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Volcanism Gulp! Tectonic Settings of Igneous Activity Figure 5.11 The World’s Active Volcanoes Fig. 5.28 Volcanism Associated with Plate Tectonics Fig. 6.19 Material ejected from volcanoes Lava: Magma that has flowed on the surface of the Earth.

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material ejected from volcanoes
Material ejectedfrom volcanoes
  • Lava: Magma that has flowed on the surface of the Earth.
  • Tephra: Fragments that solidified in the air during eruption.
looking at lava7

Viscosity: Measure of a fluid’s ability to flow.

Higher viscosity = slower flow. (Ketchup has higher viscosity than water.)

Looking at lava ….
looking at lava8
Looking at lava ….
  • What controls lava viscosity?
    • Temperature: Higher temperature = less viscous.
    • SiO2 content: Higher SiO2 = more viscous.
two main types of lava
Two main types of lava
  • Mafic or basaltic:
    • Lower viscosity
    • Faster lava flows
    • Calmer eruptions
  • Felsic or rhyolitic:
    • Higher viscosity
    • Slower lava flows
    • More violent eruptions
basaltic lava
Basaltic lava
  • Erupts at 1000° to 1200°C
  • Can flow as fast as 100 km/hr (but usually a few km/hour)
  • Can travel as much as 50 km from volcano
  • Flood basalts: Very fluid basaltic flows that spread out in sheets over the landscape. Layered.

Columbia River flood basalts in Washington and Oregon.

Figure 6.2

basaltic lava11
Basaltic lava
  • Basaltic lava flowing downhill forms pahoehoe or aa.
  • Pahoehoe: (Hawaiian “ropy”). Thin sheet of lava cools on the surface. Skin is twisted and dragged downhill to form “ropes.”
  • Aa: (Hawaiian “ouch”). Lost gas, is more viscous than pahoehoe. Cools to form thick skin. Skin breaks into jagged blocks.

Figure 6.3

basaltic lava12
Pillow lava: Pillow-like blocks of basalt. ~1 m wide. Formed underwater. Blob of basalt extruded underwater (like toothpaste), skin cools quickly (“quenches”) to form glassy rind.Basaltic lava
eruptive styles and landforms
Fissure eruptions

Central eruptions

Shield volcanoes



Stratovolcanoes (composite)

Eruptive stylesand landforms
fissure eruptions
Not all lava flows have volcanoes.

When low-viscosity lava erupts from cracks in the Earth tens of kilometers long.

Make flood basalt provinces.

Fissure eruptions

Laki fissure (Iceland) erupted in 1783 extruding the largest lava flow in human history (Fig. 6.13).

shield volcanoes
Low-viscosity lava flows (low silica, mafic).

Successive lava flows.

Gently sloping flanks (between 2 and 10 degrees)

Tend to be very large (many 10s of km in circumference)

Shield volcanoes

Fig. 6.9


Mauna Loa -- world’s largest structure -- 10 km above ocean base -- base diameter of 120 km -- took 1 million years to grow from successive lava flows



Is Mauna Loa about to erupt again?

volcanic domes
Forms above a volcanic vent

Viscous lava — usually silica-rich (or cooler magma)

Associated with violent eruptions

Volcanic domes

Fig. 6.9

mt st helens
Mt. St. Helens



Lyn Topinka/USGS


Cinder Cone

  • Formed of pyroclastics only
  • Steep sides — ~30 degrees
  • Relatively small
  • Short duration of activity

Fig. 6.9

cerro negro cinder cone near managua nicaragua in 1968 erupted again in 1995 and 1999
Cerro Negro Cinder Cone, near Managua, Nicaragua in 1968 (erupted again in 1995 and 1999)

Mark Hurd Aerial Surveys


A volcanic rock fragment ejected into the air during an eruption. Loss of gases due to pressure drop results in explosive eruption.

Classified according to size.

Volcanic ash <2 mm in diameter.

