Chapter 4

the most common intermediate rock is:

andesite

_____ rocks are silicate-rich while _____ rocks are silicate-deficient.

felsic, mafic

______ lavas tend to flow easily, while ______ lavas tare more viscous and flow sluggishly.

mafic, felsic

because felsic magmas are more viscous they are associated with:

more violent, explosive eruptions

Submarine eruptions

nearly always basaltic mid-ocean ridge eruptions -- pillow basalts basalt plateaus under the ocean pillow basalts form when pahoehoe-type lava flows into water, blobs of lava break out of the thin skin of solid basalt that quickly formed, and then blob of lava hardens quickly, forming a pillow shape.

Three factors that control viscosity:

(1) Silica Content-higher silica content produces higher viscosities because the more silica there is the more silica tetrahedrons that are cooling and arranging themselves causing the lava to thicken and become more viscous. (Most important) (2) Dissolved gases in the lava - more dissolved gases in lava, more fluid it is (mafic lava), helps lava flow, as long as the gases are not trapped by silicon tetrahedrons. Gases: Mostly water vapor and carbon dioxide, some sulfur dioxide, hydrogen sulfide, and carbon monoxide. (3) Temperature of lava relative to temperature of solidification- cooler lavas have higher viscosities (felsic, 700 °C; mafic 1200 °C) Mafic lava is 10,000 times more viscous than water. Felsic lava is more viscous because the silica tetrahedrons begin to arrange themselves immediately at a cooler temperature than mafic lava. Felsic lava is 100 million times more viscous than water.

Composite volcano

Also known as stratovolcanoes Constructed of alternating layers of pyroclastic fragments (when enough gas is present for a violent eruption) and solidified lava flows (when not enough gas is present for a violent eruption). Slopes are less than 33° from the horizontal (between shield volcano slopes of 2°-10° and cinder cone slopes of 33°) Pyroclastic layers build up steep slopes, subsequent lava flows partially flatten the cone as lava flows down slopes building up the flanks. Lava hardens and protects the pyroclastic layer from weathering and erosion. Mostly composed of intermediate magma (andesite is the extrusive igneous rock formed)

flood basalts

At continental rifts, vast bodies of basaltic lava flow forth from fissures, formin

Shield volcano

Broad gently sloping cone constructed of solidified lava built up from numerous flows; shaped like a shield. Largest volcanoes: tallest mountains on Earth when measured from their base, usually on the ocean floor. Non-explosive eruptions due to low silica content of mafic magma. Lava spreads widely and thinly from a central vent due to low viscosity (thin, not thick) [viscosity: resistance to flow] Builds slopes 2°-10° from the horizontal forming a shield shape. Forms basalt: (extrusive igneous rock formed from mafic lava) Shield volcanoes form where plates diverge and over hot spots (mantle plumes). Examples: Kilauea and Mauna Loa in Hawaiian Islands, Surtsey in Iceland, and Mt. Etna in Mediterranean.

cinder cone

Constructed of loose rock fragments ejected from a central vent. Slopes are no more than 33° from the horizontal. Steepest volcanoes: built up from ejected material landing near the vent and forming a peak. Much smaller than shield volcanoes, usually less than 500 meters tall. Erupt due to build up of gases, independent of magma composition; most are mafic but can be felsic or intermediate lava. They weather and erode quickly and easily due to unconsolidated pyroclastic material that makes them up. [pyroclastic: Greek for "fire" (pyro) and "broken into pieces"(clastic)] Form where plates converge. Example: Paricutin in Mexico

How are volcanoes predicted?

