Dikes
– Forms when magma
squeezes into a cross
cutting fracture and
solidifies
• Sills
– Magma that intrudes
between two layers and
is parallel to them
15