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Geology 12Mr. M. Gauthier

24 March 2016

Introduction: Mt. St. Helens� Before 1980 Mt. St Helens, in

Southern Washington State, had not erupted since 1857

� On March 27,1980 minor ash and eruptions were due to exploding gas blasting out of the volcano

� The Northern Flank (side) of the volcano started to bulge outward at a rate of 1.5 meters a day

� On May 18,1980 the historical blast blew the side of the volcano causing a near nuclear level blast

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Extrusive Activity� Molten rock material on the surface is called lava

(opposed to magma when it is beneath the surface)

� During volcanism igneous rocks are cooled quickly forming fine grain rocks (basalt, obsidian, rhyolite)

� During large volcanic eruption pieces of lava harden very quickly in the air and are called pyroclasts which collects into piles of pyroclast debris called tephra

� In volcanoes like Hawaii, lava flows out of the earth in large flows called lava flows

Volcanism� The process of lava solidifying on land (lava flows,

pyroclasts) forms volcanoes by a process known as volcanism.

� Volcanism can lead to a lava plateau when successive layers of lava harden

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The Growth of an Island� Lava flowing into the sea and hardening can form

islands.� The island of Hawaii has a volcano named Kilauea

which has been continuously erupting since 1983 generating 325 000 m3 of material a day which means that over 3 800 000 000m3 of material since � That’s 1.53 x 1014 dump trucks of material!

� This process has and continues to grow the island of Hawaii

� Mauna Loa is the largest mountain on earth because its base is on the bottom of the Pacific Ocean

Geothermal Energy� In areas where volcanic activity is prevalent (Iceland,

Italy, Mexico, New Zealand , Argentina, Japan, California) the heat from the earth can be used in lieu of burning fossil fuels

� This means energy is mostly clean and has a very small carbon footprint

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Effect on Weather� Occasionally a volcano will erupt large amounts of ash

high into the atmosphere.

� Winds can keep this material suspended for years

� In 1815 the eruption of Tambora in Indonesia was the largest single eruption in 1000 years.

� So much ash was ejected during this eruption that the following year, 1816, was known as the “year without summer” as snow was found in many places in June as well as frost the killed summer crops.

Fatalities� The following is a breakdown of causes of death

during an eruption:

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Violence of an Eruption� There are two major factors that control the violence

of an eruption:

1. The amount of gas in the lava or magma

2. The ease or difficulty in which a gas can escape

ViscosityThe viscosity(thickness) of the lava determines how easily the lava can escape. Viscosity is controlled by:

1. The amount of silica in the lava

2. The temperature of the lava

3. The gas dissolved in the lava

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� Mafic lavas have low silica in them so they flow relatively easy and cool and form dark rocks like basalt

� Felsic lavas have higher amounts of silica so they have a much higher level of viscosity and flow much slower

Pyroclastic flow� A pyroclastic flow is a

mixture of gas and pyroclastic debris that is so dense it hugs the ground as it flows into low areas. � These flows can cover an

area with dense ash and high temperatures leading to total destruction

https://www.youtube.com/watch?v=Cvjwt9nnwXY

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Volcanoes� The vent is the opening through what the eruption

takes place

� The Crater of a volcano is the bowl like depression on top of the vent of a volcano

� A flank eruption is an eruption where lava pours out of a vent on the side of the eruption

� A Caldera is a crater that is at least 1km wide

Shield Volcanoes� These are broad, gently sloping volcanoes constructed

from lava flows

� Lava produced here is very runny so it doesn’t build up quickly, leading to the shallow sides of the volcano

� E.g. Kilauea, Hawaii

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Pyroclastic Cones� Also called a cinder cone

� Formed from built up pyroclastic material that doesn’t get ejected very far from the vent

� This leads to a very steep sided volcano

� These are generally short lived and found on the sides of shield volcanoes

Composite Volcanoes� Also known as

stratovolcanoes

� Formed from alternating layers of ash and lava flow

� Very violent eruptions; found at subduction zones

� These are very long lived volcanoes

� E.g. Mt. St. Helens

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� Most stratovolcanoes are found along the circum-Pacific belt, or the Pacific Ring of Fire

Volcanic Domes� Volcanic Domes are steep sided dome, or spine

shaped, masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent

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Pillow Lava� This is formed when lava hardens on the sea floor (or

under water) and takes a characteristic bubblyappearance

https://www.youtube.com/watch?v=5G8W9sZtBTM