Earth Sci Wk 7
Chap 7 – Igneous Activity in the Earth
Intro with the story of Mt St Helens, state of Washington
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May 1980
Lots of force
N flank of volcano blown out
Elev reduced from 9500 ft to 8200 ft
Mudflows, ash & gas clouds
59 deaths
One of 15 big volcanos in Cascade Range
last Mt St Helens eruption was 1857
Other types of volcanoes less violent, like Mt Kilauea in Hawaii…magma
more fluid, less viscous and explosive (boiling water vs spaghetti sauce)
Kilauea has been active since 1983, but eruptions are not explosive
This chapter focus is on “whys” of volcanoes and magma.
I.
Nature of volcanic eruptions
 Explosive vs Quiet
 Viscosity is the key (syrup vs water)
Factors: - composition
- temperature
- dissolved gases
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Composition – % silica makes a big difference in viscosity Table 7.1
50% silica – magma thin (low vis)
70% silica – magma thick (high vis)
High vis a result of long silica chains…flow of lava impeded.
Granitic lavas viscous, flow distance short; explosive eruptions when
magma plugs the vent
Basaltic lavas thin, flow distances long (almost 100 mi); eruptions quiet
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Dissolved gases
- Gases provide the drive that pushes the lava
- As confining pressure is reduced near surface, gases come out of
solution
(analogy is to a Coke being popped, and CO2 comes out of sol’n as a free
gas)
- gas expands to 100x original volume
Eth Sci wk 7 p.2
With a fluid magma, expanding gas bubbles up and out of magma easily
(see those tall lava fountains in Hawaiian eruptions)
With a viscous magma, gases collect in big pockets, explode violently (like
a thick spaghetti sauce)
II.
What is extruded during eruption…
Many things in addition to lava…bombs, ash, dust, gas.
A. Lava
 Basaltic lava Fig 7.4
- pahoehoe – ropy lava – smooth skin
- aa – blocky lava - “advancing mass of rubble”
B. Gases
 Gas component only 1-5 % by weight, most as H2O vapor
 H2O vapor from volcs thought to represent the original source of water
for ocean
 Volc gases also thought to contribute to Earth atmosphere
Typical content of gas from Hawaii:
70% H2O vapor
15% CO2
5% N
5% S
and Cl, H, Ar
Volcs also a natural source of air pollution (!)
C. Pyroclastics (“fire fragments”) ejected from the volcano
 Particles that are assoc w/ viscous magma, many sizes
- ash – very fine gr, shards fuse together to form “welded tuff”
pumice too
- lapilli and cinders (walnut- and pea-sized)
- blocks (hardened already before flight)
- bombs (hardened during flight, and streamlined by air) Fig 7.6
Earth Sci wk 7 p.3
III.
Types of volcanoes
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Some common features:
“Crater” – depression at top of volc, usu connected to a lower
magma chamber by a..
“Vent” – a pipelike conduit
When crater is very large (>1 km diam) it is known as a “caldera”
Fluid lava simply stored in crater, but viscous lava can plug it, ultimately
be blown out and enlarge crater even more (like Mt St Helens)
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3 generic types of volcanoes:
1. Shield volcano (like Hawaii) Fig 7.7
- broad dome (like a warrior’s shield)
- basaltic, fluid flows
- Mauna Loa (Hawaii) 14,000 ft tall from sea level, 30,000 feet from
ocean floor (largest volc on Earth)
- took how many years to build???
- other islands like Hawaii include the Galapagos, which Darwin
studied
Take a closer look at Kilauea, a volc on the flank of the larger volc Mauna
Loa..
- >50 eruptions in recorded history, latest from 1983 - present
- magma source at depth (60 km) feeds chambers nr surf (3-5 km down)
- eqk swarms occur prior to eruption (volcano expanding as magma fills
chamber)
- often see fissure eruptions, not just point sources Fig 7.8
- lavas fluid, move quickly, not a threat to life but do a lot of property
damage, burying homes
2. Cinder cones
- built from ejected fragments of lava
- very steep sides due to high angle of repose of fragments Fig 7.9
- usu small in terms of height (1000 ft max)
Story of birth of a cinder cone in Mexico...
 Eqks prior to birth in 1943
 130 feet growth in one day
 finally grew over two years to 1300 feet high
 1944 - aa lava flow 30 feet thick covered nearby village Fig 7.10
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Earth Sci wk 7 p.4
3. Composite cones
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Composite of both lava flows and pyroclastic flows
- Mt Fuji in Japan
- Mt. St. Helens, Mt. Rainier, Mt. Shasta Fig 7.11
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Active composites circle the Pacific…the “Ring of Fire”
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Composite lavas are andesitic, thick, viscous
- Eruptive style alternates between lava flows and pyroclastic flows
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Most dangerous and explosive volcanoes
- 79 AD – Mt. Vesuvius exploded for 3 days, buried Pompeii near
Naples, Italy
- Pompeii buried for 17 centuries
- killed 2,000 out of 20,000 inhabitants
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Particularly devastating feature of composite volc is “nuee ardente”
(“glowing avalanche”)
This is a mix of hot gas and hot ash moving at avalanche speed
down mountain, up to 125 mph, and can travel up to 60 miles by riding on
a cushion of hot gas.
