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Where do we find igneous rocks?
• Divergent and convergent plate boundaries
Continental margins above subduction zones
Island arc volcanic chains above subduction zones
Divergent boundaries: mid-ocean ridges, continental rifts
• Hot spots
• Hawaii – in an ocean basin
• Yellowstone – on the N. American continent
• Northern part of the E African Rift Valley (sometimes continental rifts,
or divergent boundaries within continents, are started by hotspots)
• Note - transform plate boundaries are not generally associated with
igneous rocks and volcanism.
Where do we find igneous rocks?
Where magma is produced: known volcanoes of the world
from the Smithsonian Global Volcanism Project
Where do we find igneous rocks?
Plutonic Rocks Exposed
at the Surface
Magma injects between
or across layers of rock
giving rise to the various
plutonic rock bodies
shown at right.
Uplift and erosion over
geologic time exposes
intrusive bodies of
igneous rock.
Where do we find igneous rocks?
Terminology for Intrusive Features
• Dike – a tabular intrusion cutting across layers
(usually oriented vertically)
• Sill – a tabular intrusion in between layers
(usually oriented horizontally)
• Stock/batholith - large areas of intrusive rock that
represent old “frozen” magma chambers (plutons)
that are now exposed at the surface. By definition
a batholith covers an area of >100 km2.
Where do we find igneous rocks?
Fig 4.7
Sills do not cut across older rocks that they intrude into.
Dikes cut across older rocks they intrude into. Note
geologists on the dike for scale!
Where do we find igneous rocks?
Shiprock, NM – a volcanic neck
Dikes are often “feeders” for lava flows
Sierra Nevada Mountains:
A Large Batholith
Where do we find igneous rocks?
Volcanic rocks in the landscape
• Volcanic neck - the frozen residual “core” of
magma that was rising up through the center of the
volcano (actually a very shallow intrusive body!)
• Lava domes - a mound of lava that did not flow
• Pyroclastic deposits - ash, pumice, lapilli, bombs,
may form deposits such as welded tuff
Rhyolite ash and pumice
deposited very hot and fused
into a consolidated mass of
tuff – a compact, solid
volcanic rock composed of
pyroclastic material.
What Are Common Volcanoes Like?
• The Type of Volcano is Controlled by the Type of
Magma That is Erupting.
• Felsic (silicic) magmas are cooler, very viscous (thick,
sticky) and highly gas charged.
• Mafic magmas are hotter, more fluid and flow easily
by comparison, and have significantly less gas.
• The result is that silicic magma produces lava domes,
short thick flows and explosive (pyroclastic) rocks.
• In contrast, mafic magma produces long, thin lava
flows, and eruptions are generally not as explosive.
What Are Common Volcanoes Like?
Gases Drive Eruptions
Similar to a bottle of
champagne, a “corked”
volcano builds up
pressure as gases
come out of solution.
Eventually enough
pressure builds up that
an eruption occurs.
What Are Common Volcanoes Like?
• Viscosity: defined as the resistance to flow
• Controlled by silica tetrahedra polymerization in the
melt, so varies dramatically with composition
• High silica = high viscosity, thick, sticky magma
Gases escape more readily from low-viscosity magma
• Low silica = more fluid magma (like the thin basalt
lava flows that make up the Hawaiian islands)
What Are Common Volcanoes Like?
What Are Common Volcanoes Like?
Basic Types of Basalt Volcanoes
Cinder cone: small,
made mainly of
Eruptive phases 1
to 20 years.
Shield volcano:
vary in size, made
of thin flows of
basaltic lava. Longlived eruptions. Low
What Are Common Volcanoes Like?
Intermediate to Felsic Composition Magmas
Produce Very Different Volcanic Features
Composite volcano:
modest size, steep
profile. Andesite to
dacite lavas
interbedded with
Dome complexes:
modest size,
overlapping domes.
Dacite to rhyolite
What Are Common Volcanoes Like?
Tuff is deposited by a pyroclastic flow, these are called a nuee ardente
or glowing avalanche. They move at speeds up to 200 kph (120 mph)
and have internal temperatures up to 700-800 oC (approx. 1,300-1,500 oF).
