Volcanoes Geology 12 Citadel High School

What comes out of a volcano? Gas Most common: H 2 O - water CO 2 – Carbon Dioxide SO 2 – Sulfur Dioxide HCl – hydrochloric acid

What comes out of a volcano? Lava

A Pahoehoe lava flow A Pahoehoe lava flow

A Typical aa flow A Typical aa flow

What comes out of a volcano? Ash

A volcanic bomb A volcanic bomb Bomb is approximately 10 cm long

The Nature of Volcanic Eruptions The Nature of Volcanic Eruptions 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 Eruptions The Nature of Volcanic Eruptions 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 Eruptions The 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 (SiO 2 ) content Higher 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 volcano Materials extruded from a volcano 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 volcano Materials extruded from a volcano 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 Volcanoes 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 Volcano

Mauna Loa (Hawaii): A typical shield volcano

Cinder cone 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 near Flagstaff, Arizona

Composite cone also known as Stratovolcano 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 volcano

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

Mt. St. Helens after its 1980 eruption

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

Volcanic Hazards Lava flows Ash fall Pyroclastic flows Mudflows Volcanic Gases Tsunami

Other volcanic landforms 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 a volcano has collapsed

Other volcanic landforms 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 landforms Lava Domes Bulbous mass of congealed lava Most are associated with explosive eruptions of gas-rich magma

A lava dome on Mt. St. Helens A lava dome on Mt. St. Helens

Other volcanic landforms Other 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 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 feed hot spots; multiple volcanoes result  like Hawaii.

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

Distribution of magnitude 5 or greater earthquakes,

Volcanoes and Climate Volcanoes 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

Connections It is not by chance that earth quakes and volcanoes are located in the same areas around the earth! Plate tectonic boundaries are the location of most volcanoes and most earthquakes.