I. Effusive eruptions: relatively quiet, non-explosive mostly basaltic lava, flows freely.

A. Central Vent Eruptions—lava flows out (sometimes fountaining) from one central vent, then the lava solidifies in approximately the same volume all around.

Shield volcano: a low, broad, cone-shaped structure - looks like a warrior’s shield

Low angle slopes of 1-10

Composed primarily of basalt lava flows

Largest volcanoes in volume

Volcanic Landforms

Mauna Kea - 13,792 ft above sea levelMauna Loa - 13,678 ft above sea level

Both ~30,000 ft from their base

Volcanic Landforms

Shield Volcanco

Low angle slopes of 1-10

Composed primarily of basalt lava flows

Volcanic Landforms Effusive eruptions – Shield volcanoes

Volcanic Landforms Effusive eruptions – Shield volcanoes

B. Fissure Eruptions on Land—basalt may flow out of large cracks in the ground (fissures)

Flood Basalts—large volume of very fluid basaltic lava may gush out at speeds of 25 miles per hour

Columbia River Basalts (CRB) 170,000 km3 about 17-14 Mya

Over 60 individual flows, covering some areas in over 2 km of basalt!

One flow alone could pave I-90 from Seattle to Boston 575 feet deep!)

Siberian Flood basalt 900,000 km3 about 245mya

Lava Plateaus—thick plateaus of lava spreading over areas thousands of kilometers

Volcanic Landforms

Fig. 7.18a

W. W. Norton

B. Fissure Eruptions on Land:Flood Basalts

basalt may flow out of large cracks in the ground (fissures)

Volcanic Landforms

B. Fissure Eruptions on Land:Flood Basalts

Columbia River Basalt: basalt may flow out of large cracks in the ground (fissures)

Volcanic Landforms

II. Pyroclastic Eruptions: Explosive, involve viscous, gas-rich magma.

The more gas-rich it is the higher the tephra column; less gas results in pyroclastic flows.

A. Cinder Cones: formed by gas-rich lava of any composition (usually basaltic).

Built of tephra that is remarkably vesicular (pumice to scoria)

Generally short lived eruptions - weeks to a few years until the magma is degassed, then it solidifies in the pipe and flows form from the base

After they’re done, they never erupt again!

Smallest volcanic features have large craters with steep slopes of 30-40

Volcanic Landforms

Paricutin, Mexico, cinder cone soon after its birth in 1943 in a Mexican cornfield.

B. Composite Cone or Stratovolcano

Volcanoes on continents over subduction zones

Built up by alternating layers (lava and pyroclastic deposits)

Steeper slopes 10-25

Cascades, Andes, Aleutian Islands

Built over tens to hundreds of thousands of years

Volcanic Landforms

Izalco, El Salvador, December 1949.

Small steam eruption and a view of the older lava flow on the side of the cone in the foreground.

Composite composite cone

Lava flow

Blast cloud

Summit crater

Volcanic neck

Layers of lava flows & pyroclastics

Lava flow

Pyroclasticflow

Eruption onflank of upbuildingcomposite cone

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1.Lava Dome

Degassed magma may erupt in the crater and harden there without flowing anywhere

Produces a plug in the volcanic vent which must be blown away before future eruptions can occurTraps gases inside so they build up pressure.

2. Calderas

Energetic eruption, blasts out everything, then collapses

Volcanic Landforms

Caldera

A large amount of magma erupts explosively to form ash fall and ash flow deposits, partially emptying the underlying magma chamber.

There is essentially a big "hole" the overlying rock collapses, leaving a depression.

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1.Calderas

The following diagrams show the formation of Crater Lake during the climactic eruption of Mount Mazama.

Eruption deposits airfall pumice and ash, blown by winds to north and east.

Volcanic Landforms

1. Vent enlarges and eruption column collapses.

2. Pyroclastic flows deposit the Wineglass Welded Tuff on north and east flanks of Mt. Mazama

Volcanic Landforms

Caldera has been partly filled with pumice and ash from the eruption with blocks of rock from the caldera walls

Weak, dying explosions within the caldera deposit ash on the caldera rim

Pyroclastic-flow deposits develop fumaroles and gradually cool.

