GEO 305 Midterm - Living with active Cascade Volcanoes
Keys to protecting people and property are:
Assessment - could it erupt? If so, how violently?
Monitoring - watching for signs of impending eruption
Mitigation - how to prepare for and respond to an eruption
Positive aspects of living with volcanoes
Volcanic soils are some of the most fertile in the world.
Snow cover on volcanoes provides summertime stream flow.
Heat inside volcanoes can be a source of geothermal energy.
Volcanoes are sometimes associated with deposits of gold, silver, copper, and other precious metals.
Negative aspects of living with volcanoes
Eruptions may be violent, destroying lives, infrastructure, and natural resources.
Effects may be widespread, especially due to ash fall.
Airborne ash poses hazards to airplanes.
Effects may last for years to decades because loose volcanic deposits are easily eroded and remobilized as mudflows.
The Cascade Volcanoes
The Cascades are a volcanic mountain chain stretching from northern California to southern British Columbia.
They are a broad (50-80 mile-wide) volcanic platform with ~16 major high (10,000-14,000 ft elevation) volcanic cones.
All historic volcanic eruptions in the lower 48 states have been Cascade volcanoes. Baker, Rainier, and St. Helens have been the most active volcanoes in historic times.
The Cause of Cascades Volcanism
Internal Structure of Earth
Crust (Solid & Rigid)
Mantle (Solid but deformable)
Outer Core (Liquid)
Inner Core (Solid)
Solid and Rigid
Rich in light elements (Si, Al, K, C, O, etc.)
Only 0–90 km thick (0–11 km for oceanic crust, 25–90 km for continental crust)
Solid but deformable
Mixed elements (mostly Si, O, Mg, Fe)
From base of crust (0–90 km) to 2900 km; accounts for 2/3 of the entire mass of Earth
Heavy, molten Fe and Ni metal. Circulation of liquid Fe creates our magnetic field.
From 2900–5155 km deep
Heavy, hot Fe and Ni, but the pressure is so high that is is solid.
From 5155–6370 km deep
Where does lava come from?
The liquid metal of the outer core is way too deep and dense to ever rise to the surface.
Under the right conditions, parts of Earth’s solid mantle and crust can melt, creating lava that is hot enough and buoyant enough to rise to Earth’s surface.
Internal Structure Observation of Earth
Total mass minus mass near surface leaves a lot of mass that must be hidden in its deep (dense) interior.
Earth’s strong magnetic field requires a large body of magnetic material such as a convecting zone of liquid iron.
Meteorites fall to Earth are pieces of planetary bodies that were once similar to Earth, but were destroyed by collisions.
Earthquake waves that cannot travel through liquid, and they do not travel through the central part of Earth’s interior.
The sliding plates (called “Plate Tectonics”) is one of the greatest revolutions in Earth Science
Most earthquakes and volcanoes occur at plate boundaries.
The outer 100 km or so of Earth is a rigid, brittle shell
The lithosphere is broken up into pieces or “plates”, like a cracked shell on a hard-boiled egg.
These plates can slide around on the underlying weak mantle
Is solid, but deformable (like plastic)
Greek “tecton”: builder / architect
The study of large features on Earth’s surface and the processes that formed them.
Large features:continents, ocean basins, mountain ranges
Processes:earthquakes, volcanic eruptions
Due to movement of plates ofEarth’s outer shell.
1912 Alfred Wegener
Stated that there once existed a large “supercontinent” (called Pangea) that split into the pieces that are our present continents.
Wegener based his idea on 4 observations
1. Fit of the continents
2. Crust appears to “float” on the mantle
3. Correlation of fossils, rocks, glacial depsoits, and mountain belts
4. Diversity of Species
Fit of the continents
The coastlines of many of the continents appear to fit together like pieces of a jigsaw puzzle.
Crust appears to “float” on the mantle
Continental crust is thicker than oceanic crust, but most of that extra thickness is hidden below the surface as a deep “root”.
Correlation of fossils, rocks, glacial deposits, and mountain belts
Areas of specific fossils or rocks, or mountain belts, became continuous across what is now open ocean if the continents were fitted back together as their coastlines suggested.
