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 How much silica is contained in the magma

water vapor and carbon dioxide are dissolved in 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 Vapor Water may be added to magma when ocean crust rock is melted at convergent plate boundaries

Composition of magma 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

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: 3 basic types of volcanoes 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

Fissure eruptions Common at divergent plate boundaries, mid-ocean ridges

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 Paricutin

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

Cinder cone example: Paricutín, Mexico 1943

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

Tephra Names are based on size: Ash Lapilli/Cinders Bombs/Blocks

Volcanoes 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 or Bombs-larger than 64mm

Ash

Lapilli/Cinders Mt Pinatubo, Philippines 1991

Bombs

Blocks