What Controls Volcanic Eruptions?

Ch 12.2

Some eruptions are quiet…

Some eruptions are EXPLOSIVE!

Mt. Pinatubo

Mont serrat Mt. St. Helens

What Controls Volcanic Eruptions?

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

2. How much silica is contained in the magma

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

Composition of magma

• Silica-rich magma is called granitic 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. Kilauea in Hawaii

Basaltic magma may result in different lava types:

• Pahoehoe

• AA

• Pillow

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

Name That Lava Type!

Name That Lava Type!

Name That Lava Type!

Name That Lava Type!

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

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

Mauna Kea, Hawaii

Shield volcanoes

Mauna Loa, Hawaii

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 volcano

Fissure eruption

Fissure eruptions

• Common at divergent plate boundaries, mid-ocean ridges

Columbia Plateau

2. Cinder Cone Volcanoes

Cinder Cone Volcanoes

Cinder Cone Volcanoes

Cinder cone example: Paricutín, Mexico

1943

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

Tephra

• Names are based on size:

• Ash

• Cinders

• Bombs

• Blocks

Ash

Cinders

Bombs

Blocks

Cinder Cone Volcanoes

3. Composite Volcanoes

• Steep slope (30o)

• Made of multiple lava and ash flows

• Explosive

Composite Volcanoes

Composite Volcanoes

Composite Volcanoes

Composite Volcanoes:

Mt. Rainier

Mt. Fuji

Composite Volcanoes:

Mt. Kilimanjaro

Pinatubo

Composite Volcano examples:

Soufrière Hills, Montserrat

Composite Volcano examples:

Mt Rainier, Washington

What city is near Mt. Rainer?

Consider:

• 100,000 people live on the solidified mudflows of previous eruptions

• The danger is mostly from laharslahars traveling down river valleys at a speed of 25mph and destroying everything in its path

Lahar (mudflow):

mixture of ash, eroded land, and water flowing down river valleys

• Lahar flows average every 500 years500 years and have gone as far as the Puget Sound lowlands (1 in 7 chance of it happening during your lifetime)

• Mount Rainier has erupted 4 times4 times in the last 4000 years with the last eruption 200 years ago