Chapter 5 Volcanoes Most volcanic activity is concentrated

Chapter 5 Volcanoes Most volcanic activity is concentrated near plate boundaries

Magma Sources and Types • Magma sources tend to be 50 to 250 km deep into the crust and upper mantle • Temperatures increase as depth increases • Some of the internal heat is left over from the earth’s formation; more heat is generated by the decay of radioactive elements in the earth • Volcanoes are generated at: – Divergent Plate Boundaries – Convergent Plate Boundaries – “Hot Spots”

Figure 5. 2

Magma Sources and Types • Magma compositions vary in Si. O 2 , iron, magnesium, and volatile gases • Mafic magma – low in Si. O 2 (45 -50 %) but high in iron, and magnesium • Felsic magma – high in Si. O 2 (up to 75 %) but low in iron, and magnesium • Intermediate magma – intermediate range of Si. O 2 (50 -65 %), iron, and magnesium • Amount of volatile gases will affect explosive characteristics of eruptions

Magma Sources and Types • Mafic magmas produce basalt lavas – Intrusive equivalent is gabbro • Intermediate magmas produce andesite lavas – Intrusive equivalent is diorite • Felsic magmas produce rhyolite lavas – Intrusive equivalent is granite

Figure 5. 3

Magma at Divergent Plate Boundaries • Magma produced at a Divergent Plate Boundary is typically melted asthenosphere material • Asthenosphere is extremely rich in ferromagnesian (ultramafic) and a melt from it is mafic (or ultramafic) • Basalt is emplaced as new seafloor at the spreading ridge or a rift • Rift systems in continental crust may melt granitic crust and produce andesite or rhyolite lavas – A bimodal suite of extrusive igneous rocks characterize rift volcanoes

Magma at Convergent Plate Boundaries • Magmatic activity at convergent boundaries is complex • The composition of the subducted plate determines the composition of the lava – Subducted continental crust may melt and produce rhyolite lava – Subducted oceanic crust may melt and produce basalt or andesite lava – Subduction of sediments derived from the top of the subducted slab may produce a variety of lavas

Magma at Hot Spots • Magmas associated with a hot spot volcano in an ocean basin will produce a basalt lava • Magmas associated with a hot spot volcano under continental crust generally will produce a felsic lava (and often an explosive one)

Figure 5. 4

Types and Locations of Volcanoes • Seafloor Spreading Ridges – Most voluminous volcanic activity – About 50, 000 km of ridges around the world – Mostly under the oceans - except at Iceland – Generally, harmless mafic fissure eruptions • Continental fissure eruptions – Pour out of cracks in lithosphere – Result in large volume of “flood basalts” – Columbia Plateau (over 150, 000 km 2 and 1 km thick) – Other locations include India and Brazil

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Figures 5. 7 a and b

Types and Locations of Volcanoes • Shield volcanoes – – Very large, flat, with abundant thin basalt flows Basalt is less viscous than andesite or rhyolite Shield like shape - larger area relative to height Examples: Hawaiian Island chain • Volcanic Domes – Composed of more viscous andesite or rhyolite • these lavas do not flow – Ooze out onto surface from a tube and pile up close to the vent – Compact, small, and steep sided – Various locations around Pacific Ring of Fire

Figures 5. 8 a, b , and c

Figures 5. 9 a and b

Figure 5. 10 a and b

Types and Locations of Volcanoes • Cinder Cones – Minor explosive volcano – Batches of lava shot into the air as pyroclastics – Size of pyroclastics range from ash (very fine), cinders, bombs, or blocks (very coarse) – Pyroclastics fall close to the vent creating a cone shaped volcano – Example: Particutin, Mexico

Figures 5. 12 a and b

Figures 5. 11 a, b, c, and d

Types and Locations of Volcanoes • Composite Volcanoes (Stratovolcanoes) are built up of layers of lava and pyroclastics – Mix of lavas and pyroclastic layers allows for a tall volcano to form – Usually associated with subduction zones – These tend to be violent and explosive – Example: Mount St. Helens, Cascade Range, Northwest U. S. A.

Figures 5. 13 a and b

Hazards Related to Volcanoes • Lava, the principal hazard? But not lifethreatening generally • Pyroclastics, more dangerous than lava flows • Lahars, a volcanic ash and water mudflow • Pyroclastic Flows - Nuées Ardentes • Toxic Gases • Steam Explosions • Secondary Effects; Climate and Atmospheric Chemistry

Figures 5. 1 a and b

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Figures 5. 15 a and b

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Predicting Volcanic Eruptions • Classification by activity – Active: erupted in recent history – Dormant: no historic erupts but not badly eroded – Extinct: no historic eruptions and badly eroded • Volcanic Precursors – Seismic activity – Bulging, tilting or uplift – Monitoring gas emissions around volcano

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Present and Future Volcanic Hazards in the United States • Hawaii: active or dormant volcanoes • Cascade Range: a series of volcanoes in the western United States and southwestern Canada resides above the Pacific Northwest subduction zone • The Aleutians: South-central Alaska and the Aleutian island chain sit above a subduction zone • Long Valley and Yellowstone Calderas

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Figures 5. 33 a and b

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Figures 5. 35 a and b

Fig. 5. 36 Track of North America across Yellowstone hotspot