What controls volcanic eruptions
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What Controls Volcanic Eruptions?. Ch 12.2. Some eruptions are quiet…. Kilauea, Hawaii. Some eruptions are EXPLOSIVE!. Mt. Pinatubo. Montserrat. Mt. St. Helens. What Controls Volcanic Eruptions?. How much water vapor (a gas) and other gases are trapped in the erupting magma

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What Controls Volcanic Eruptions?

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What Controls Volcanic Eruptions?

Ch 12.2

Some eruptions are quiet…

Kilauea, Hawaii

Some eruptions are EXPLOSIVE!

Mt. Pinatubo


Mt. St. Helens

What Controls Volcanic Eruptions?

  • How much water vapor (a gas) and other gases are trapped in the erupting magma

  • How much silica is contained in the magma

Trapped Gases

  • water vapor and carbon dioxide are dissolved in magma

    • When magma rises to surface, gases escape because they are under less pressure

    • Some magma is thin and fluid- this lets gases escape easily- resulting in quiet eruption

    • Other magma is thick and stiff- when gases erupt it results in explosive eruption

Water VaporWater may be added to magma when ocean crust rock is melted at convergent plate boundaries

Silica-rich magma is called granitic/rhyolitic magma

Results in explosive eruptions

Ex. Soufriere Hills in Montserrat

Characteristic of convergent plates

Silica-poor magma is called basaltic magma

Results in quiet eruptions

Ex. Iceland

Characteristic of divergent plates

Composition of magma

Basaltic magma may result in different three lava types:

  • Pahoehoe

  • Aa

  • Pillow (cools

    the fastest)

Pahoehoe lava:

  • Hot, thin, fast flowing

  • harden with a relatively smooth surface

  • Often has a ropy or wrinkled appearance

Pahoehoe lava

Aa lava:

  • Cooler, thicker, slow moving

  • Hardens with a rough, jagged, sharp edge surface

Pillow Lava:

  • Lava cools underwater

  • resembles stuffed pillows

Explosive Magma/Lava

Granitic/Rhyolitic Magma

high in silica

high amount of trapped gases

very viscous

lead to explosive type eruptions

Soufriere Hills volcano-active since 1995

At what type of boundary would granitic/rhyolitic magma form?

convergent boundaries

Composition of magma

  • Andesitic magma – more silica than basaltic magma, but less than granitic magma

    • Common at convergent boundaries

    • Named after the Andes Mountains in South America

    • Also violent eruptions

    • Ex. Krakatau- one of the most violent eruptions in history

Andes Mtns

Krakatau 1883

Sound heard 3,000 mi away!

13,000 x more powerful than nuclear blast at Hiroshima in WWII

Anak Krakatau (Child of Krakatau)

Forms of Volcanoes

  • Depends on:

    • Type of magma it was formed from

    • Type of eruption- quiet or explosive

  • 3 basic types of volcanoes

    • Shield

    • Cinder cone

    • Composite

1. Shield volcanoes

  • Result of quiet eruptions of basaltic lava flows (no ash)

  • May form volcanic cones or fissures- long cracks that result if flood basalts (Ragged Mtn, Lamentation Mtn)

  • Ex. Hawaiian Islands and the Columbia Plateau of Northwest US

Shield volcanoes

Mauna Kea, Hawaii

Shield volcanoes

Mauna Loa, Hawaii

Shield volcanoes

5 Shield volcanoes make up the island of Hawaii

Fissure eruption

Common at divergent plate boundaries, mid-ocean ridges

Fissure eruptions

Columbia Plateau

2. Cinder Cones

  • Small base, steep-sided, loosely consolidated

  • Up to 1000 feet tall

  • Life span of a few years

  • Commonly built from solidified rock thrown through the air called tephra

  • Has violent eruptions, dangerous when close

2. Cinder Cone Volcanoes

Paricutin, Mexico (1943-1952, newest volcano in Western Hemisphere, 1000 killed)

Cinder Cone Volcanoes


Cinder Cone Volcanoes

Pu'u ka Pele on flank of Mauna Kea volcano, Hawaii

Cinder cone example: Paricutín, Mexico


Cinder Cone Volcanoes

Composite Volcano (aka stratovolcano)

  • most beautiful, but most deadly

  • characteristic shape

  • made of alternating layers of lava and tephra- periods of quiet, then explosive eruptions

Mt. Fuji, Japan- one of the most photographed volcanoes in the world because of its near perfect symmetry

Ex. Mt. St. Helens

Composite Volcanoes

What are the three types of magma in order from least viscous to most viscous?

  • a. Basaltic

  • b. Andesitic

  • c. Granitic


  • Names are based on size:

  • Ash

  • Lapilli/Cinders

  • Bombs/Blocks


Volcanoes eject different types of material. This material is known as tephra or pyroclastic material.

There are 3 main types:

Ash-less than 2 mm in diameter

Lapilli/cinders-2mm-64 mm in diameter

Blocks orBombs-larger than 64mm



Mt Pinatubo, Philippines 1991



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