Establishing a volcano's history: Active, Dormant, Extinct Active - if currently or recently eruptive (Approximately 500 in the world today) Dormant - if it hasn't erupted in many thousands of years but is expected to erupt in the future Extinct - haven't erupted in many years and show no signs of any future eruptions Monitoring changes and anomalies (something that deviates from what is standard, normal, or expected): Earthquakes Shape or elevation of volcano Gas emissions Ground/water temperature Water acidity

Volcanic benefits

Fertile Soil -weathering of volcanic materials (Hawaii) Growth of volcanic islands (Hawaii) Tourist attractions - Hawaii, Mt. Vesuvius, Mt. St. Helens Geothermal energy: natural steam harnessed as a clean energy resource underground heat generated from superheated water at depth; sometimes considered a renewable energy source Climatic effects: very large eruptions can result in measurable global cooling, resulting crop failures and famines from ash and sulfur dioxide in the atmosphere, offsets global warming

Products of Volcanic Eruptions:

Lava - produced when magma reaches Earth's surface Pyroclastic flows - mixtures of hot gas and pyroclastic debris (fast, 450 mph; hot, 1830 °F) explosive eruptions can produce rapidly cooled rock fragments called pyroclasts, size ranges from dust (ash) to boulders (blocks and volcanic bombs) calm oozing of magma out of the ground produces lava flows pyroclasts and lava flows form extrusive igneous rocks lava flows and pyroclasts pile up to form volcanoes

primary volcanic hazards

Lava flows - magma overflows a volcanic vent--incinerate everything in their paths Ash falls: tremendous quantity of rock fragments, natural glass fragments, and gas blown high in the air--damage to vegetation and infrastructure Pyroclastic flows: most lethal effect of volcanic eruptions; ash, rock, volcanic glass fragments, and gases are blown out of a vent and move rapidly down the sides of a volcano; fast moving and very hot. ----incinerate everything in their paths Poisonous gases: water vapor, carbon dioxide, carbon monoxide, sulfur dioxide, hydrogen sulfide, hydrogen chloride, and hydrogen fluoride are emitted (water and carbon dioxide make up 90% of the gases emitted). ------Effects: sulfur dioxide reacts in the atmosphere to produce acid rain, Vog is formed which is a thick, acidic haze produced by volcanic gases reacting with oxygen and moisture in the atmosphere (causes asthma, headaches, sore throats, flu-like symptoms), and water and soil contamination.

plateau basalts

Layers of basalt flows that have built up to great thicknesses.

secondary volcanic hazards

Mud and Debris Flows (Lahars): produced when large volumes of loose volcanic ash becomes saturated with water, becomes unstable ,and moves down slope. Debris flows consist of larger sized material than mudflows.-----can reach 50 miles away from volcano and be up to 500 feet deep, can cause tsunamis if occur in an ocean.

Three types of volcanoes

Shield volcano Composite volcano Cinder cone

Volcanic Landforms

Vent - opening through which lava erupts (central vent at top of volcano) Crater - basin-like depression over the vent at the summit of the volcano usually no more than 300 meters in diameter or 100 meters deep Volcanic domes - extremely high viscosity (thick), degassed, often felsic lava that solidifies above a volcanic vent (often glassy, e.g., obsidian) Caldera - large crater forms when the top of a volcano is blown off or when a crater collapses into an empty magma chamber, usually more than 1500 meters in diameter and several 100 meters deep.

Summary: Plate Tectonics and Volcanism

Volcanic Activity at Divergent Boundaries: Decompression Melting Calm eruptions of basaltic magmas and pillow lavas Formation of most of the sea floor Mid-oceanic ridges, Iceland Volcanic Activity at Convergent Boundaries: Most large well-known volcanoes Explosive composite volcanoes Viscous andesitic lavas Within-Plate Volcanic Activity: Mantle Plumes (Hot Spots) - Hawaii, Yellowstone Basaltic magma/lava

viscosity

a fluid's resistance to flow (degree of thickness) High viscosity: thick, does not flow easily Low viscosity: runny, flows easily

Shield and Cinder Cone volcanoes are composed of mostly ______, while Composite volcanoes are composed of mostly ________

basalt, andesite

Composite volcanoes are distributed mainly along which two belts?

circum-Pacific or "Ring of Fire"--the larger of two Mediterranean belt