Lecture 7: Volcanism

I. What comes out of a volcano?

• Lava

  • Molten rock effusively erupted by volcano

  • Generally more degassed than its magma counterpart

  • Types:

    • Pahoehoe Lava - occurs in basaltic lava; shows smooth, undulating and ropy surface; thin flows, slow velocity

    • A’a Lava - rough and rubbly surface (clinkery), faster movement, higher velocity; may occur in other types of magma

    • Pillow Lava - basaltic to andesitic magma; associated with water; pillow-shaped lobes with quickly chilled (glassy) margin filled with molten material; convex roof and concave to flat-floor; associated with MOR volcanism

• Tephra

  • Umbrella term for the pyroclastic material (rock & glass fragments) explosively erupted by a volcano

• Pyroclastic Fragments (Pyroclasts)

  • Individual pyroclastic (rock/glass) fragment classified based on size

  • Pyroclastic Flows:

    • Pyroclastic Flow - high velocity avalanche of hot gases infused with incandescent ash and pyroclastic fragments

      • Nuee Ardente

        • glowing avalanches composed of a low-density cloud of hot expanding gases containing hot fine ash and ground-hugging slurry of pyroclastics

    • Lahar Flow - slurry or volcanic mudflow from saturated volcanic and pyroclastic debris

  • Pyroclastic Density Currents (PDCs) - the umbrella term

    • Fall

      • ash eventually settles after being carried high above the volcano in a vertical plume

    • Flow

      • fast, concentrated density current of steaming pyroclastic material; travels faster near bottom

    • Surge

      • extremely fast (hurricane speed), dilute density current of steaming pyroclastic material; concentrated density current of volcanic debris; travels fastest at top; very dangerous

• Volcanic Gases

  • Gases exsolved & separated from magma as they reach the surface

  • gas exsolved from the magma composed of SO2, CO2, H2O, or H2S

  • Fumaroles - fracture or opening from which volcanic gases and steam escape

  • Solfatara - fumarole emitting SO2 or H2S, precipitating sulfur deposits

II. Volcano Morphology

Volcano

• Opening or rupture in the crust where lava, volcanic gases and pyroclasts are released from a magma chamber beneath the surface

• Is a vent, hill or mountain from which molten or hot rocks with igneous material have been ejected

• Can also be craters, depressions, hills or mountains formed by removal of pre-existing material or by accumulation of ejected materials

Types of Volcano

• Shield Volcano

  • Dome-shaped, low gradient and gently sloping (<15°)

  • Effusive and low viscosity basaltic flows

  • Examples:

    • Kilauea: Eruption of a shield volcano Kilauea, the most active and intensely studied shield volcano in the world, is located on the island of Hawaii in the shadow of Mauna Loa

    • Mauna Loa is one of five overlapping shield volcanoes that together comprise the Big Island of Hawaii

    • Extensive study of the Hawaiian Islands confirms that they are constructed of myriad thin basaltic lava flows averaging a few meters thick intermixed with relatively minor amounts of pyroclastic ejecta

• Stratovolcano / Composite Volcano

  • Tall, conical with steeper profile (15-30°)

  • Alternating layers of lava flows and pyroclastic materials

  • Intermediate magma

  • Examples:

    • Mt. Fujiyama, Japan

    • Mt. Mayon, Albay, Philippines

• Pyroclastic cones

  • Produce fragments of lava that accumulates around the volcanic vent to produce scoria mounds and other topographic forms

  • Types:

    • Scoria cones: mounds of scoria, (< 200-300 m high), with a crater in the middle. Young scoria cones have slopes of 33°C, which is the angle of rest for loose scoria. Once the eruption ceases, solidification seals off the volcanic vent and the volcano never erupts again

    • Maars form in a similar way to scoria cones, but in this case involving the interaction between magma and a water-bearing stratum—an aquifer. The result of this combination is explosive.

