Chapter 6
Volcanoes and Igneous Rocks
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
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
Eruptions, landforms and materials
• Eruption types
– Shield volcano
• A broad, flat volcano with gently sloping sides, built of successive
lava flows
• Produce flood basalts or basalt plateaus
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
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
Eruptions, landforms and materials
Eruptions, landforms and materials
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
Ash
Agglomerates
Tuff
Eruptions, landforms and materials
• Other volcanic
features
–
–
–
–
–
Craters
Resurgent dome
Thermal spring
Geysers
Fumaroles
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
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
Predicting Eruptions
• Establish a volcano’s history
– Active
– 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
– Ocean crust: temperature rises twice as rapid
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
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
How, Why and Where Rocks
Melt
How, Why and Where Rocks Melt
• The tectonic setting
– Lava characteristics influenced by location
• Oceanic, divergent margins
– Lava is thin with a steep geothermal gradient
• Subduction zones
– Typically have high water content and melt at lower temperatures
• Hot- spots
– Lava tends to be hot and basaltic
– Build giant shield volcanoes
• Continental divergent margins are all different
– High silica lava
How, Why and Where Rocks
Melt
How, Why and Where Rocks
Melt
How, Why and Where Rocks
Melt
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
Rate of Cooling
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
Chemical composition
• Igneous rocks subdivided into three categories
based on silica content
– Felsic
– Intermediate
– Mafic
Fractional Crystallization
• Separation of crystals from liquids during
crystallization
– Bowens reaction series
– Predictable melting and cooling of minerals
Fractional Crystallization
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
Mount Saint Helens
Critical Thinking
• What factors might prevent magma from reaching
Earth’s surface?
• What reasons can you think of for living near a
volcano? Do you think the advantages outweigh the
disadvantages?
• If you were to heat up a glass beaker full of crushed
rock, the beaker would melt before you could finish
studying the rock melting process. How do you think
geologists study rock melting?