Volcanoes eruptive style and associated landforms Viscosity Resistance

Volcanoes: eruptive style and associated landforms

Viscosity • Resistance to flow Which test tube contains the fluid with high viscosity? Left? Right?

Viscosity • Which eruption was produced by high viscosity lava? What are the clues? Eruption A Eruption B

Why does one type of lava have a higher viscosity than the other?

Why does one type of lava have a higher viscosity than the other? • Tectonic setting • Source of lava • Composition Basalt: asthenosphere and oceanic crust Andesite: sediments, water, oceanic crust and continental crust Intermediate composition Lower percentages of silicon and oxygen

The Silicon Tetrahedron • Acts as a thickening agent • Building block to all rock forming minerals • Higher percentage = higher viscosity Rhyolite > 65% Andesite = 55 -65 % Basalt < 55%

Rhyolite is the lava type with the highest percentages of silicon and oxygen • Most violent eruptions

Hot spot under continental crust Notice the direction of plate movement

Andesite • Intermediate composition lava

Landforms associated with viscous lava Andesitic lava produces stratovolcanoes Rhyolitic or dacitic lava produces plugs.

Mt. Rainier

Mt. St. Helens: before the 1980 eruption Bulge: plug that is pushed out by magma within the conduit.

Mt. St. Helens: after the eruption Plug dome

Mt. St. Helens: dome plug The plug is nearly the height of the Washington Monument and the width of four football fields.

Plug dome: andesitic to rhyolitic in composition

Lassen Peak • Lassen Peak is a plug dome volcanic landform • Built from felsic lava • One of the largest on Earth • Carved by glaciers during the Ice Age

Crater Lake: volcanic caldera

Caldera formation and subsequent plug 1. Volcanic eruption 2. Large volume of material extruded 3. Magma chamber empties 4. Volcano collapses into the empty magma chamber

Yellowstone: hot spot under continental crust • Three large eruptions in the last 2 million, 1. 3 million and 600, 000 years ago Calderas formed when felsic lava produced enormous eruptions.

Yellowstone caldera formation

Long Valley Caldera • An enormous eruption 760, 000 years ago, forming a caldera

Landforms associated with low viscosity lavas Basaltic lava flows produce shield volcanoes and lava plains or flood basalts.

Shield volcano Mauna Loa is 9 miles high Built over a long period of time Associated with basaltic lava

Modoc Plateau, northeastern California (extension) • Medicine Lake volcanic field • Mt. Shasta is in the background – Tectonic setting?

Basaltic lava flows from fissures Columbia River Basalts Layer upon layer of lava flows Covers continental crust 14 -16 million years old

What happened in Iceland? • Eyjafjallajokull's eruption creates an ash cloud that closed Europe’s airports for weeks • Shield volcano eruption under a layer of ice

Size comparison

Cinder cones: found in most setting Hawaii • Short lived events • made of cinders • generally about 1000 feet high Mojave Desert

Composition, Viscosity and Eruptive Style Composition Basalt Fluid Quiet Hot Andesite Viscosity Eruptive Style Temperature Rhyolite Pasty Violent Cool

The three Vs Viscosity Strombolian Icelandic Volatiles Volume Plinian

Volcanic material Pyroclastic debris Lava flow • Pieces of older rock and magma • Ash size to bombs • Smooth or chuncky

Volcanic Explosivity Index • Volume of material • How high the eruption column reached • How long the main eruption occurred