VOLCANOESVOLCANOES

Formation of MagmaFormation of Magma Even though the mantle is so hot it’s still mostly Even though the mantle is so hot it’s still mostly

solid.solid.• Why? Why? • High Pressures keep heated rock in a solid stateHigh Pressures keep heated rock in a solid state

Magma forms under three conditionsMagma forms under three conditions1.1. Temperature of rock > melting temperature for the Temperature of rock > melting temperature for the

minerals in the rockminerals in the rock2.2. When a rock is at it’s melting point, but under high When a rock is at it’s melting point, but under high

pressures, it remains a solid. If that pressure is pressures, it remains a solid. If that pressure is removed, the rock will melt.removed, the rock will melt.

3.3. Adding fluids (like water) may lower the melting point Adding fluids (like water) may lower the melting point of some minerals and cause the rock to melt of some minerals and cause the rock to melt

Where do volcanoes form?Where do volcanoes form?

Subduction Zones

Mid Ocean Ridges

Hot Spots

Global Distribution of VolcanoesGlobal Distribution of Volcanoes See any patterns?See any patterns?

• Plate BoundariesPlate Boundaries• Ring of FireRing of Fire

What are Hot Spots?What are Hot Spots? A hot spot is an area of constant volcanic activity in A hot spot is an area of constant volcanic activity in

the middle of a lithospheric platethe middle of a lithospheric plate

They do not move, but the crust moves over them!They do not move, but the crust moves over them!

General Structure of a Volcano

• Tephra (pyroclastics) – ash and rock ejected from a volcano

• Crater – the opening at the top of a volcano

• Vent – conduit or pathway for magma

• Strata – layers of rock

• Magma chamber – storage place for magma

Why do Why do volcanoes volcanoes

erupt?erupt?

Volcanic EruptionsVolcanic Eruptions

There are only three general types There are only three general types of eruptions.of eruptions.

• Quiet Eruptions, Lava Flows, and Quiet Eruptions, Lava Flows, and Violent EruptionsViolent Eruptions

What separates one type of What separates one type of eruption from another?eruption from another?

ViscosityViscosity ViscosityViscosity = resistance = resistance

to flowto flow• High viscosity fluids High viscosity fluids

are thick, slow are thick, slow moving, high in moving, high in dissolved gasses dissolved gasses (Oxygen), felsic, and (Oxygen), felsic, and explosiveexplosive

• Low viscosity fluids Low viscosity fluids that are runny, low in that are runny, low in dissolved gasses dissolved gasses (oxygen), mafic, and (oxygen), mafic, and less explosiveless explosive

The Eruption of Mt. St. Helens

Kilauea Eruptions

More Kilauea Eruptions

But Different

Explosive EruptionsExplosive Eruptions Called Called pyroclasticpyroclastic

• Latin for “broken fire” Latin for “broken fire” • ContinentalContinental

Felsic lavaFelsic lava High percentage of trapped gasses in the lavaHigh percentage of trapped gasses in the lava

• Water vapor and carbon dioxideWater vapor and carbon dioxide• When the volcano erupts the trapped gasses When the volcano erupts the trapped gasses

make the volcano explosivemake the volcano explosive• Examples: Mt. St. Helens, Mt. PinatuboExamples: Mt. St. Helens, Mt. Pinatubo

• Pyroclastic materialPyroclastic material is solid material is solid material ejected from a violently erupting volcanoejected from a violently erupting volcano

• Three sizes of pyroclasticsThree sizes of pyroclastics1. Ash1. Ash2. Lapilli2. Lapilli3. Bombs3. Bombs

Volcanic Ash – the Volcanic Ash – the smallsmall stuff stuff

Volcanic Ash is small, but it is the most deadly.

LapilliLapilli

Volcanic BombsVolcanic Bombs

Material from Quiet Eruptions and Material from Quiet Eruptions and Lava FlowsLava Flows

There are two types of lava flowsThere are two types of lava flows

1.1. Pahoehoe – smooth, slow cooling Pahoehoe – smooth, slow cooling lava flow that has a ropey look.lava flow that has a ropey look.

2.2. Aa – sharp and jagged lava. Cools Aa – sharp and jagged lava. Cools fast.fast.

PahoehoePahoehoe

AaAa

Types of VolcanoesTypes of Volcanoes

There are three main types of volcanoesThere are three main types of volcanoes1.1. Cinder ConesCinder Cones

2.2. Shield Volcanoes Shield Volcanoes

3.3. Strato (or composite) VolcanoesStrato (or composite) Volcanoes

Cinder ConesCinder Cones

• Cinder cones build from erupting lava that breaks into small pieces as it blasts into the air. • As the lava pieces fall back to the ground, they cool and harden into cinders that pile up around the volcano's vent.• Cinder cones are very small cone-shaped volcanoes.• Form over continental hot spots

WHERE DO CINDER CONES FORM?WHERE DO CINDER CONES FORM?

Continental Hot Spots

• What type of eruption and why?

• Continental – felsic – pyroclastic = Cinders

Shield volcanoesShield volcanoes form from eruptions of form from eruptions of flowing lava. flowing lava.

The lava spreads out and builds up volcanoes The lava spreads out and builds up volcanoes with broad, gently sloping sides. The shape with broad, gently sloping sides. The shape resembles a warrior's shield.resembles a warrior's shield.

Form over oceanic hot spotsForm over oceanic hot spots

WHERE DO SHIELD VOLCANOES FORM?WHERE DO SHIELD VOLCANOES FORM?

Oceanic Hot Spots

• What type of eruption and why?

•Oceanic – mafic – lava flow = Pahoehoe, Aa

•Stratovolcanoes/Composite build from eruptions of lava and pyroclastics that pile up in layers, or strata, which evidence previous eruptions.•These volcanoes form asymmetrical cones with steep sides.•From at oceanic-continental convergence zones

WHERE COMPOSTITE/STRATOVOLCANOES FORM?WHERE COMPOSTITE/STRATOVOLCANOES FORM?

Subduction Zones

• What type of eruption and why?

•Oceanic and Continental – pryroclastic

MOUNT ST. HELENS

Comparing the Sizes of Volcanoes

Mt. EtnaMt. EtnaSicily, ItalySicily, Italy

Mayon is a stratovolcano on the island of Luzon in the Philippines

Mt. Egmont, NZ

Mount Rainier is a large stratovolcano in the Cascade Range of western Washington.

Mt. Hood, Washington

Mt. EdzizaMt. EdzizaNorthwestern British Columbia Northwestern British Columbia

Olympus Mons on MarsOlympus Mons on Mars

Mt. Pinatubo, PhilippinesMt. Pinatubo, Philippines

Pago VolcanoPago Volcano