Diveristy of Species
Animals that existed before Pangea split apart were similar between continents, whereas animals that existed after the continents separated are often unique to certain continents (such as the unusual mammals of Australia).
Wegener’s Continental Drift idea had two critical weaknesses
No reasonable mechanism for why the continents would drift
We knew that Earth’s crust beneath the oceans was solid rock. How could the continents possibly plow through the ocean crust as they moved around?
(SOund Navigation And Ranging)
During and after WWII revealed some oceans had long ridges down the middle (“mid-ocean ridges”) that looked like the rift valleys that we see on land (e.g. Iceland, Rio Grande Valley, Red Sea)
Typical Rates in Plate Tectonics
Plates move at 1–10 cm/yr (a few inches/yr) [rate of fingernail growth]
~ 3-14 m in a human lifespan (70 yr) = 10-100 km/million years
Rates confirmed over: Geologic time (magnetic stripes on seafloor), Modern times (surveys, GPS measurements)
Two Important Factors in the Development of Landscapes
1) Type of Plate boundary
2) Type of Crust
Type of Plate Boundary
Type of Crust
Continental (thick; more buoyant)
Oceanic (thin; less buoyant)
Tectonic (Lithospheric) Plate
Continental Crust: thick and rides high
Hard plate of Lithosphere is made of crust & uppermost mantle
Oceanic Crust: thin and sits low
Plates rides on softer part of mantle (Asthenosphere)
Divergent Plate Boundaries
Plates move away from one another.
Plates (Lithosphere) created
Convergent Plate Boundaries
Plates move toward one another.
Plates Destroyed and Recycled
Transform Plate Boundary
Plates slide past one another.
Lithosphere neither created nor destroyed.
Pacific plate slides past the N. American plate along the San Andreas Fault in Cali.
Plates rides over plume of hot mantle.
Important, but not a plate boundary.
The hawaiian Islands form as teh Pacific Plate moves northwestward over a stationary hotspot.
Three Stages of Divergent Plate Boundary Development
Continental Rift / Mid-Ocean Ridge
New Ocean (Red Sea)
Mature Ocean (Indian Ocean)
Mid-Ocean Ridges Circle the Globe
East Pacific Rise
Basin and Range Province
A continental rift zone in the western U.S. and Mexico.
San Andreas Fault
Pacific Plate and Norht American Plate
Transform Palte boundary is a broad zone of shearing between two plates
Observations along Hawaiian Island Chain
Islands are older in a norhtwest direction.
Islands are more eroded in a northwest direction.
Islands are shorter in a northwest direction.
Yellowstone National Park, Wyoming
Geothermal features form due to regions lies about the Yellowstone hotspot
Formed as a plate capped with oceanic crust plunges down at a subduction zone.
Subducting Juan de Fuca Plate froms two Parallel mountain ranes in the Pacific Northwest
Oceanic sediment and basalt scraped off subducting plate, forming Coastal Mountains.
Puget Sound and the Willamette Valley are low-lying regions between the rising mountains.
In OR and WA, subduction causes two different mountain ranges.
The Coastal Ranges contain materials that were manufactured in the sea, then scraped off the subducting Juan de Fuca Plate
Layers lifted out of the Sea.
Lava erupts into cold ocean water
Lava squeezes in tube
Why are the Cascade Volcanoes in such a straight line?
The Cascade Volcanoes lie above the line at which the top of the Juan de Fuca plate is deep enough (~50 miles), so that it is hot enough to dehydrate (sweat)
The Cascades are volcanoes formed above the sweating plate
Leads to the highest topography on earth.
Cascadia Subduction Zone
Juan de Fuca Plate subducts underneath the North American Plate at ~3 cm/yr.
At the Surface: magma reaches the surface (called lava), it erupts & forms volcanoes.
In the Upper Crust: magma is hot enough & finds a pathway through the crust it will make it to the surface or stay in crust & cool, & not erupt. In this case it will form a large body of igneous rock called a pluton.
