Volcanism 2012 Pearson Education Inc Volcanism Eruptive Style

Volcanism © 2012 Pearson Education, Inc.

Volcanism • • • Eruptive Style Volcanic Materials Volcanoes Other Volcanic Landforms Plate Tectonics and Igneous Activity © 2012 Pearson Education, Inc.

Eruptive Style Explosive Effusive © 2012 Pearson Education, Inc.

Eruptive Style • Why do volcanoes have different eruptive styles? – Pressure vs. resistance © 2012 Pearson Education, Inc.

Eruptive Style • A volcano is like a giant pop can that’s been shaken up – Pressure builds up – When pressure is released, material is pushed out © 2012 Pearson Education, Inc.

Eruptive Style • Pressure comes from gases dissolved in magma within the volcano – Mostly water vapor and carbon dioxide – When magma rises toward vent, gases come out of solution and expand © 2012 Pearson Education, Inc.

Eruptive Style • Resistance to this pressure comes from magma’s viscosity • Viscosity = resistance to flow (“thickness” or “stickiness”) • Higher viscosity = thicker, more resistant © 2012 Pearson Education, Inc.

Eruptive Style • Explosive eruptions – High-viscosity magma resists gas pressure • Effusive eruptions – Low-viscosity magma offers little resistance © 2012 Pearson Education, Inc.

Eruptive Style • Why are some magmas more viscous than others? – Temperature – Silica content • Silica = silicon and oxygen dissolved in magma © 2012 Pearson Education, Inc.

Eruptive Style • Why are some magmas more viscous than others? – Temperature • Hotter = less viscous (runnier) © 2012 Pearson Education, Inc.

Eruptive Style • Why are some magmas more viscous than others? – Silica content • More silica = more viscous (thicker) © 2012 Pearson Education, Inc.

Eruptive Style This volcano is fed by high-silica, low-temp magmas with high viscosity This one’s magma is low -silica and high-temp, and low viscosity © 2012 Pearson Education, Inc.

Eruptive Materials © 2012 Pearson Education, Inc.

Eruptive Materials • Lava Aa lava flow Pahoehoe lava flow © 2012 Pearson Education, Inc.

Eruptive Materials • Gases – Water vapor – Carbon dioxide – Smaller amounts of other gases © 2012 Pearson Education, Inc.

Eruptive Materials • Pyroclastics • • • Ash and dust – fine, glassy fragments Lapilli – walnut-sized material Cinders – pea-sized material Blocks – hardened or cooled lava Bombs – ejected as hot lava © 2012 Pearson Education, Inc.

Eruptive Materials • Pyroclastic flow – Hot, fast-moving cloud of pyroclastic material © 2012 Pearson Education, Inc.

Eruptive Materials • Lahar – Volcanic mudflow – Mixture of water, soil, and ash – Triggered by melting of snow during eruption © 2012 Pearson Education, Inc.

Eruptive Materials • Three Japanese lahars © 2012 Pearson Education, Inc.

Volcanoes © 2012 Pearson Education, Inc.

Volcanoes • Shield volcanoes – Largest type – Dome-shaped – Effusive eruptions – Much lava, few pyroclastics © 2012 Pearson Education, Inc.

Volcanoes Mauna Kea, a Hawaiian shield volcano © 2012 Pearson Education, Inc.

Volcanoes • Composite cones – Smaller than shield volcanoes – Classic “volcano shape” – Explosive eruptions – Much pyroclastic material, little lava © 2012 Pearson Education, Inc.

Volcanoes Mt. Fuji, a composite cone in Japan © 2012 Pearson Education, Inc.

A Composite Volcano Interbedded pyroclastic deposits and small lava flows © 2012 Pearson Education, Inc. Figure 4. 11

Volcanoes • Cinder cones – Smallest type – Loose pyroclastic materials © 2012 Pearson Education, Inc.

A Size Comparison of the Three Types of Volcanoes Figure 4. 14 © 2012 Pearson Education, Inc.

Other Volcanic Landforms © 2012 Pearson Education, Inc.

Other Volcanic Landforms • Calderas – Pits caused by magma chamber collapse – Three types • Hawaiian-type • Crater Lake-type • Yellowstone-type © 2012 Pearson Education, Inc.

Other Volcanic Landforms • Hawaiian-type calderas – On shield volcanoes Olympus Mons Kilauea © 2012 Pearson Education, Inc.

Other Volcanic Landforms • Crater Laketype calderas – Catastrophic eruptions © 2012 Pearson Education, Inc.

Other Volcanic Landforms • Yellowstone-type calderas – Largest © 2012 Pearson Education, Inc. Caldera outline (diameter approx. 30 mi)

Other Volcanic Landforms • Basalt plateaus – Very large, flat areas covered with basalt – Outpourings of low-viscosity lavas from fissure eruptions © 2012 Pearson Education, Inc.

Other Volcanic Landforms Fissure eruption in Hawaii Basalt plateau in Washington © 2012 Pearson Education, Inc.

Plate Tectonics & Igneous Activity © 2012 Pearson Education, Inc.

Plate Tectonics & Igneous Activity • Most volcanism occurs along tectonic plate boundaries – Divergent boundaries: decompression melting – Subduction zones: hydration melting © 2012 Pearson Education, Inc.

Plate Tectonics & Igneous Activity The “Ring of Fire” is a chain of active volcanoes around the Pacific Ocean. It marks the boundaries of tectonic plates. © 2012 Pearson Education, Inc.

Plate Tectonics & Igneous Activity • Hot spot (intraplate) volcanism – Not near plate boundaries – Fed by magma reservoirs beneath the crust © 2012 Pearson Education, Inc.

Plate Tectonics & Igneous Activity • Hawaiian Islands were formed by hot spot volcanism North Kauai 5. 1 my Oahu 3. 7 my Molokai 1. 9 my Maui 1. 3 my Hawaii (Big Island) < 1 million yrs old © 2012 Pearson Education, Inc.

Plate Tectonics & Igneous Activity • Hot spot animation © 2012 Pearson Education, Inc.

End of Chapter 6 © 2012 Pearson Education, Inc.