Volcanic Landforms

Crater Lake today

Volcanic Landforms

C. Ash-flow eruptions

Eruption not from a cone, felsic magma

1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock.

2. Creates ring fractures over the bulge.

3. May collapse, magma forced into fractures and erupts

Forms large calderasLargest and most devastating

eruptions in history

Volcanic Landforms

C. Ash-flow eruptions

Eruption not from a cone, felsic magma

1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock.

2. Creates ring fractures over the bulge.

3. May collapse, magma forced into fractures and erupts

Forms large calderasLargest and most devastating

eruptions in history

Volcanic Landforms

C. Ash-flow eruptions

Eruption not from a cone, felsic magma

1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock.

2. Creates ring fractures over the bulge.

3. May collapse, magma forced into fractures and erupts

Forms large calderasLargest and most devastating

eruptions in history

Volcanic Landforms

Volcanic Landforms

B. Composite Cone or Stratovolcano

Mt. St Helens pyroclastic eruption on the volcano flank.

Lateral blast

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C. Ash-flow eruptions

Eruption not from a cone, felsic magma

1. Felsic magma pushes up into the crust near the surface, bulging the overlying rock.

2. Creates ring fractures over the bulge.

3. May collapse, magma forced into fractures and erupts

Forms large calderasLargest and most devastating

eruptions in history

Volcanic Landforms

C. Ash-flow eruptions

Examples:

Toba, Indonesia75,000 years agoCaldera is 30 x 60 milesCovered 10,000 square miles!

—1000 feet thick!

Yellowstone3 major eruptions in last 2

million years

Approximately 1000 times larger than Mt. St. Helens!

New felsic magma may be pooling, thermal features are heated by the magma

Volcanic Landforms

Volcanic Landforms

The major eruptions of the volcanic field were exceedingly voluminous, but their products are only surficial expressions of the emplacement of a batholithic volume of rhyolitic magma to high crustal levels in several episodes.

The total volume of magma erupted from the Yellowstone Plateau volcanic field since 2.5 million years ago probably approaches 6,000 cubic kilometers.

C. Ash-flow eruptions

Examples:

Long Valley caldera near Mammoth Lakes ski resort in California, north of Bishop, CA

Last erupturion 700,000 years ago

Over the past 20 years the floor has risen 9 inches

Magma recently risen from 5 miles depth to 2 miles

Eruption very likely, but timing not certain

Volcanic Landforms

C. Ash-flow eruptions

Examples:

Long Valley caldera near Mammoth Lakes ski resort in California, north of Bishop, CA

Last erupturion 700,000 years ago

Over the past 20 years the floor has risen 9 inches

Magma recently risen from 5 miles depth to 2 miles

Eruption very likely, but timing not certain

Volcanic Landforms

Volcanic Landforms

Imagine the effects of a large caldera forming eruption

Ash covering the US!

Clogs all air filters, engines no cars, no electricity, not air travel

Abrades all moving parts(ash + water is very heavy building collapses

Centimeters of ash on all crops crop failure and famine

Volcanic Landforms

Imagine the effects of a large caldera forming eruption

Ash covering the US!

Clogs all air filters, engines no cars, no electricity, not air travel

Abrades all moving parts(ash + water is very heavy building collapses

Centimeters of ash on all crops crop failure and famine

Volcanic Landforms

p.194-195c

original artwork by Gary Hincks

9 km

3 km

0.3 km

1.5 km

15 km

150 km

Shield volcano (e.g. Hawaii)

Composite volcano (e.g. Vesuvius)

Cinder cone (e.g. Sunset crater)

Volcanic Landforms

Non-violent vs. explosive eruptions

Basalt: flows onto the surface

Andesite/Rhyolite: explode, huge eruptive clouds

Depends on the viscosity of the lava- resistance of lava to flow (water vs. molasses)

Viscosity is controlled by:

1) silica content

2) temperature

3) gas content

Santa Maria, Guatemala. Santa Maria had a huge eruption in 1902, from a vent on the other side of the cone as viewed from this direction.

The 1902 eruption was not from the summit. Starting in 1929, a lava dome began to grow in the 1902 crater, and it is still active today. It is named Santiaguito.

Basaltic lava

Basaltic lava

Silica content

As silica tetrahedra bond together they make the magma thicker, or more viscous.