In the Upper Mantle/Lower Crust: Water driven off of the descending plate migrates upwards lowering melting temperature, causes melting of the overlying mantle, forming magma that rise throught the mantle & crust.
Separates the Western Cascades from the High Cascades.
Provide pathways to the surface for magmas
Helps explain the abundance of volcanism in the central Cascades of Oregon.
The low area created by the down-dropped graben contains many lakes.
3 Ways to make rocks melt
1. Increase in temperature
2. Decrease in pressure
3. Addition of impurities
Melting point of rock is lower at lower pressure.
Chain of volcanoes above a subductionzone
= arc-shaped on a map (earth is round)
What happens when rock melt?
Rocks are made up of minerals, and different minearls have different melting temperatures.
Chemical composition of the partial melt depends on what minersals melted to create it.
only part of the rock melts at any one time
less dense than the rock it melted from, so it will start migrating upwards through the mantle.
magmas formed by between 1% and 20% partial melting of mangle rocks
magma erupts onto Earth's surface
magma solidifies underground
Extrusive (Volcanic Rock)
magma erupts and solidifies
molecule is the mian component in most magma
silicon ion (Si4+) bonded to 4 oxygen ions (O2-) in the shape of a tetrahedron. =SiO2
magma cools, forming chains and networks
The arrangement depends on temperature, pressure, and other available elements
(such as Mg, Fe, Al, Ti, Na, K, etc.)
Si and O ions make up about 50%–90% of most magmas and Al, Ca, Fe, K, Mg, Na, and Ti
Ba, Cr, Mn, Ni, P, Sc, Sr, Zn, and many others.
Each type of mineral
different crystalline structure & chemical composition
characteristic range of T, P, and magma composition at in which it will grow.
Some crystallize at higher temperatures
If cools too quickly, no minerals will have time to grow.
Magmas can have complex cooling histories
larger crystals crystallized early while the magma was still underground =cooled slowly
smaller crystals formed only after the lava erupted = cooled quickly.
The ordered structures of molecules
As a magma continues to cool, the polymers settle into ordered structures as they search for their minimum energy state.
As a magma cools, the minerals that crystallize first are those with the highest melting temperatures.
As the magma continues to cool, addition minerals with lower crystallization temperatures begin to crystallize.
Bowen's Reaction Series
Olivine & Pyroxene
higher in heavy elements (Mg, Fe, and Ca) lower amounts of light elements (such as Na and K).
Separation of the crystals from the magma
a fraction of the original magma is removed when the crystals are separated from it
Fractional crystallization takes magmas that were high in Mg-Fe (mafic), and turn them into magmas low in Mg-Fe, but high in K, Si, etc. (silicic)
Igneous Rocks classified according to two parameters
1. Texture (grain size, vesicularity)
2. Chemical Composition (indicators are minerals and color)
Classified by composition
which is mostly determined by how much partial melting created them
how much fractional crystallization has occurred
Si content is often used to summarize the classification because Si increases with increasing fractional crystallization.
Lowest Si, highest Fe and Mg
Ultramafic magmas that do not reach the surface crystallize to peridotite,
If they reach the surface they form a rare volcanic rock called komatiite.
Low Si, high Fe, Ca and Mg
In all tectonic settings, but most abundant in hotspot & continental rift/mid-ocean ridge settings.
Mafic magma crystallizes into an intrusive rock called gabbro or volcanic rocks called basalt and basaltic andesite
Moderate Si, high Ca and Al
Erupt in subduction zones.
Intermediate lava crystallizes into diorite, or a volcanic rock called andesite (named after the Andes mountains) and dacite.
High Si, Al, K, Na (low Fe, Mg).
Erupts on thick crust in subduction zones.
Silicic lava crystallizes into rhyodacite and rhyolite, or granite if it is intrusive.
Key Properties of Magma
Viscosity: A measure of how much something resists flowing
Silica content: More viscous with increasing silica content
Temperature: Magmas, like many fluids, get more viscous as temperature decreases.
Density: Mafic magmas are denser because they contain more of the heavier elements
Volatile content: All magmas have some volatile gasses dissolved in them, water, CO2 , S, F and Cl.
