Chapter 9

Volcanoes

9.1 What causes volcanoes?

• Volcano: an opening in the Earth’s surface which forms a mountain when layers of ash & lava build up

• More than 600 active volcanoes

• Most volcanoes are inactive

Two Examples of Volcanoes

• May 18, 1980– Mount St. Helens in Washington– One of North America’s most damaging

eruptions– 59 people were killed—most by landslides &

flooding due to snow on the mountain melting

Two Examples of Volcanoes

• Mount Kilauea in Hawaii– World’s most active volcano– It has been erupting continuously since 1983

Volcano Terms

• Magma: melted rock INSIDE THE EARTH

• Melts due to high levels of heat & pressure

• Composed of a mixture of molten rock, minerals, & gases

• Melting occurs in the mantle– Which layer of the Earth is this?

Volcano Terms• Density: m/v (which means?)• Mass divided by volume• This means how much something weighs

(m), in relation to how much space it takes up (v)

• Magma is less dense than the rock which surrounds it…so…

• Magma rises to the surface of the Earth

How do volcanoes form?• After long periods of time magma may

reach the surface of the Earth• Then it will flow out of an opening called a

vent• Lava will flow out & cool continuously• This can make large mountains made of

lava & ash packed together• Crater: steep-walled depression around a

volcano’s vent

Anatomy of a Volcano

• Magma Chamber: area underneath the Earth where magma comes from

• Lava: magma which reaches the surface of the Earth

• Vent: opening in the crust of the Earth where magma is released

• Crater: top of a volcano (bowl-shaped) which is connected to the magma chamber by the vent

Where do volcanoes occur?

• Most volcanoes occur along plate boundaries—areas where plates interact with one another

• 1. Divergent Boundary: where plates move apart

• Plates move apart and magma is forced upward through faults (?)

• Rifts: openings in plates caused by divergent boundaries

• Ex. Iceland—Mid-Atlantic Ridge

• 2. Convergent Boundary: plates come together

• Plates move together and the plate which is more dense will sink and melt under the other one.

• Then the magma will rise up.

• 80% of volcanoes are convergent

• Ex. Pacific Ring of Fire, Mt. St. Helens

• 3. Hot Spots: areas in the mantle which are hotter than others

• Plates move over a hot spot and this causes the plate to melt—then magma rises to the surface

• Ex: Hawaiian Islands

• Hot spots also form…

• Seamounts: underwater islands

• Once an island is past a hot spot, its volcanoes will become dormant (meaning they will no longer erupt)

9.2 Types of Volcanoes

Styles of Eruptions

• Some volcanic eruptions are explosive & some are quiet.

• There are 3 factors of magma that affect the type of eruption.

• 1.Trapped Gases

• Gases such as water vapor & CO2 are trapped in magma by the pressure of the surrounding magma & rock

• Gas escapes easily from some magma during quiet eruptions.

• Other eruptions are explosive because the gas builds up to a very high pressure

• 2. Magma Composition:

• Magma low in silica (an explosive compound found in magma that traps gas) is fluid (flows easily) & produces quiet eruptions

• Basaltic magma

• Lava pours from vents (Hawaii)

• Magma with a lot of silica produces very explosive eruptions (Mt. St. Helens)

• Silica-rich magma is thick & gas gets trapped causing pressure to build up

• Granitic magma

• 3. Magma Water Content: High water content will cause explosive eruptions

• Magma at some converging zones contains a lot of water vapor

Types of Volcanoes

• The appearance of a volcano depends on the material that forms it and the type of eruption.

• 1. Shield Volcano: mountain with gently sloping sides and a circular base

• Forms when layer upon layer of lava accumulates after non-explosive eruptions

• Ex: Kilauea (Hawaiian Islands)

• 2. Cinder-Cone Volcano: generally small volcanoes with steep sides

• These volcanoes throw rock fragments called tephra high into the air

• The tephra lands around the vent and begins to pile up creating a steeply-sided, loosely packed pile.

• 3. Composite Volcano: very large, steep-sided mountains

• Begin with extremely explosive eruptions (magma with high silica content)

• Tephra is thrown into the air

• Next there is a quiet period with lava flows

• This repeating cycle of tephra-lava, tephra-lava forms the mountain

• Ex: Mt. St. Helens

9.3 Igneous Rock Features

Intrusive Rock Activity• Because magma rises but only sometimes

reaches the surface, it will intrude on rock layers which already exist

• These igneous (“born of fire”) structures are called plutons

• Can range in size from 1km to 100 km• These areas can be seen in places that have

been eroded or uplifted

• Batholith: largest intrusive igneous rock bodies

• Form when magma cools underground before reaching the surface

• One that can be seen is in Yosemite National Park

• Dikes and Sills:

• Dikes: magma that is squeezed into a vertical crack which cuts across other rock layers

• Sills: magma squeezed into a horizontal crack between rock layers and hardens

• Volcanic neck:

• Forms when a volcano stops erupting

• Magma hardens inside the vent

• Soft cone erodes away first leaving behind a solid igneous core—volcanic neck

• Ex: Ship Rock, New Mexico

• Many times the tops of volcanoes will sink into magma chambers.

• The hole left behind is called a Caldera.

• The caldera usually fills with water

• Example: Crater Lake in Oregon

Damage From Volcanoes

• Many snow-capped volcanoes will melt during eruptions.

• This melting snow causes severe landslides, mudslides, and flooding.

• Many homes are destroyed due to fire caused by hot ashes or lava coming into contact with homes.