Similar to slushy as ice bonds start to form. Slushy is thicker and more viscous than water.

Thus more silica = more viscous

Mafic magmas = <50% silica

Intermediate magmas >60-65% (Andesite)

Felsic magma’s = >65% (Ryolite).

Felsic (granitic) magmas = more viscous based on silica content.

Sarigan volcano, Northern Mariana volcanoes. It has not had any recorded eruptions but it is very young. That is just a regular cloud over its summit.

Temperature

The hotter a liquid is the less viscous it is. Example: heating honey or molasses to make them flow more readily. Breaks bonds.

Temperatures required to melt the minerals in mafic vs. felsic magmas.

Minerals in mafic magmas have higher melting temperatures- therefore the magmas must exist at higher temperatures.

Minerals in felsic (granitic) magmas have lower melting temperatures- cooler, thus more viscous.

Lascar Volcano, Chile, The most active stratovolcano in the central Andes. Note the two massive andesite flows exhibiting thick flow margins tens of meters high and well-developed lava levees. Courtesy of Peter Francis.

Gas Content

Gases in magmas = mostly water vapor, also SO2, H2S, CO2, HCl, …

If the magma has a low viscosity (e.g., basaltic magmas) the gases can escape easily.

If they magma has a high viscosity the gases are trapped, they build up until they explode (felsic magmas)

Colima Volcano, Mexico. Thick, short andesite flow on the flanks of Colima. Courtesy of J.C. Gavilanes, Universidad de Colima.

Buoyancy

There are several factors that are important in allowing magma to move to the surface and erupt as a volcano.

The first is buoyancy. Buoyancy is the tendency for a less dense substance to move up or float.

Generally, a liquid is less dense than a solid of similar chemical composition.

Pacaya, Guatemala is a volcanic complex of two small strato-volcano cones and older lava domes.

It has erupted over twenty-two times since its birth in 1565 and nearly annually since 1965.

Buoyancy

Since magma is liquid rock, surrounded by solid rock, the magma will tend to move up through the crust toward the surface.

As a magma is moving toward the surface, it is moving into cooler areas of the crust (geothermal gradient)

Begins to cool down. Pacaya, Guatemala. Eruptions are generally characterized by explosions, but recent eruptions have also produced lava flows. Here, an ash eruption shortly after the February 4, 1976, magnitude 7.5 earthquake.

Buoyancy

What happens to magma when it cools down?

It begins to crystallize. If the magma gets to about 50% crystallized, it will stop moving up. ( “crystal mush”).

All those crystals make the liquid too sluggish to flow very easily, and it simply stops moving.

Therefore, whether or not a magma makes it to the surface is really a race between how fast it moves up and how fast it crystallize

The hotter a magma starts out, the more likely it is to get to the surface before it has reached that 50% crystallization.

Eruption

Analogy to soda bottle:

When soda can is closed and the soda is under pressure the carbon dioxide (CO2) is dissolved in the soda.

Open the bottle, release the pressure, the carbon dioxide comes out of solution and bubbles out.

Gas expands and escapes.

Can happen slowly, controlled, or violently.

Magmas generally have a high gas content. Like soda, the gas is dissolved within the magma when the magma is under pressure.

Pressure builds up until there is an eruption, this releases the pressure.

Again, gases expand and escape.

If gases escape easily and gradually (non-viscous) it is a non-violent eruption (basalt),

If gases can't escape easily and gradually it results in a violent eruption (felsic magma).

Obsidian flow, Long Valley Caldera, California

Magmas generally have a high gas content. Like soda, the gas is dissolved within the magma when the magma is under pressure.

Gas bubbles and froth on surface of the lava, similar to bubbles on top of soda.

Produces distinctive texture in the rock.Obsidian flow, Long Valley Caldera,

California, was created by crustal collapse associated with an explosive eruption about 650,000 years ago.

Since that time, felsic eruptions of de-gassed magma have generated viscous rhyolitic domes and short felsic flows.

Types of explosive volcanoes:

Composite or stratovolcanoes

these types of eruptions, which often alternate with more effusive eruptions, produce composite or stratovolcanoes.

Very steep sided volcanoes that are characterized by interbedded or alternating deposits that result from explosive (pyroclastic rocks) and effusive eruptions (lavas).

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