Properties of Magma
Magmas that are hotter, less viscous (runnier), and less volatile-rich (less gassy) tend to erupt effusively (calmly).
Magmas that are stiffer (more viscous) and more volatile-rich (gassier) tend to erupt explosively
Basalt and Basaltic Andesite = Usually Effusive
Rhyolite and Rhyodacite = Usually Explosive
Andesite and Dacite = Either
Innards of a Volcano
Volcanic Vent or Crater
Volcanic Conduit (Neck)
Volcanic Vent or Crater
Where lava leaves the volcano is called the vent, or crater if it is a depression. May be located at the top, side, or edge of the volcano
Volcanic Conduit (Neck)
This is the path that magma takes to get to the surface. After the volcano has gone extinct and the outer parts are eroded, the solidified conduit that is exposed is called a “volcanic neck” (Mt. Washington, Mt. Thielsen).
This is where most fractional crystallization occurs, so magma here contains many crystals and is sometimes called a “crystal mush”.
Volcanic rock and/or tephra (loose volcanic debris) that surrounds the vent. It can be very steep or gently sloped, depending on the material.
Created by a single eruption (may be over a number of years, but once it goes dormant it does not reactivate). Typical of small volcanoes such as scoria cones (aka cinder cones).
Created by multiple eruptions, even over thousands or millions of years. Most volcanoes, and probably all large ones, are polygenetic.
Essential material and driver
Any erupted material, such as lava, tephra, and volcanic gases, that actually came from that volcano’s magma chamber during the eruption. This is what creates and drives the eruption, and usually makes up the largest portion of erupted material.
Not Directly related to the eruption
The magma chamber may become incorporated into the magma on it way out of the volcano. These bits of other rocky debris are called accidental, and can come from a previous eruption or from other rocks under the volcano.
Due to rapid escape of volatiles from the magma.
Silicic lavas (dacite, rhyolite) have the high viscosity and volatile contents that typically lead to explosive eruptions.
Create abundant tephra.
Can also occur when magma encounters external water such as an ocean, lake, glacier, rain, or groundwater. (hydromagmatic eruption).
Fairly calm eruptions that produce lava flows and lava domes.
Typical eruptive style for mafic lavas with low viscosity and low volatile contents (basalt, basaltic andesite),
Can also occur in silicic lavas that have degassed their volatiles.
The eruption may be centralized at or near the top of the volcano.
Some eruptions are on the flank of the volcano.
An eruption can occur simultaneously on the summit and the flank.
An eruption can occur on the periphery of a volcano, not directly related to activity at the main vent.
Lava that erupts from cracks a few hundred feet to a few miles long.
Early stage may include tall spurts of lava (“fire fountaining”) that produce some minor basaltic tephra.
Mostly produces thin, runny basalt lavas that can travel 10–100 km or more.
The largest lava eruption in historic times (1783, Laki, Iceland) was a fissure eruption.
Massive floods of basaltic lava covering large areas (flood basalts) are caused by unusually intense fissure eruptions. (Columbia River Basalts)
Enormous edifice, low profile, gentle slopes.
The largest volcano on Earth (Mauna Loa) is a shield volcano.
Made of low viscosity and low volatile content basalt lava flows that are able to spread over a large area.
Summit eruptions and rift eruptions along the flanks of the volcano are common.
Scoria Cone (Cinder Cone)
Small, steep-sided cone made of basaltic tephra (called cinders or scoria) ejected from a vent. Many have a lava flow extending from their base.
The loose material piles up near the vent at the angle of repose
Composed of basalt or basaltic andesite, with low viscosity but high enough volatile content to eject tephra.
Composite volcano (stratovolcano) Simple cone
Often a beautifully conical, radially symmetric mountain. Like a grand cinder cone.
Typically formed by repeated eruptions over a long time (polygenetic), but all from the summit vent.
Composed of relatively low viscosity basalt and basaltic andesite flows and tephra.
Long-lived volcanic center with a complex eruptive history: multiple lava and tephratypes from multiple vents over 10,000 to 1,000,000 years or more.
