Volcanoes Lec

VOLCANOES AND OTHER IGNEOUS ACTIVITY (Ch. 4) What are the tectonic contexts of volcanoes? Convergent Boundaries Island Arcs Continental Arcs Divergent Boundaries Rift Valleys Bimodal volcanism Stratovolcanoes and fissure eruptions Oceanic ridges Intraplate Activity Hot spots – mantle plumes Continental - Yellowstone Oceanic Hawaii Tensional Terranes Basin-Range Province of western US

What factors determine the nature of volcanic eruptions? Viscosity Magma composition [ SiO2 content ] Rhyolitic 70% Basaltic 50% Volatiles- decrease viscosity; increase explosiveness Temperature - directly affects viscosity Eruptive style Lava/pyroclastic ratio

What are the products of volcanic eruptions? Lavas Pahoehoe Aa Pillow basalts Flood basalts Pyroclastic materials Ash, Lapilli, Bombs

Pumice, cinders Pyroclastic flows – Nuée Ardente Welded tuff Lahars

The three kinds of volcanoes: behavior and form Crater- slopes – size - composition Shield Volcanoes Seamounts- Islands Kilauea, Mauna Loa (40,000 km3 lava, 9 km high) Lava tubes – lateral growth Steep caldera Late stage more pyroclastic, viscous Cinder (Scoria) Cones Lapilli dominate ash and bombs Vesicular clasts (scoria) Deep crater, step slopes Short-lived small Composite Cones (Stratovolcanoes) Tallest, symmetrical Mainly andesitic, but variable Interbedded lava and pyroclastics Fissure eruptions, parasitic cones Nuee ardente eruptions (Mt Pelée, 1902) Vesuvius, Mt St Helens, Fujiyama Calderas Yellowstone Mt Mazama – Crater lake (7,800 BP) 50-70 km3 pyroclastics 1,500 m collapse Hawaiian-type Subsidence Fissure Eruptions and Basalt Plateaus Columbia River Deccan Traps (India) 66 Ma 500,000 km2

2,000,000 m3 of lavas in 1 mill years

How do intrusive igneous bodies vary? How do they form? Size and Relations to Host Rocks Batholith Mountain cores, regional extent Sierra Nevada, So California, Idaho, Coast Range Laccolith Conves upper surface, domes overlying rocks Sill Horizontal, between stratified layers (concordant) Dike Vertical (discordant) Pipe-Neck