Volcanic bombs: Blobs of lava that cool as they fly trough the air. Can be as big as houses.

pyroclasic eruption at arenal volcano costa rica
Pyroclasic Eruption at Arenal Volcano, Costa Rica

Gregory G. Dimijian/Photo Researchers

Fig. 6.5

volcanic bomb
Volcanic Bomb

Fig. 6.5

Science Source/Photo Researchers

lithification of pyroclasts
Lithification of pyroclasts

Volcanic tuffs: Rocks created from smaller fragments.

Volcanic breccias: Rocks formed from larger fragments.


Volcanic Breccia

Fig. 6.7

Fig. 5.8

Doug Sokell/Visuals Unlimited

welded tuff california
Welded Tuff: California

1 foot

Gerals and Buff Corsi/Visuals Unlimited

composite volcano
Alternating pyroclastic layers and lava flows

Slopes intermediate in steepness

Intermittent eruptions over long time span

Mostly andesite

Circum-Pacific Belt (“Ring of Fire”), Mediterranean Belt

Composite volcano

Fig. 6.9

mt fujiyama japan
Mt Fujiyama, Japan

Fig. 5.15

Raga/The Stock Market

phreatic eruption
Phreatic Eruption

An extremely explosive eruption that occurs when hot lava encounters cool seawater. Huge quantities of steam are released.

Phreatic eruption on a Pacific island south of Tokyo.

Fig. 6.11

hot spots
Hot Spots

Fig. 6.19

hot spots38
Hot Spots

What is a hot spot?

A hot spot is the surface expression of a mantle plume.

What is a mantle plume?

A narrow, cylindrical jet of hot material, rising from deep within the Earth (perhaps the core-mantle boundary) that gives rise to surface volcanism.

hot spots39
Hot Spots

Fig. 6.22

hot spots40
Hot Spots

Fig. 6.20

hot spots41
Hot Spots

Fig. 6.20

hot spots42
Hot Spots

Fig. 6.20

types of volcanic hazards
Lava Flows: e.g. Hawaii, 1998

Gas: e.g. Lake Nyos (Cameroon), 1984

1700 people killed

Ash fall: e.g. Mt. Pinatubo, 1991

Pyroclastic flows: e.g. Mt. Pelee, 1902

28,000 killed

Lahars (mudflows): e.g. Nevado del Ruiz, 1985

23,000 killed

Tsunami: e.g. Krakatoa, 1883

36,417 killed

Types of Volcanic Hazards

Before May, 1980

Emil Muench/Photo Researchers


After May, 1980

David Weintraub/Photo Researchers

mudflow or lahar
Mudflow or Lahar
  • A mixture of water and pyroclastic material and sand, gravel, and boulders, in a concrete-like slurry capable of moving up to 100 km/hour
  • Flow is supported by collisions between clasts

The only remaining buildings in Armero, Colombia, 72 km dowstream from Nevado del Ruiz volcano, destroyed and partially buried by lahars on November 13, 1985. Lahars reached Armero about 2.5 hours after an explosive eruption sent hot pyroclastic flows across the volcano's broad ice- and snow-covered summit area. Although flow depths in Armero ranged only from 2 to 5 m, three quarters of its 28,700 inhabitants perished.

escaping a pyroclastic flow at mount unzen japan 1991 fig 6 8
Escaping a Pyroclastic Flow at Mount Unzen, Japan, 1991(Fig. 6.8)

Pyroclasticflow (nueé ardente)

  • Mixture of hot gases, ash, and rocks forming a super-heated and dense current capable of moving 150 km/hr.
  • Buoyancy due to heated gas, density due to ash- turbulence keeps particles suspended in flow

Pyroclastic flows erupted by Mount Pinatubo on June 15, 1991, buried the Marella River valley (SW of Pinatubo) with pumice, ash, and other volcanic rocks to depths of between 50 and 200 m. This eruption was one of the largest in the 20th century, depositing about 5.5 km3 of rock debris over nearly 400 km2.