Andesite, dacite and rhyodacite lavas (high viscosity and high volatile content), tephra, and pumice. Explosive eruptions common.
Most subduction-related (volcanic arc) volcanoes
Tuff Ring & Tuff cone
Circular feature constructed of basaltic tephra.
Interaction of basaltic lava with external water near the surface,
Like scoria cones that erupt in water
Steam explosions propel tephra high into the air, & steep pile of wet tephra forms where it lands
Sides are steeper than a cinder cone because wet tephra is stickier than dry tephra
Tuff rings are hollow and broad, tuff cones are more cone-shaped.
Underground explosion due to rising magma interacting with external water (usually groundwater).
There may be very little juvenile material involved in the explosion.
Explosion excavates a pit (usually <1 km across and 100 m deep) that looks like a meteorite impact crater.
Small, steep-sided, dome-shaped feature.
High viscosity lava (dacite, rhyodacite or rhyolite) low in volatile content.
Usually caused by a single eruption from a single vent beneath the dome.
Erupts effusively and very slowly, but the dome can collapse and form a dangerous, fast-moving flow of hot rocks and ash.
Steep-sided feature formed during a sub-glacial eruption.
Lava erupts under a glacier cools very quickly and cannot travel far
It is constrained by the glacier into a steep-sided hill.
Can either melt all the ice or emerge through the top of the ice to create normal-looking basalt.
Inward collapse of the summit area of a volcano, after the rapid emptying of a magma chamber by a large eruption. Collapse feature at least 1 km in diameter
Calderas and craters are sometimes confused because both appear after violent eruptions, but careful study of the deposits on the floor of a caldera reveals that the top of the volcano collapsed down into the caldera.
involves external water such as an ocean, lake, glacier, rain, or groundwater.
This can affect the style of the eruption because water explodes into steam when it encounters magma, which can turn an otherwise effusive eruption into an explosive one
Two types of hydromatic eruptions
Phreatic – explosions of steam only, with no juvenile material.
Phreatomagmatic – explosions of steam that include juvenile material
is a measure of the size and explosiveness of an eruption
Eruptive Power define by:
Dispersal: total area covered by tephra (in km2).
Fragmentation: size of tephra pieces.
Intensity: rate at which material erupted (in kg/sec).
Magnitude: total volume of erupted material (in km3)
Volcanic Explosivity Index (VEI)
Chris Newhall & Steve Self (1982)
based on total volume of erupted tephra
Relationship between the size of eruptions & the length of time between eruptions of that size – larger eruptions occur much less frequently than smaller ones.
Basalt and Basaltic Andesite: Usually aphanitic or porphyritic, sometimes vesicular.
Typical eruption temperature is approximately 1100°C.
1. Found in all tectonic settings
2. Especially common at mid-ocean ridges & intraplate (hotspot) islands
3. Low viscosity and usually low volatile content
4. Usually erupt effusively.
5. Some mafic volcanoes are volatile-rich
6. External water can also cause explosive eruptions
Phase 1: Inflation
Phase 2: Dense Fumes
Phase 3: Fragmental Ejecta
Phase 4: Curtain of Fire
Phase 5: Central Vent
Relatively small eruption (low on VEI) that produces a small, cauliflower-shaped eruption column.
Common as flank eruptions on larger volcanoes.
Typically create cinder cones.
Usually basalt or basaltic andesite
Usually basalt, but may be andesite or dacite.
Erupt tephra, lava flows uncommon.
Typically occur at composite volcanoes
Lots of interaction with external water
Jets of black tephra with white steam
95% tephra and ash
Forms a tuff ring or tuff cone
A flow with a rough rubbly surface composed of broken lava blocks (called clinkers) surrounding a thick, hot core.
Flows like a tank tread rolling over earlier clinkers.
This causes rubble zones above and below the lava flow
Lava with a smooth or ropy surface. Advances as a series of small lobes and toes that continually break out from the leading edge of the cooling crust
Active lakes have a partially cooled crust 5–30 cm thick,
Only a few minutes or hours old.
