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How is the location of volcanoes related to plate tectonics? (In general, where do we expect to find volcanoes?)
Divergent Plate Boundaries
- Convergent Plate
- Boundaries (o/c, o/o)
Give specific examples of volcanoes at each tectonic setting.
O/C Convergent Boundaries: Mt. St. Helens
Hot Spots: Mauna Loa
- Broad, gentle sloping shape
Fluid basaltic lava
- Made of layers of basalt
- Very little pyroclastics or pyroclastic flow
E.g. Mauna Loa, Kilauea, Iceland
Tectonic Setting: Hot Spots
Large, conical shape
Interbedded lavas and pyroclastics
Silica rich magma (doesn’t flow far), often of andesitic composition
Tectonic Setting: O/C Convergent Boundaries
E.g. Mt. St. Helens, Mt. Rainier
Generally a single event (thus small size)
Steep slope angle (30-40 degrees)
Built from pyroclastics (ejected lava fragments)
Occasionally may eject lava, usually from base
Gas-rich basaltic magma
Tectonic Setting: Anywhere there is a pocket, or chamber, of basaltic magma
Discuss why some volcanoes are explosive and some have relatively mild eruptions. Give specific examples.
- Volcanoes that have explosive eruptions contain more viscous magma. High pressure caused by the viscosity of the magma cause it to burst out explosively. Volcanoes with
- mild eruptions contain magma that is more fluid, thus it has less pressure.
Mt. St. Helens: Explosive
Mauna Loa: Mild
Discuss viscosity, silica content, volatiles, and
temperature as each relates to magma composition.
Increase Silica, more viscous
Identify and briefly describe the hazards associated with explosive volcanic eruptions.
- Pyroclastics – Fragments
- of rock in the air from a volcanic eruption
Pyroclastic Flows – Mix of ash and hot gases flowing down the volcano
Lahars – Volcanic mudflows, ash mixed with water
List the various materials that erupt from volcanoes.
Volcanic Neck – forms when magma hardens within a vent. Shiprock
Calderas – Collapsed magma chamber (e.g. magma chamber) Crater Lake
Lava Domes – Old composite cone volcanoes. Mt. St. Helens
Basalt Plateaus – Form from successive flows of lava. Cascade Mountains
List and briefly describe benefits of volcanoes.
Rich agricultural soil: rich soil in Idaho (potatoes)
Mineral resources: from igneous rocks
- Geothermal resources: heat and
- electricity: Iceland
water and atmosphere
rock resources (e.g. pumice)
List the various types of plutonic igneous bodies (including whether each is massive or tabular and concordant or discordant).
Batholith – massive, discordant: irregular mass of intrusive igneous rock
Laccolith – massive, concordant
Dike – tabular, discordant: A sheetlike discordant igneous intrusion that cuts across layers of bedded country rock.
Sill – tabular, concordant: sheetlike body formed by the injection of magma between parallel layers of preexisting bedded rock.
Discuss the origin, composition, and location of
batholiths and give a California example.
- Batholiths form when magma pushes aside the surrounding rock and solidifies before it rises to
- the surface.
They are felsic.
What is the ophiolite complex?
The structure of the ocean crust