    • Tuff rings are produced by near-surface subterranean explosions where magma and water mix, but instead of being holes in the ground they are surface accumulations of highly fragmented basaltic scoria

    • Tuff cones are smaller and steeper versions of tuff rings

      

  • Examples:

    • Paricutin, Michoacan, Mexico

    • Sampalok Lake, San Pablo, Laguna

    • 7 Lakes of San Pablo

    • Diamond Head, Oahu, Hawaii

• Lava Dome

  • Steep domal mound structures

  • Slow extrusion and piling of viscous lava

  • Associated with felsic lavas, but may also occur in other type of magmas

• Fissure Eruptions and Basalt Plateau

  • Flood basalt

  • Extensive basaltic lava flows exhibiting staircase geomorphology (trap)

  • Trappe - Swedish word for staircase

  • Examples:

    • Siberian Traps (Siberia) - Permo-Triassic Extinction event

    • Ontong Java Plateau - coincides with the Early Aptian Anoxic Event

    • Deccan Traps (India) - Previously linked to the K-Pg extinction

    • Columbia River Plateau (USA) - Linked to the Middle Miocene Climatic Optimum

    • The Philippine Rise (also called the Benham Rise) lies east of Luzon and is a Large Igneous Province (LIP)

    • Located in the Philippine Rise, the newly discovered Apolaki Caldera may be the world’s largest known caldera

    • The rich biodiversity and abundant natural resources in the Rise make it necessary to enforce our claim over it

• Caldera

  • Examples include Laguna de Bay, Taal Volcano, Crater Lake, Oregon

  • Large depressions (> 2km in dia) formed from:

    • Summit collapse from explosive eruption of a composite volcano (Crater Lake)

    • Shield collapse from subterranean drainage of central magma chamber (Hawaiian)

    • Large-area collapse from discharge of colossal volumes of pyroclasts (Yellowstone)

III. Volcanic Eruptions

• generally categorized into effusive and explosive eruptions

Volcanic Explosivity Index (VEI)

• Used to describe the explosivity of an eruption estimated with the volume of tephra ejected and eruption column height

• Takes into account observable and measurable parameters during an eruption

• General eruption types:

  • Explosive - gas-driven with associated magma extrusion and tephra column

  • Effusive - oozing of lava flows

Dense Rock Equivalent (DRE)

• One way to characterize the size of an eruption

• Takes into account the amount of erupted magma (effusively and explosively) from volume estimates of erupted products (lava and/or tephra)

• Allows better size comparison between effusive and explosive eruptions

Eruption Types

• Based on the driving mechanism / components involved

  • Magmatic eruption

    • Hawaiian

      • quiescent, effusive eruption begins as fissures evolving to central vent flows and lava fountains forming shield volcanoes

      • low viscosity and volatile basaltic magma

      • VEI 0-1

    • Strombolian

      • mild blasts, incandescent cinders, lapilli and lava bomb; least violent explosive eruption

      • intermittent bursts of partially molten basaltic to andesitic lava

      • VEI 1-2

    • Vulcanian

      • discrete ejection of mostly solid materials and gases from more viscous magmas that results from the fragmentation and explosion of a plug of lava in a volcanic conduit, or from the rupture of a lava dome

      • Intermediate magma

      • VEI 2-3

    • Peléan

      • large amount of gas, dust, ash and lava fragments driven by lava dome collapse from a larger eruptive column which associated incandescent pyroclastic flow (nuée ardente)

      • occurs in viscous rhyolitic to andesitic magma

      • VEI 3-4

    • Plinian

      • largest, most violent caused by fragmentation of gassy magma associated with very viscous magma

      • High viscosity and volatile andesitic to rhyolitic magma

      • VEI 5-6

  • Phreatic eruption

    • Eruption caused when surface / near-surface water is heated by magma, hot rocks, or hot volcanic deposits

      • Water vapor

      • Tephra from fragmentation of surrounding rocks

  • Phreatomagmatic eruption

    • generated by the interaction of magma with abundant surface water; explosive eruption of water vapor, pyroclasts (from magma and/or surrounding rocks) as well as lava

      • Surtseyan

        • occurs in an undersea volcano that has grown large enough to break the water’s surface

        • wet equivalent of Strombolian eruption and occur in basaltic volcanoes

      • Submarine

        • volcanic eruptions that occur underwater along mid-oceanic ridges and hotspots

        • gives rise to seamounts and volcanic islands

        • occurs in basaltic magma and associated with pillow lavas