Chapter 5: Volcanoes

5.1: Introduction

• Eruption - the release of lava and/or pyroclastic debris from a volcanic vent.

• Volcano - erupting vent through which molten rock reachers the Earth's surface or the hill or mountain built from the products of eruptions.

5.2: The Products of Volcanic Eruptions

• Lava flow - sheets or mounds or lava that flow onto the ground surface or seafloor in molten form and then solidify.

• Character of a lava reflects its viscosity (resistance to flow), which depends on chemical composition.

• Silica lava can't move easily.

• Basaltic lava is less viscous than andesitic lava.

• Andesitic lava can flow farther than rhyolitic lava.

• Viscosity depends on…

  • Chemical composition

  • Temperature

• Lava tube - the empty spot left when a lava tunnel drains; happens when a surface of a lava flow solidifies while the inner part of the flow continues to stream downslope.

• Andesitic Lava Flows

  • When erupted, its lava first forms a mound above the vent. advances slowly, blocky lava.

• Rhyolitic Lava Flows

  • Most viscous, most felsic, coolest, accumulates above vent in a lava dome or in short and bulbous flows.

  • Sometimes freezes while in vent and pushes upward as a column-like spire

  • Broken, blocky surfaces

• Most magma has gases such as water, carbon dioxide, and sulfur dioxide

• Low viscosity magma = gas bubbles rise more quickly, erupt large quantity of gas without much lava.

• Pyroclastic Debris from Basaltic Eruptions

  • Basaltic magma rising in a volcano may contain dissolved gas. As such magma approaches the surface, the gas comes out of solution and forms bubbles. When the bubbles reach the vent of the volcano, they expand and burst, producing enough pressure to drive columns or sprays of basaltic lava upward in dramatic lava fountains, from which clots of lava rain down onto the volcano.

• Pyroclastic flow - fast moving avalanche that occurs when hot volcanic ash and debris mix with air and flow down the side of a volcano.

• Pyroclastic debris - unconsolidated fragments resulting from an eruption

5.3: Structure and Eruptive Style

• Magma chamber - large, underground, irregularly shaped zone with magma

  • Melt and crystals forms a crystal mush

• Crater - circular, bowl-like depression on top of volcano

• Mafic = fast flow

• Intermediate = viscous lava, slower and thicker flow

• Felsic = high silica, high viscous, lava domes, fills craters

5.4: Geologic Settings of Volcanism

• Mid ocean ridges

• Convergent boundaries

• Continental rifts

• Oceanic hot spot volcanism

• Continental hot spot volcanism

• Flood basalts

5.5: Volcanoes are Hazards

• Natural hazards

• Lava flows

• Pyroclastic flows

• Falling ash and lapilli

• High altitude ash to aircraft

• Blast

• Landslides and Lahars

• Earthquakes and Tsunamis

• Gas

5.6: Protection from Vulcan's Wrath

• Active volcanoes - have erupted recently or likely to erupt (Kilauea Hawaii)

• Dormant volcanoes - vents that have not erupted for hundreds to thousands of years but may erupt again (Mt Fuji)

• Extinct volcanoes - will never erupt again (Devils Tower)

5.7: Effect of Volcanoes on Climate and Civilization

• Volcanic eruptions can produce cooling effects due to ash and aerosols

• Largest explosive eruption: Toba volcano in Indonesia

5.8: Volcanoes on Other Planets

• There is volcanic activity on other planets

• Types of volcanoes (shield, cinder cone, stratovolcano, supervolcano, hot spot, mid-ocean ridge) and examples of each.

  • Shield volcanoes

    • Broad, gentle domes whose shape resembles a soldier's shield lying on the ground

    • Form when the products of eruption have low viscosity and can't build into a mound at the vent.

    • Flow easily and spread out in thin sheets over large areas

    • Example: Kilauea (Hawaii)

  • Cinder cones

    • Scoria cones

    • Cone-shaped piles of basaltic lapilli and blocks.

    • Steep slopes, maximum slope that the loose fragments can sustain before sliding down

  • Stratovolcanoes

    • Composite volcanoes

    • Largest, most explosive, viscous

    • Interleaved layers of lava, tephra, and pyroclastic debris

    • strato- emphasizes internal layering (stratification).

    • Example: Mt. St. Helens (Washington, Oregon)

  • Mid-ocean ridge

    • Develop along fissures parallel to the ridge axis

    • Erupt basalt which cools quickly underwater and forms pillow lava mounds

    • Water heats up as it circulates through the crust near the magma chamber bursts out of hydrothermal vents along the mounds

  • Hot spots

    • Oceanic hot spot volcano forms on oceanic lithosphere, basaltic magma erupts at the surface on the seafloor

    • Continental hot spots and rifts produce both effusive and explosive eruptions

• Calderas - A large circular depression with steep walls and a fairly flat floor, formed after an eruption as the center of the volcano collapses into the drained magma chamber below .

• Viscosity of magma/lava, effusive vs. explosive

  • Effusive eruption - low viscosity lava spills or fountains steadily from a vent or fissure

  • Explosive eruption - pyroclastic debris blasts forcefully into the air

    • Large explosions happen in felsic or andesitic volcanoes

• Types of effusive basaltic lava (pahoehoe, A’a’)

  • Pahoehoe

  • A'a'

• Columnar jointing - a type of fracturing that yields roughly hexagonal columns of basalt; columnar joints form when a dike, sill, or lava flow cools.

  • Can form in dikes and sills

• Pillow basalt - when lava cools quickly in water, submarine basaltic lava travels short distances before freezing, which produces a glass-encrusted blob. Glass rind of pillow momentarily stops the flow, then pressure from the lava squeezing into the pillow breaks the rind and a new blob squirts out. Process repeats.

• Explosive volcanic textures (pyroclastic, tuff, obsidian – volcanic glass)

  • Pyroclastic - mix of rock fragments, pumice, and volcanic ash

  • Tuff - volcanic ash and fragmented pumice, when debris accumulates and cements together

  • Obsidian - glassy

• Explosive volcanic features - lahar, debris flow, ash fall, pyroclastic flow

  • Lahar - A muddy, rapidly flowing slurry caused by ash-rich debris becoming very wet.

• Volcanism at tectonic plate boundaries & where mafic, felsic, intermediate rocks occur (basalt, andesite, rhyolite).

Extra Notes

• Lava is molten rock that solidifies at the surface.

  • Often, volcanic eruptions don't produce any lava.

  • Lava has different chemical compositions.

  • Lava usually moves slowly.

• Mafic lava can travel the most distance from a volcano before solidifying.

  • Intermediate can move the second farthest distance.

  • Felsic is the most viscous, so it travels the least distance.

• Due to a sudden release of accumulated gas pressure within a volcano, an explosive eruption occurs, blasting lava upward rather than allowing it to flow down the slope of the volcano.

• The shape of the volcano and presence of a lava dome suggest relatively viscous magma. Keep in mind that the composition of a magma controls its viscosity. Felsic magmas are more viscous than mafic magmas. If the lava were mafic, it would not build into a dome.

• Volcanic islands form over hot spots. Over time, the volcano is carried away from the hot-spot source by the movement of the tectonic plate, causing it to go extinct. As the island ages, it cools and subsides. The island is eroded until it is no longer emergent and becomes an underwater seamount.