Crust continually forms, circulates, breaks, and sinks into the moving lava below (like a miniaturized version of plate tectonics).
Lavas reach water, cool quickly & pile up into steep benches.
Not stable & very dangerous to walk on.
Hawaiian lava benches have collapsed into the ocean without warning, killing the tourists standing on them.
Lava encounters water, violent steam explosions
Forms basaltic tephra that piles up on the shoreline.
Similar to a cinder cone except that it is rootless (its lava source is a vent somewhere upslope, not beneath it).
Rapidly cooling blobs of lava that quickly form a chilled glassy margin. Lava breaks through the crust and squirts out to form another pillow.
large volumes of basalt lava that erupted in a short period of time
shooting a column of ash high into the atmosphere.
Long-term solutions to understand likely effects of an eruption & prepare residents for those effects
Short-term solutions to detect an eruption & remove people from the vicinity or divert the eruption away from population.
Four Key Components make for successful mitigation
1. Good science and research
2. Outreach and education
3. Practice and drills
4. Monitoring and early warning
Volcanoes create new land
Lavas erupt and tephar is deposited, new lands can be created.
Home Reef, Tonga
Volcanoes create fertile soil
Volcanic materials are rich in mineral nutrients, so volcanic soils are very fertile.
Good rice growing regions in Indonesia
The civilizations of Rome and Greece excellent vineyards & olive groves.
Coffee, pineapple, sugar cane, & macadamia nut plantations in Hawai’i
Water is heated by hot volcanic rocks or magma, creating two important benefits of volcanic activity: energy and mineral deposits
High temperature - Electricity is produced in geothermal fields by circulating water through hot rocks until it turns into steam, which is pumped back to the surface to drive turbines connected to generators.
Low temperature - Hot water can be pumped directly into the heating systems of buildings. 70% of the homes in Iceland are heated this way.
Byproducts - Some geothermal systems also produce byproducts such as gold, mercury, silver, and sulfur.
The Geysers, California
Near Santa Rosa, is the largest geothermal field in the world
Produces 2000 megawatts (MW) of electricity
Enough to power 2 cities the size of San Francisco
Volcanic Mineral Deposits
Valuable mineral deposits form from geothermal activity in volcanic rocks. Includes Cu, Sn, Au, Ag, Zn, Ni, Cr, S, and many others.
Form beneath the surface. Hot water leaches tiny amounts of metals from large bodies of hot rock. Waters cool upon migration into cooler surrounding rocks. Metals come precipitate into enriched veins & metallic bodies.
Most diamonds are mined from a rare volcanic rock calledkimberlite
Chilean Copper Belt
Berkeley Pit Mine (Butte, Montana) - Superfund Site
Bingham Canyon Copper Mine (Utah)
Hot fluids percolate through the volcanic system at a ridge and bring up hot, mineral-rich fluids.
Minerals rich in S, Fe, Cu, Zn, Pb, & Ni precipitate into towers referred to as “black smoker chimneys”
Industrial Products of Volcanoes
One of the most important is sulfur used to produce sulfuric acid and fertilizer.
A sulfur mine near the summit of Volcan Aucanquilcha in Chile (~5800 m or ~19,000 ft).
This mine removed sulfur as it was being deposited on the summit and slopes by active geothermal activity
Used as abrasive and cleaning product, including making stone-washed jeans. Pumice is mined in Italy, South America, and California, Oregon.
Bentonite is a clay formed from altered volcanic ash.
It has an exceptional ability to absorb water, ammonia, and odors.
Used in drilling mud for oil and water wells, polishing, papermaking, and as a clumping agent in cat litter.
It is also used as an emulsifying agent in soft-serve ice cream
Used as a road-building material, and is spread on icy roads in winter to improve traction
Most volcanic rocks, when unaltered, make excellent building stones.
Basalt, granite, and welded tuff are especially strong and hard, and are often decorative as well
Tourist economies build up around volcanoes that are popular destinations for recreation such as skiing, hiking, and climbing.
Volcanoes with religious significance are visited on pilgrimages.