VOLCANOES

Formation of Magma Even though the mantle is so hot it’s still mostly solid. Even though the mantle is so hot it’s still mostly solid. Why?Why? High Pressures keep heated rock in a solid stateHigh Pressures keep heated rock in a solid state Magma forms under three conditions Magma forms under three conditions 1.Temperature of rock > melting temperature for the minerals in the rock 2.When a rock is at it’s melting point, but under high pressures, it remains a solid. If that pressure is removed, the rock will melt. 3.Adding fluids (like water) may lower the melting point of some minerals and cause the rock to melt

Where do volcanoes form? Subduction ZonesMid Ocean Ridges Hot Spots

Global Distribution of Volcanoes See any patterns? See any patterns? Plate BoundariesPlate Boundaries Ring of FireRing of Fire

What are Hot Spots? A hot spot is an area of constant volcanic activity in the middle of a lithospheric plate A hot spot is an area of constant volcanic activity in the 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 volcanoes erupt?

Volcanic Eruptions There are only three general types of eruptions. There are only three general types of eruptions. Quiet Eruptions, Lava Flows, and Violent EruptionsQuiet Eruptions, Lava Flows, and Violent Eruptions What separates one type of eruption from another? What separates one type of eruption from another?

Viscosity Viscosity = resistance to flow Viscosity = resistance to flow High viscosity fluids are thick, slow moving, high in dissolved gasses (Oxygen), felsic, and explosiveHigh viscosity fluids are thick, slow moving, high in dissolved gasses (Oxygen), felsic, and explosive Low viscosity fluids that are runny, low in dissolved gasses (oxygen), mafic, and less explosiveLow viscosity fluids that are runny, low in dissolved gasses (oxygen), mafic, and less explosive

The Eruption of Mt. St. Helens

Kilauea Eruptions

More Kilauea Eruptions But Different

Explosive Eruptions Called pyroclastic Called pyroclastic Latin for “broken fire”Latin for “broken fire” ContinentalContinental Felsic lava Felsic lava High percentage of trapped gasses in the lava High percentage of trapped gasses in the lava Water vapor and carbon dioxideWater vapor and carbon dioxide When the volcano erupts the trapped gasses make the volcano explosiveWhen the volcano erupts the trapped gasses make the volcano explosive Examples: Mt. St. Helens, Mt. PinatuboExamples: Mt. St. Helens, Mt. Pinatubo Pyroclastic material is solid material ejected from a violently erupting volcanoPyroclastic material is solid material ejected from a violently erupting volcano Three sizes of pyroclasticsThree sizes of pyroclastics 1. Ash 2. Lapilli 3. Bombs

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

Lapilli

Volcanic Bombs

Material from Quiet Eruptions and Lava Flows There are two types of lava flows There are two types of lava flows 1. Pahoehoe – smooth, slow cooling lava flow that has a ropey look. 2. Aa – sharp and jagged lava. Cools fast.

Pahoehoe

Aa

Types of Volcanoes There are three main types of volcanoes There are three main types of volcanoes 1.Cinder Cones 2.Shield Volcanoes 3.Strato (or composite) Volcanoes

Cinder 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? Continental Hot Spots What type of eruption and why? Continental – felsic – pyroclastic = Cinders

Shield volcanoes form from eruptions of flowing lava. Shield volcanoes form from eruptions of flowing lava. The lava spreads out and builds up volcanoes with broad, gently sloping sides. The shape resembles a warrior's shield. The lava spreads out and builds up volcanoes with broad, gently sloping sides. The shape resembles a warrior's shield. Form over oceanic hot spots Form over oceanic hot spots

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? Subduction Zones What type of eruption and why? Oceanic and Continental – pryroclastic

MOUNT ST. HELENS

Comparing the Sizes of Volcanoes

Mt. Etna Sicily, 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. Edziza Northwestern British Columbia

Olympus Mons on Mars

Mt. Pinatubo, Philippines

Pago Volcano