- examples:
Mt. St. Helens – 1980 Fig 7.12
Mt. Pelee in the Caribbean on Martinique Fig 7.13
Extremely destuctive – killed 28,000
Destroyed town of St. Pierre “in moments”
Very strong lateral force pushed over 3-foot thick
walls
IV.Sizes of Volcanoes - Fig 7.14
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3 basic sizes –
1. Small – cinder cones
- 1000 feet tall
- short eruption history
2. Intermediate – composite volcs
- 10,000 feet tall
- long eruption history (Mt Rainier 700,000 yrs old)
Earth Sci wk 7 p.5
3. Large – shield volcs
- some almost 30,000 feet tall (Mauna Loa taller than Mt. Everest)
- large volume of lava
- broad, low slopes, huge size
V.
Volcanic Landforms
A. Volcanic neck often only remnant of ancient volcano…a resistant rock
plug, compared to erodible ash & cinder
 Example of neck - Ship Rock, NM Fig 7.15
B.
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Craters and Calderas
Crater at summit or near summit of volc
Caldera is a crater > 1 km in diam
Good example is Crater Lake, Oregon
Largest in US is in Colorado..named “LaGanita”
- some calderas huge, believed to be result from collapse after
eruptions occur and magma chamber empties
Crater Lake 6 miles across, 4000 feet deep
LaGanita 17 miles across
- some other calderas form more quietly on shield volcanoes
Mauna Loa
Kilauea
these are 2-3 mi across, 650 feet deep
C. Fissure Erutions and Lava Plateaus
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Columbia plateau built from fissure (cracks or fractures) eruptions
- basalt flows dreated plateau up to 1 mile thick
- thin, fluid lava flowed as much as 90 miles away from source
- known as “flood basalts”
D.
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Pyroclastic Flows
Silica-rich, ash and pumice
Can often resemble lava flows
Usu assoc with large calderas
Example is Yellowstone Plateau in Wyoming
Earth Sci wk 7 p.6
VI.
Intrusive Igneous Rocks
 Large bodies of intrusives known as “plutons”, after Pluto, lord of the
underworld
 Terminology and Definitions Fig 7.19
- tabular(sheetlike) vs massive (large bulky blob)
- discordant (cut across bedding) vs concordant (parallel to bedding)
A. Dikes – sheetlike, vertical slabs, usu injected along fractures
Often follow conduits that were used by magma to feed lava flows
B. Sills – sheetlike , horizontal slabs, usu injected between sedimentary
beds Fig 7.20
Sills usually basaltic..fluid magma able to penetrate between layers
Usu emplaced at fairly shallow levels; not much weight of overlying sed rx
C. Laccolith – more viscous magma than sill, has convex upward top Fig
7.19
D. Batholith – largest of the intrusive bodies
- Defined as > 100 sq km in area (stock is a small batholith or
portion of batholith)
- Idaho batholith – 49,000 sq km, tens of km thick
- Batholiths often at the cores of mt ranges
- Baths can take millions of years to form and cool
Example – Sierra Nevada bath emplaced over a 130-million year
time span
VII. Igneous Activity and Plate Tectonics
2 themes here:
origin of magma
global distribution of volcanic activity
A. Origin of magma
Source of magma has been a puzzle, given that both crust and mantle
are believed to be solid rock.
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Theory is that magma originates from this solid rock
- most straightforward way to generate magma is to melt the rock by
raising temperature
need 750oC to melt granite, 1000oC to melt basalt
Earth Sci wk 7p.7
How do we get the heat to melt the rock?
- one poss is radioactive decay of elements
- miners note that it gets hotter as you go deeper into the Earth
we theorize that oceanic lithosphere descends into hot mantle, and
mantle melts the oceanic crust, which generates basalt magma
we also theorize that this hot basalt magma, hotter than granitic crust,
can melt the granitic crust, which in turn becomes explosive andesitic
magma such as that which came out of Mt St Helens or Mt Pinatubo
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But pressure important too..
- as confining pressure incr,so too does the melting temp of rock
(higher press tends to keep things in the most dense state possible)
- the converse is also true..as rock ascends and moves into zones of
decreasing confining press, the melting point of the rock decreases,
and melting can begin
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Concept of partial melting important too
- silica-rich minerals melt first, mafic mins melt later
- very important consequence here….a melt from a parent rock is
usually more silica-rich than the parent rock…this helps us understand
the chemistry of magmas globally
B.
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Distribution of activity
Most volcanoes are assoc with convergent margins
Divergent margins also sites of extensive extrusion of magma
3 main focal pts for volc activity: Fig 7.22
1. on spreading center ridges
2. next to trenches
3. within plates
1. Spreading centers
- largest volume of lava extruded here, fissure eruptions mostly
- partial melting of peridotite thought to produce basalt
- occasional volc cones develop Fig 7.23
2. Subduction zones
- partial melting thought to occur at depth of 60 miles
- both basaltic and andesitic magmas produced
- magma migrates slowly upward toward surface because it is less dense
than surrounding rx
- oceanic island arcs produced by volcanoes that extrude the magma
Earth Sci wk 7 p.8
When subduction occurs under a continent, the hot ascending magma
tends to melt the granite crust, so magma becomes more silica-rich
andesite, named after Andes Mts, which have this composition
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The Ring of Fire around which ocean?
Why are the volcanoes there?
3. Intraplate (within the plate) Volcanism
- processes remain difficult to determine
- very different lithologies (rock types) can overlie each other
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Suggestion is that hotspots in mantle cause these “point source”
eruptions
- composition of lavas and ash inland in continents is granitic,
suggesting that re-melting of granitic crust occurs to produce this
secondary magma
- mechanisms for hotspot activity still not well understood