Volcanoes Unit 5 Natural Disasters. Volcano Locations Volcanoes are associated with certain plate boundaries: Subduction Zones Rift Valleys Mid-Ocean.

Volcanoes

Unit 5

Natural Disasters

Volcano Locations Volcanoes are associated with certain plate boundaries:

Subduction Zones Rift Valleys Mid-Ocean Ridges Hot Spots

Volcanic eruptions include any escape of lava and/or gases from the Earth.

Lava can escape from rifts, fissures and vents.

Volcano Locations

First some definitions…

Magma = molten rock inside the Earth Lava = molten rock that reaches the Earth’s

surface Volcano = the place in the Earth where magma is

transported to the Earth’s surface Vent = the place in a volcano where lava flows

Most volcanoes have more than 1 vent

Factors Affecting Eruptions

Magma compositionViscosity = resistance to flow (ex: maple syrup = high

viscosity)Explosive eruption = more viscousMore silica in magma = greater viscosity

Magma temperature Greater heat = less viscous

Factors Affecting Eruptions Dissolved Gases

Gases in magma = force to eject molten rock As magma gets closer to surface --> pressure in magma is

reduced Dissolved gases released suddenly

Fluid basaltic magmas = gases bubble and escape easily Quiet eruptions (ex: Hawaii)

Highly viscous magmas = slow movement of gases Gases collect in bubbles and increase in size Explosive eruptions (ex: Mount St. Helens)

Factors Affecting Eruptions

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Lava Flows

Volcanoes emit lava Hot basaltic = very fluid

10-300 meters per hourHardens = form smooth skin that wrinkles as it moves

= pahoehoeAa = rough, jagged blocks with sharp edges

Silica-rich lava = slow

pahoehoe

aa

Volcanic Particles During volcanic eruptions,

many rock fragments are blown into the air

Dust = smallest of volcanic debris (less than .25 mm)

Usually pushed up into the atmosphere

Ash = next biggest (.25 mm- 5 mm)

Can reach places on the other side of the globe from the volcano

ash

Volcanic Particles Cinders = small (golf ball sized) particles thrown from the volcano

Usually liquid when they leave the volcano but solid when they land Bombs = large cinders to blocks of rock (few cm – several meters)

Liquid when they leave the volcano, harden as they travel

bombs

Anatomy of a Volcano

Volcano Types:

Cinder Cone Composite (or Stratovolcano) Shield

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Cinder Cones: form by explosive eruptions of volcanic material the size

of cinders into the air Settles around the volcano’s vent to form steep, loosely arranged

sides of the cone Narrow base and steep sides Not really tall Single eruption that lasts a few weeks-few years

Eruption ends = magma in pipe solidifies and it won’t erupt again

examples include: Pari’cutin, Mexico

Cinder Cones (cont):Pari’cutin, Mexico

Composite Cones: Built up of alternating

layers of pyroclastic deposits & lava explosive eruption, lava blanket, explosive eruption, lava blanket, and so on.

Generate viscous lavas that travel only short distances

Composite/Stratovolcano (cont.): When a violent eruption

occurs, tephra is ejected at very high speeds in an event known as a pyroclastic flow.

Explosive eruption Most located in the Ring of

Fire Usually capped with a

crater Classic looking volcanoes Examples: Mt. St. Helens,

Mt. Vesuvius

Composite/Stratovolcano (cont): Mt. St. Helens

During: After:

CompositeMt. Mayon, Philippines

Mt. Agua, Guatemala

Composite Cones--Dangers Pyroclastic flows = hot

gases, glowing ash, rock fragments Sometimes flows can go

downslope very quickly

Mount St. Helen’s Pyroclastic Flow

Composite Cones--Dangers Can make mudflows =

lahars Volcanic debris saturated

with water Consistency of wet concrete

Mt. Rainier?

Lahar on Mt. St. Helens

Ring of Fire Narrow zone that rims the

Pacific Includes Andes and

Cascades

Ring of Fire

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Shield Volcano: gently-sloping dome formed from non-explosive,

runny lava Not explosive because the lava

does not have dissolved gases (like composite cones).

lava forms basalt rock form over hot spots Wimpiest of all volcanoes Examples: Kilauea, Hawaii,

Olympus Mons on Mars

Shield Volcano (cont):

Mauna Loa, Hawaii

Volcanic Landforms Caldera = big depression in a

volcano often trap rain water to form

mountain top lakes or fill-in forming islands.

Formation1. Collapse of the top of a composite volcano after eruption2. Collapse of the top of a shield volcano after the magma chamber is drained

Ex: Crater Lake, Oregon

Volcanic Landforms Lava Plateaus

Low-viscosity lava flows from fissures and cover a wide area

Ex: Columbia Plateau

Volcanic Activity Active = volcanoes that either erupt continually or periodically

Kilauea in Hawaii = continually Mt. St. Helens = periodically

Dormant = has been active in modern times but is not currently showing signs of activity Mt. Rainier

Extinct = have not shown signs of activity and have not erupted in modern times Sometimes trick us Kohala volcano, Hawaii

Where do Volcanoes Occur? Subduction Zones

Slabs of oceanic crust are pushed down into the mantle

Melting --> magma forms and migrates upward forming volcanoes

Found all over Pacific Ocean (Ring of Fire)

Where do Volcanoes Occur? Ocean-ocean subduction

Oceanic slab descends under another

Chain of volcanoes on ocean floor

Ex: volcanic island arcs = Aleutians

Where do Volcanoes Occur? Divergent Plate

Boundaries Magma from within the

mantle below rises up to erupt and fill in the space left behind as the plates move apart

New ocean crust formed At mid-ocean ridges and on

land (African continent spreading apart)

Where do Volcanoes Occur? Hot Spots (Intraplate)= place

where a lot of magma is produced, which rises to the Earth’s surface Stay stationary over long periods

of time but plates move Can be anywhere (don’t have to

be on plate boundaries) Ex: Hawaiian volcanoes Big Island of Hawaii lies above

the mantle plume

Effects of Volcanoes on Humans Houses, roads, fields covered

with ash Hard to breathe

Pyroclastic flows = can kill you Fast = can’t outrun them

Destruction of a city Produce rich soils for farming Beauty

Climate Effects of Volcanic Eruptions Indirect role in reducing ozone levels

Short lived Emit CO2 which enhances global

warming Offset by global cooling caused by

particles that block out sunlight (haze effect)

Enhance haze effect Block sunlight, lower temps.

Billowing eruption plume being

carried in a westerly direction

Effects of Volcanoes on Earth’s Systems

Biosphere Killed by lava flows, pyroclastic

flows, gases, tsunamis Die from famine, forest fires,

earthquakes Aquatic life = increased acidity,

change in temperature Influence bird migration, flying

ability, feeding activity

Effects of Volcanoes on Earth’s Systems

Hydrosphere = Increase temperatures Falling ash contaminates streams and ponds

Can be water supply for many people Eruptions can result in floods Create tsunamis

Lithosphere Create new islands and land Create mineral deposits Destroy habitats and crops Create beautiful landscapes