Volcanic activity sites along boundaries

Where do volcanoes occur—and why?

• Divergent [MOR] boundaries – About 75% of erupted lava is found here– Basaltic magma; low viscosity, therefore it flows easily!– Oceanic crust ranges from about 2 km to 10 km thick

• Convergent plate boundaries– Forms about 10% of all erupted magma (on continents or ocean floor)– Ocean-continent or ocean-ocean plate collisions– Volcanoes contain mainly andesite, intermediate in composition

• Isolated hot spots– Under oceanic plates - Hawaii– Under continental plates – Yellowstone NP– At plate boundaries - Iceland

Lithosphere and asthenosphere

Volcanic features

• Composite volcano • Basaltic lava flow

Volcanoes in Ring of Fire

Hawaiian Islands and seamounts

Distance vs age in Hawaiian Islands

Products of volcanic activityMagma becomes lava and other products of eruption• Lava flows

– Basalt flows faster and farther– Higher silica content means slower flowing lava, such as

rhyolites

• Pyroclastic debris– Due to high gas content of magma

• Volcanic gases– Many not-so-nice gases

Components of Volcanic Eruptions

• Role of gases• Volcanic explosivity• Mineral composition– Role of silica content– Presence of different elements

Layers of basalt along Columbia River

Mauna Loa, HI, shield volcano

Cinder cone• General structure of cinder cone • Sunset Crater, AZ

Composite or stratovolcano

Domes

Elden Mt in Flagstaff Dome inside Mt St Helens

Observe flow down hillside

CalderaLake Atitlan in central Guatemala

Types of Volcanic Hazards

• Gas emissions• Lava flows• Lahars• Pyroclastic flows• Directed blasts• Landslides and tsunami• Earthquakes

Gas emissions

• Up to 9% of composition• Water vapor (ranges from 50 to >80%)• Carbon dioxide• Sulfur dioxide• Hydrogen sulfide• Lesser gases – HCl, HF, Cl• Creates vesicles when trapped in lava

Fig. 7.05a

W. W. Norton Consider distribution offallout material away fromsource

Lava flows

• Extruded magma creates lava• Movement controlled by composition and

temperature• More basaltic, greater flow due to simple

chemistry (most common type)• Can flow hundreds of kilometers

Hawaii basalt lake

Lahars due to snowmelt

Nevado del Ruiz, ColombiaToutle River, near Mt St Helens

Pyroclastic flows

Combination of gases, ash and other debris

Up to 100s of meters thick

Can travel at >200 km per hour

Mount Rainier, WA

Activity around Mt Rainier, WA

Directed blasts

Landslides and tsunami

Earthquakes

• Caused by rising magma• Usually small

magnitude events• Mostly shallow source

Human toll through history

• Mount Vesuvius, Italy 79 AD• Mount Tambora, Indonesia 1815 (famine)• Krakatua, Indonesia 1883 (tsunami)• Mount St. Helens 1980 (last in continental US)• Lake Nyos 1986 – expulsion of carbon dioxide• Mount Pinatubo, Philippines 1991

Many others listed in Table 3.3 (p.72 of text)

Events are Mt St HelensLook at the stages of development of the eruption

What were the precursors?

Relatively few died (about 60)

Precursors to help mitigate damage

• Active, dormant, or extinct structures• Possible signals of eruptions– Seismic activity changes– Surface heat– Surface bulge– Gas emissions

Type of volcano

Viscosity

(fluidity)

Volatiles(gas

content)

Volume Examples

Shield Low Low Large Hawaii

Flood basalt Low Low Very large Deccan, Columbia River

Scoria Medium Medium Small Paricutin, Stromboli

Stratovolcano (composite)

High High Large Vesuvius, Rainier,Fujiyama

Caldera High High Very large Crater Lake, OR, Krakatoa

Lava domes High Low Small In crater of Mt St Helens; Mt Elden

• Sample Questions– Convection cells in the mantle rise and generate

what feature on the ocean floor above the cells? (a) continental plates, (b) active subduction, (c) convergent boundaries, (d) the mid-ocean ridge

– The concept of plate tectonics is classified as a(n): (a) theory, (b) hypothesis,(c) law or principle, (d) unaccepted idea.