Pyroclastic flow on Merapi volcano, Indonesia
What Are Common Volcanoes Like?
Pyroclastic flows are characteristic
of eruptions from andesite, dacite
and rhyolite volcanoes.
Pyroclastic flows on Mt. Unzen
volcano in Japan
What Are Common Volcanoes Like?
Pyroclastic flow deposits may appear to be cold, but can remain very hot
for many weeks after they are formed. Thermal camera images capture this.
Recently formed pyroclastic flow on Montserrat volcano, Carribean
What Are Common Volcanoes Like?
Panum Crater, California – A small rhyolite lava dome with surrounding
pyroclastic apron from the initial explosive phase of the eruption.
What Are Common Volcanoes Like?
Felsic (silicic) magma forms short, thick lava flows
What Are Common Volcanoes Like?
Mt. Fuji, Japan – A classic example of a dacite to andesite composition
composite volcano – often called a stratovolcano. These are built up
from explosive and effusive eruptions producing alternating layers of
pyroclastic rocks and lava flows.
What are common volcanoes like?
Basalt magma forms pahoehoe lavas (smooth and with a “ropy”
surface) or a’a’ lavas (rough and broken into sharp fragments).
What sounds would you make if you walked barefoot on these two lavas?
Pahoehoe lava in the process of forming….
Formation of Collapse Calderas
• Two basic types: smaller ones produced on basalt
volcanoes, and much larger ones produced by felsic
• In either case withdrawal of magma from a nearsurface chamber leads to collapse of the rock above
forming a large crater – called a caldera.
• Basalt volcanoes – usually small, ~1-3 km diameter.
• Silicic volcanoes – can be very large, the La Garita
caldera in the San Juan Volcanic Field is ~30x80 km
(20x50 miles). Approx. 5,000 km3 of magma was
erupted to form this caldera (in comparison Mt. St.
Helens was about 1 km3 of magma erupted)
How and why do rocks melt?
• If temperature is raised enough rocks will melt.
• Silicate minerals melt across a wide range from
approximately 800oC to around 1900oC.
• Increasing pressure increases the melting
temperature – Decreasing pressure does
• At the surface olivine melts at 1890oC, whereas at 100
km it melts at 2050oC
• Mixtures of minerals melt at a different temp than
individual minerals (due to reactions between minerals).
• Olivine mixed with pyroxene and plagioclase melts at
• Adding water decreases the melting temperature.
How does magma generation relate to
plate tectonics?
• Where do conditions for
melting occur?
• Where decompression
occurs due to rising rock
• Divergent boundaries:
Mid-ocean ridges
• Also, at hotspots
• Convergent boundaries
• Addition of water to the
“mantle wedge” as wet
ocean crust is subducted
and dehydrates
How and why do rocks melt?
• When a rock melts completely (temperature high
enough to melt all minerals) it can cool and
recrystallize into the same rock.
• However, incomplete melting - or partial crystallization
- can yield different composition rocks than the
starting composition rock or magma.
• Example: partial melting may produce magma from
pyroxene and plagioclase, most olivine is still solid.
• Example: during partial crystallization olivine forms
first, depleting the remaining magma in the chemical
components that olivine needs to grow.
How and why do rocks melt?
What makes igneous rock
compositions so diverse?
Fractional crystallization: As minerals crystallize out, the remaining melt
changes in
comparison to
the original
Example: Olivine
crystallizing from
a basalt magma
depletes the
magma in Mg
and Fe.
What makes igneous rock
compositions so diverse?
The first crystals to form will either settle downward (if denser), or stay
where they form—on the walls (cooling occurs from outside in). If
remaining magma solidifies or erupts it’s composition will be different
from the original magma composition.
Fig 4.23
What makes igneous rock
compositions so diverse?
• Melting rocks produces magma of differing
compositions. The extent to which the rocks
melt also determines composition.
• Fractional crystallization can change a magma
into one with a different composition.
• Rocks surrounding a magma chamber may
partly melt and become assimilated.
• Two or more magmas may mingle to produce a
hybrid magma of intermediate composition.