NATURE OF VOLCANIC ERUPTIONS

VIOLENCE OF VOLCANIC ERUPTIONS Factors That influence the Violence of Volcanic Eruptions Composition of the Magma Temperature of the Magma Amount of dissolved Gases in the Magma

Composition & Viscosity

MAGMA & LAVA VISCOSITY As silica content Decreases Viscosity “gooeyness”- a material’s resistance to flow (how thick it is) related to Silica content of magma As silica content Increases viscosity Increases - thicker magma As silica content Decreases viscosity Decreases - runny magma

VISCOSITY FACTORS

VISCOSITY FACTORS (CONT.) COMPOSITION Low % Silica content – Low Viscosity (More fluid) Basaltic lava Intermediate % Silica content Andesitic lava High % Silica content – High Viscosity (Thicker) Granitic lava

VISCOSITY FACTORS TEMPERATURE Hotter the magma the Less Viscous (more fluid) Hawaiian lava: 1170° C - 1175° C very fluid (runny) Mt. Saint Helens lava: 500° C - 700° C very viscous (stiff)

VISCOSITY FACTORS (CONT.) DISSOLVED GASES H2O & CO2 Gases escape, as magma moves toward the surface and the pressure is reduced. How easily do gases escape from magma? Low Viscosity (Fluid) magma quieter the eruption Gases can escape easily Viscous (Thick) magmas produce violent eruptions Gases become trapped within cooling magma and build up pressure

Volcanic MATERIAL

MATERIALS ASSOCIATED WITH VOLCANIC ERUPTIONS

LAVA FLOWS Aa Lava Pahoehoe Lava Basaltic lavas are more fluid Types of Lava Aa Lava Rough, jagged blocks Pahoehoe Lava Resembles twisted braids in ropes Aa Pahoehoe

GASES Magma contains 1% - 5% gas Water vapor - H2O (70%) Carbon dioxide - CO2 (15%) N2 and nitrogen compounds (5%) Sulfur compounds (5%) minor amounts of Cl, H2 & Ar

PYROCLASTIC MATERIAL Pyroclastic – means “Fire Fragments” Pyroclastic – means “Fire Fragments” Types of Pyroclasts

ASH - fine, glassy fragments

PUMICE - forms “frothy” lava

CINDERS -“pea-sized” with voids

LAPILLI - “walnut” size

BLOCKS hardened lava > 64mm

VOLCANIC BOMB ejected as semi-molten hot lava > 64 mm

What materials are ejected from volcanoes? Bellringer You have 5 minutes to answer the questions on a sheet of paper and turn into the basket What materials are ejected from volcanoes? Depending on the type of eruption, volcanoes may produce lava flows or eject pyroclastic materials, or both. Also all eruptions emit large amounts of gas.

“A volcano is a mountain with hiccups” VOLCANOES “A volcano is a mountain with hiccups”

ANATOMY OF A VOLCANO

Parasitic cone Lava Pyroclastic material ANATOMY OF A VOLCANO Crater Caldera Vent, Conduit (pipe) Parasitic cone Lava Pyroclastic material

Sratovolcano (Composite) VOLCANO TYPES Shield Volcano Cinder Cone Volcano Sratovolcano (Composite) Lava Dome

SHIELD VOLCANO Broad & slightly domed Covers large area Broad & slightly domed Covers large area Quiet eruptions produce large volumes of Basaltic lava Dark colored lava Low viscosity (very runny or fluid)

STRATOVOLCANO (COMPOSITE) Large in size Iayered - lava & pyroclastics Most adjacent to Pacific Ocean Most Violent eruptions Produce “Lahars” Type of mudflow Produce “Nuée ardente” Fiery cloud Hot gases mixed with ash Flows down sides at speeds up to 200 kph

CINDER CONE Built from ejected lava fragments (pyroclastics) Built from ejected lava fragments (pyroclastics) Have Steep Slopes Small in size Frequently occur in groups

THREE VOLCANO PROFILES

DANGERS OF COMPOSITE CONES MOUNT St. HELENS Before: 5/18/1980 After: 5/18/1980

Nuée ardente & Lahars

*What are the three types of volcanoes? Bellringer You have 5 minutes to answer the question on a sheet of paper and turn into the basket *What are the three types of volcanoes? The three main types of volcanoes are shield volcanoes, cinder cones, and composite cones.

Other Volcanic Landforms EXTRUSIVE IGNEOUS FEATURES

VOLCANIC LANDFORMS Crater Steep walled depression at summit of volcano

CALDERA Collapsed crater generally larger than 1 km in diameter Collapsed crater generally larger than 1 km in diameter Formation: Collapse - drained magma chamber Proclastic Explosion - blasts out material

VOLCANIC VENT & Conduit Vent (pipe) – conduit that connects crater to magma chamber

VOLCANIC NECKS Magma that hardened in the volcanoes pipe (vent) Magma that hardened in the volcanoes pipe (vent) Remains standing after volcano has eroded away Devils Tower (Wyoming) Shiprock (New Mexico) Tsé Bit' A'í - Rock With Wings

FLOOD BASALTS Lava flows that cover many square miles of area fissures that open up in the crust lava flow out onto the surface

FISSURES Fissure - crack in the crust allows lava to flow onto the surface

VOLCANIC PLATEAU Fissure Eruptions Fissure Eruptions Volcanic material flows from fractures in crust Flood Basalts Yellowstone Plateau Columbia Plateau