Igneous rocks and volcanic structures
Igneous rocks
Formed from magma
How is lava and magma different?
Two basic types of igneous rocks
Intrusive
Extrusive
Question: How do these two “types” differ?
How does this affect the minerals that create the rocks?
Answer:
Different cooling histories
Slow vs. fast
Changes the texture of the rock
Visual texture not physical
“Texture” of Igneous Rocks
Intrusive
slow cooling
minerals grow large (can see unaided)
Extrusive
cools quickly
often hard to see minerals unaided
Other differences
Intrusive:
No evidence of flow
Never vesiculated
Extrusive:
Water tends to degasàOften has vesicles
Often see flow structures…banding, jointing etc…
Composition of Igneous Rocks
Based on total % of silica
Felsic: high percentage of silica (SiO2) (>65%)
Intermediate: silica from 55-65%
Mafic: silica from 40-55% and large amounts of magnesium (Mg) and iron (Fe)
Ultramafic: silica <40% and abundant iron and magnesium
Based on total silica in rock…all minerals together
Some common rock forming minerals found in igneous rocks
Quartz:
SiO2 (all silica)
Plagioclase NaAlSi3O8 to
CaAl2Si2O8
Orthoclase: KAlSi3O4
Augite: Ca(Mg,
Fe)Si2O3(Al,Fe)2O3
Biotite: K(Mg,
Fe)3AlSi3O10(OH)2
Olivine: (Mg,Fe)2SiO4 (low silica)
Felsic (often light in color)
intrusive: Granite
extrusive: Rhyolite
Intermediate (Salt and
Pepper)
intrusive: Dorite
extrusive: Andesite
(
Mafic (Dark)
intrusive: Gabbro
extrusive: Basalt (Ocean floor)
Ultramafic
intrusive: Peridotite (Mantle)
Extrusive: Komatiites (no eruptions of this composition today)
How do we get different igneous rock types?
Why are there different compositions of magma?
Possible processes?
Other ways to change magma compositons
Assimilation
Incorporate into a melt the surrounding country rock
Country rock: rock that is found in an area before magma intrudes. Pre-existing rock.
Filter pressing
Squeeze magma through a fracture and leave crystals behind
Gravity settling
Heavy crystals settle and separate from magma
Magma mixing
Two magmas mix and create new composition
Partial melting
Not all minerals melt at the same temperature
Partially melting a rock removes some minerals and leaves others
Partially melting a rock will always produce a rock that is richer in silica
WHY?
Bowen’s Reaction Series
Shows ideal mineral crystallization sequence
Shows temperature minerals are stable
Indicates which minerals are found in which rocks
Can also use to see which order minerals melt
Peridotite (mantle)
Rich in iron and magnesium (low in silica)
Melting peridotite creates basalt and gabbro
Creates ocean floor…Where does this happen?
Continental material
Rich in silica and aluminum
Often creates rhyolite and granite
Found on continents
Continental material + basalt = intermediate rocks OR Melt basalt
Often found as volcanoes from subduction zones
Different magmas are created by melting different sources
EXERCISE for Bowen’s reaction series
1) Based on temperature, which rock forms from a “hotter” magma, Granite or
Gabbro?
2) Two magmas, one is granitic in composition and one is gabbroic in composition,
which would be more viscous?
3) Which of the two rocks mentioned above (granite or gabbro) have the minerals
olivine and a calcium rich plagioclase in it
4) What minerals would the other rock have in it?
5) If a basaltic rock were to experience partial melting, which two minerals would
be the first to melt and what would the new magma composition be?
How can we get a rock to melt?
Add Heat
Lower pressure
Add water
What happens at different
plate boundaries to cause volcanism?
Divergent boundaries
Subduction zones?
Continental to oceanic
Oceanic to oceanic
At all locations, both intrusive and extrusive rocks are found
Some Igneous rock Names
Intrusive
Granite
Diorite
Gabbro
Periodotite
Extrusive
Rhyolite
Andesite
Basalt
Scoria
Pumice
Obsidian
Intrusive igneous rocks
Granite
(Felsic)
Diorite
(Intermediate)
Gabbro
(Mafic)
Peridotite (Ultra mafic)
Magma cools slowly over long period of time
Hundreds of thousands to millions of years
Creates larger crystals
Magma rises by assimilation and eventually forms plutons
Pluton: large volume of intrusive igneous rock
Uplift, erosion and exposure create batholiths and stocks
Batholiths: granitic
Area exposed at surface is > 100 km2
Examples:
cooling time = ~130 mil years
Stocks: silicic to intermediate composition
Area exposed at surface is < 100 km2
Often partially exposed batholith
Dikes
Magma filled fractures
All compositions of magma
Often resistant to erosion
Sills
Magma intrudes between layers of rocks (often sedimentary)
Extrusive/Volcanic Rock types
Basalt—lava flow
Andesite—lava flow
Rhyolite—lava flow
Obsidian—lava flow (glass)
Pumice—pyroclastic
Scoria—pyroclastic
Ash Tuff--Pyroclastic
Physical properties of magma
How can the physical properties of magma vary?
Temperature
% silica
% of water
All of the above can affect the viscosity of a magma
Increasing viscosity
Lower temperature
Increase silica
Lower the amount of water
water interrupts the silica structure and allows easier flow
Silica and Temperature: important for type of volcanic eruptions
Low silica = high temp = low viscosity
allows gasses dissolved in solution to easily escape
creates fluid lava flows or fire fountains
ex//
High silica = low temp =high viscosity
water or gas dissolved in magma can’t escape
pressure builds and explosion occurs
ex//
Types of volcanoes
Shield Volcanoes
created from basaltic lava flow
high temperature and fluid flows
Cinder cones
Basaltic in composition
created by fountaining effect
Composite cone
silicic volcanism (andesite and rhyolite)
alternating layers of ash and lava
conical looking
Pumic cone
Silica volcanism
All pumice
Volcanic products
Lava
basalt, andesite, rhyolite
Pyroclastic material
ash, cinders, bombs, blocks
Gasses
primarily H2O, CO2, N and SO2
Pyroclastic flows
ignimbrites or nue ardents
Lahars
Basaltic Volcanism
Basaltic rock is most abundant rock on earth
eruptions are often non-violent
Low viscosity
flows travel long distances
Magma is produced by melting mantle (peridotite)
Where is basaltic volcanism found?
Features of Basaltic volcanism
Lava flows
pahoehoe
channelized flowsà leads to lava tubes
flood basalts
Columular joints
tree casts
pillow lavas
bombs
NW Basaltic Volcanism
Flood basalts due to continental extension
17-6 million years ago
originated near Idaho/Oregon/Washington border
flowed to the coast in thick flows (haystack rock)
Boring lavas
small shields & cinder cones
4-1 million years
Newberry volcano
about 1300 years since last eruption
shield volcano with many cinder cones
Also obsidian
Silicic Volcanism
Creates andesite and rhyolite volcanoes
associated with subduction zones
very viscous lava flows
short flows
very explosive
Products of Silicic volcanoes
Ash and lapilli
Composite cones
Lava domes
volcanic plumes
pyroclastic flows
collapsed calderas
lahars
NW Silicic Volcanism
Cascade volcanoes
3 episodes of eruption
Western Cascades (20 mybp)
Ancestral Cascades (7 mybp)
High Cascades (1 mybp)
recent activity
South Sister (2001 to present)
Volcanic Hazards
Basaltic Volcanism?
Silicic Volcanism?
How can Volcanic eruptions help change the climate?
1815 Tambora
year without a summer