NW Silicic Volcanism
Cascade volcanoes
Cascades are sitting on a base of Basalt
Cascades are sitting in graben
3 episodes of eruption
Western Cascades (35-20 mybp)
Ancestral Cascades (7 mybp)
High Cascades (1 mybp)
Activity
South Sister
Since 1997 uplift of the west flank of S. Sister by as much as 10 inches. (2 in/year)
Probably caused by magma
Depth ~4 miles
Volume ~ 50 million cubic yards
Located by Harmonic tremors
Most recent report (March 2007): slowed growth 1 in/year
Radar interferogram. Measures changes in position. Each full color band (from blue to red) represents uplift of about 2.8 cm. Total uplift has been about 10 cm.
Mt St Helens
Young volcano
Formed about 40,000 years ago
Previous eruption before 1980: 123 yrs ago
Eruption began with a series of earthquake
March 20th
Fractures develop due to quakes
March 24th and 25th
Phreatic eruptions began
March 27th and 28th
Harmonic tremors started in April
Small eruptions through May 14th
North side had been developing a bulge the whole time
May 18th 8:32am eruption began
Started as mag. 5.1 earthquake triggered the slide on North side
Lateral blast followed by upward blast
Debris avalanche from landslide covered 23 square miles with a depth of 150-600 ft. Moved at 70-150 mph
Lateral blast reached 17 miles from crater. Moved at 300 mph with a temp of 600oF
4 bil board feet of timber blown down
Eruption column reached 80,000 ft in less than 15 minutes. Circled earth in 15 days
Pyroclastic flows coved 6 miles2 and reached 5 miles from crater
Multiple flows 3-30 ft thick reaching 1300oF and moving 50-80 mph
Lahars: Flowed down river valleys and damaged 27 bridges, ~200 homes and
~185 miles of hwys.
Traveled at 10-25 mph
and
reduced channel depth from 40ft to 14ft
~55-30 mybp
Created by old subduction zone
Primarily ash and lahar deposits
GREAT FOSSIL RECORD!
Newberry
Stratified Magma Chamber
2 possible scenarios
Start erupting felsic and change to mafic
Felsic material is more buoyant so at top of chamber
Mafic material is erupted at end as deep parts of chamber is tapped
Possibly occurred during eruption at Mt St. Helens about 2000 yrs ago
Start erupting mafic and change to felsic
Early eruption controlled by rifting
When conduits are established and subduction zone is near(i.e. Cascadia subduction) begins erupting felsic material
This is what happened at Newberry
Last eruption was Rhyolite and Obsidian 1300 years ago
Other:
Bimodal eruption
Silica rich then basalt
Silicic Volcanism Hazards
Pyroclastic flows
Ash
Lahars (biggest Hazard…why?)
Other Notable Volcanoes
Krakatoa
Mt Pinatubo
Tambora
http://volcano.und.edu/vwdocs/current_volcs/current.html
Basaltic Volcanism
Basaltic rock is most abundant rock on earth
70% of earth’s surface is covered with basalt
Basaltic magma is mafic in composition
High iron and magnesium
Low in silica
Produced by melting mantle (peridotite)
Where is basaltic volcanism found?
Types of Basalt
Primitive basalt
MORB (mid ocean ridge basalt)
Island arc basalt
Continental rift basalt
Continental flood basalt
Variety of compositions is based on
% of mantle that melts
What region of mantle melts
Erupt immediately or evolve
Assimilate other crust
Or combination of any of these
Eruptions are often non-violent
Low viscosity
Flows travel long distances
Gasses escape easily
Fire fountaining
Higher temperature (1200-1400oC)
On explosivity scale: Low (Hawaiian style)
Special features created by basaltic volcanism
Shield volcanoes
Magma chamber
Rift zones
Fumeroles
Created by layers of lava
Pyroclastics
Material thrown out of the vent during the eruption
Pele’s hair
Pele’s tears
Bombs
Spatter
Creates
Cinder cones
Found in vents or calderas
Lava Flows
Eruptions begin as very fluid lava
Flows can travel LONG distance from vent if eruption is long lived
Generally start on steep slope (on flanks or near top of shield)
Lavas can create Lava Rivers when they flow down a steep slope
Lava Rivers
Channelized flow
Erodes its bed downward like a river
Flow rates are around 40mph
Levees build from spatter
Lava rivers can evolve into several types of flows/features
Lava tubes
Pahoehoe lava
‘A’a lava
Tumuli
Lava Tubes
As lava river flows top cools down and crust starts to form
As crust forms over top it gets thicker as lava cuts down into its bed
Creates an insulating tube for the lava
When eruption ends and tube drains tunnel or cave is left behind
If lava is left in tube get radial joints
Pahoehoe
As lava flow moves down onto more level topography (base of shield) break outs occur
Creates many different ropy flows and small “toes” of lava (average ~1mph)
Flows are emplaced as thin lava flows which can inflate
Often upper channelized flows are classified as pahoehoe
‘A’A lava
Created as channelized flows start to crystallize
Turbulence of flow incorporates upper crust
Turbulence promotes crystallization of elongated minerals (plagioclase)
Creates very jagged lava on top with a molten core
Very thick lava flows
Lava flow inflation
Thin lava flow to start with
Crust on top of flow
Lava flowing underneath “pushes” up on overlying crust causing it to swell
Lava grows over time as more lava is emplaced under
Creates Tumuli
Summary
Lava rivers: High slope, near vent
generally thought of as pahoehoe
Down slope channelized flows change to
lava tubes
‘a’a flows
breakout pahoehoe flows
These often lead to inflation features
Other Features
Kipukas
Benches
Tree casts
Created as lava flow moves through a forest
Trees often smolder and eventually burn away leaving a hole
Intercanyon flows and jointing
Lava is fluid so follows stream valleys
Fills in stream valleys and creates intercanyon flow
Thick lava flow
Cools and causes joints
Jointing
Columular joints
Biscuit Joints
Entabulature joints
Pillow basalts
Created as basalt flow enters water