VolcanoesVolcanoes

Geology 12

Citadel High School

What comes out of a volcano? GasMost common:

H2O - waterCO2 –Carbon DioxideSO2 –Sulfur Dioxide

HCl – hydrochloric acid

What comes out of a volcano?

Lava

A Pahoehoe lava flowA Pahoehoe lava flow

A A TypicalTypical aa flow aa flow

What comes out of a volcano? Ash

A volcanic bombA volcanic bomb

Bomb is approximately 10 cm long

The Nature of Volcanic The Nature of Volcanic EruptionsEruptions

• Factors determining the “violence” or explosiveness of a volcanic eruption

• Temperature of the magma • Composition of the magma• Dissolved gases in the magma

• The above three factors control the viscosity of the magma which in turn controls the nature of an eruption

The Nature of Volcanic The Nature of Volcanic EruptionsEruptions

• Viscosity is a measure of a material’s resistance to flow (e.g. molasses is more viscous then water) Most viscous to least viscous

The Nature of Volcanic EruptionsThe Nature of Volcanic Eruptions• Factors affecting viscosity

– Temperature - Hotter magmas are less viscous (more runny)

– Dissolved Gases– Gas content affects magma mobility– Gases expand within a magma as it nears the Earth’s surface due

to decreasing pressure– The violence of an eruption is related to how easily gases escape

from magma

– Composition (what its made of) - Silica (SiO2) contentHigher silica content = higher viscosity – like molasses (e.g., felsic lava

such as rhyolite – St. Helen’s)Lower silica content = lower viscosity or more fluid-like behavior (e.g.,

mafic lava such as basalt – Hawaii) -

Materials extruded from a Materials extruded from a volcanovolcano

• Lava Flows• Basaltic lavas are much more fluid

• Types of basaltic flows– Pahoehoe lava (resembles a twisted or ropey

texture)– Aa lava (rough, jagged blocky texture)

Materials extruded from a Materials extruded from a volcanovolcano

• Pyroclastic materials – “Fire fragments”Types of pyroclastic debris

• Ash and dust - fine, glassy fragments

• Pumice - porous rock from “frothy” lava

• Lapilli - walnut-sized material

• Cinders - pea-sized material

• Particles larger than lapilli– Blocks - hardened or cooled lava

– Bombs - ejected as hot lava

Shield Shield VolcanoesVolcanoes

• Types of Volcanoes• Shield volcano

– Broad, slightly domed-shaped

– Composed primarily of basaltic lava

– Generally cover large areas

– Produced by mild eruptions of large volumes of lava

– Mauna Loa on Hawaii is a good example

Shield VolcanoShield Volcano

Mauna Loa (Hawaii):A typical shield volcano

Cinder coneCinder cone Volcanoes Volcanoes

• Types of Volcanoes continued• Cinder cone

– Built from ejected lava (mainly cinder-sized) fragments

– Steep slope angle

– Rather small volcanoes in size

– Frequently occur in groups

Sunset Crater – a cinder cone Sunset Crater – a cinder cone near Flagstaff, Arizonanear Flagstaff, Arizona

Composite cone Composite cone also known as also known as StratovolcanoStratovolcano

• Types of volcanoes continued• Composite cone (Stratovolcano)

– Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)

– Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)

– Composed of embedded lava flows and layers of pyroclastic debris

A composite volcanoA composite volcano

Mt. St. Helens – a typical Mt. St. Helens – a typical composite volcanocomposite volcano

Mt. St. Helens after its 1980 eruption

Mt. St. Helens following the Mt. St. Helens following the 1980 eruption 1980 eruption

Volcanic HazardsLava flowsAsh fallPyroclastic flowsMudflowsVolcanic GasesTsunami

Other volcanic landforms Other volcanic landforms include:include:

• Calderas• Steep-walled depressions at the summit• Size generally exceeds 1 km in diameter

• Pyroclastic flows• Consists of ash, pumice, and other

fragmental debris

Caldera – a hole that forms after Caldera – a hole that forms after a volcano has collapseda volcano has collapsed

Other volcanic landforms Other volcanic landforms include:include:

• Pyroclastic flows continued• Material is propelled from the vent at a high

speed

• Fissure eruptions and lava plateaus• Fluid basaltic lava extruded from crustal

fractures called fissures• e.g., Hawaii, Iceland

Other volcanic landformsOther volcanic landforms

• Lava Domes• Bulbous mass of congealed lava• Most are associated with explosive eruptions

of gas-rich magma

A lava dome on Mt. St. A lava dome on Mt. St. HelensHelens

Other volcanic landformsOther volcanic landforms

• Volcanic pipes and necks• Pipes are short conduits that connect a

magma chamber to the surface• Volcanic necks (e.g., Ship Rock, New

Mexico) are resistant vents left standing after erosion has removed the volcanic cone

Formation of a volcanic neckFormation of a volcanic neck

1. At spreading centers (on the ocean floor), low pressure triggers

mantle melting—fluid basaltic magma rises.

2. Explosive volcanoes form at …subduction zones

3. Within plates, rising plumes of hotter mantle feedhot spots; multiple volcanoes result like Hawaii.

Distribution of some of the Distribution of some of the world’s major volcanoes world’s major volcanoes

Distribution of magnitude 5 or Distribution of magnitude 5 or greater earthquakes, greater earthquakes, 1980 - 19901980 - 1990

Volcanoes and ClimateVolcanoes and Climate

• Explosive eruptions emit huge quantities of gases and fine-grained debris into the atmosphere which filter out and reflect a portion of the incoming solar radiation

• Examples of volcanism affecting climate• Mount Tambora, Indonesia – 1815• Krakatau, Indonesia – 1883• Mount Pinatubo, Philippines - 1991

ConnectionsConnections

It is not by chance that earth quakes and volcanoesare located in the same areas around the earth!

Plate tectonic boundaries are the location of mostvolcanoes and most earthquakes.