Volcanic Eruptions



Oldoinyo L’engai, Tanzania


  • volcanoes are unpredictable & dangerous

  • volcanic activity can build a towering, snow-capped mountain or it can blast one apart

  • eruptions can provide fertile soil that enables a civilization to susrvive, or it can generate a rain of destruction that can snuff one out


The Products of Volcanic Eruptions

  • lava flows

  • pyroclastic debris

  • gas


Lava Flows

  • viscosity determines lava characteristics

  • viscosity depends primarily on chemical composition

    • silica in lava tends to bond together into large molecules that can't move easily

    • mafic (basaltic) lava is less viscous than intermediate lava which is less viscous than felsic lavas

  • viscosity also depends on temperature


Basaltic (mafic) Lava Flows

  • has very low viscosity when it first emerges from a volcano because:

    • it contains relatively little silica

    • it is very hot

  • on steep slopes near the summit of a volcano, lava can move at speeds of up to 30 km per hour

  • slows down to a walking pace when it starts to cool

  • most basaltic lava flows measure a few kilometers long but some extend tens of kilometers from its source


How can lava travel long distances?

  • rapid cooling at the surface of a lava flow causes the surface to crust over

  • the solid crust acts as insulation, allowing the hot interior of the low to remain liquid and continue to move

  • as more of the interior solidifies, molten lava moves only through a tunnel-like passageway called a lava tube

  • in some cases lava tubes drain and become empty tunnels


A fast-moving lava flow coming from Mt. Etna, Sicily




A basaltic lava flow covers a road in Hawaii













Hallmundarhraun Lava Flow, Iceland




Basaltic Lava Flows (continued)

  • pahoehoe lava flows (Hawaiian for "smooth, unbroken lava":

    • a smooth, billowy, undulating, or ropy surface.

    • These surface features are due to the movement of very fluid lava under a congealing surface crust.

    • A pāhoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust.

    • It also forms lava tubes where the minimal heat loss maintains low viscosity.


  • a'a' lava flows

    • is basaltic lava characterized by a rough or rubbly surface composed of broken lava blocks called clinker.


  • pillow lava

    • Pillow lava is the lava structure typically formed when lava emerges from an underwater volcanic vent or subglacial volcano or a lava flow enters the ocean.


  • columnar jointing

    • hexagonal cross-section jointing in lava flows

    • caused by slow cooling of solid lava that contracts and causes cracks (joints)



Toes of a pāhoehoe advance across a
road in Kalapana on the east rift zone
 of Kīlauea Volcano in Hawaii,
United States




Pāhoehoe lava from Kīlauea volcano, Hawaii



Pāhoehoe lava




Glowing ʻaʻā flow front advancing over pāhoehoe
 on the coastal plain of Kilauea in Hawaii



A'a Lava Flow, Hawaii




Pillow Lava



Cross-section of pillow lava near Oamaru, New Zealand



Pillow Lava Formation, Hawaii (video)





Columnar jointing develops when the interior of a flow
cools and cracks. Devils Postpile, California






Intermediate (Andesitic) Lava Flows

  • greater viscosity means the lava can't flow as easily

  • when erupted, intermediate lava first forms a mound above the vent

  • this mound slowly advances down the volcanoes flank at a rate of 1-5 meters per day

  • has a lumpy, bulbous snout

  • called blocky lava, looks like a jumble of rubble





Felsic (Rhyolitic) Lava Flows

  • most viscous of all lavas because it is the most felsic

  • tends to accumulate either above the vent in a lava dome or short bulbous flows

  • sometimes the lava freezes while still in the vent and then pushes upward as a column-like spire up to 100 meters above the vent







Volcano Deposits


Paricutin Eruption


  • Volcanoes can erupt fragments of igneous material referred to as:

    • tephra or pyroclastic debris: includes the following materials:

      • specks of glass

      • pieces of rock formed from drops or clots of molten lava that solidified in midair

      • chunks of recently solidified lave or pumice blasted out of the volcano's vent

      • lapilli are pea-sized to golf ball-sized glassy lava and scoria


    • volcaniclastic debris includes:

      • tephra

      • debris that tumbled down the sides of a volcano in landslides long after an eruption

      • debris that mixed with water to form a muddy slurry

      • debris that is carried away and sorted by streams

      • ash is usually formed during explosive eruptions and is less than 2 mm in diameter

      • tuff is ash or ash mixed with lapilli that is transformed into rock


Blocks and lapilli on the flank of a Hawaiian volcano




Pyroclastic debris billowing from the 2008 eruption of Chaiten in Chile














Pyroclastic flow, Mt. Marapi, Indonesia, 2006









  • volcanic debris flows that are mixed with snow, ice and water

  • can move as fast as 50 km/hour

  • will harden like concrete


A lahar fills a river bed in New Zealand after an eruption in 2007





This mudline left behind on the trees on the banks of the Muddy River
after the 1980 eruption of Mount St. Helens shows how high the lahars reached here.




The aftermath of a lahar from the 1982
eruption of Galunggung, Indonesia.





The lahar from the 1985 eruption of Nevado del Ruiz
that wiped out the town of Armero in Colombia.






Volcanic Gas & Aerosols


  • Most magma contains dissolved gases such as:

    • gasses cowater

    • carbon dioxide

    • sulfur dioxide

  • up to 9% of a magma is gas

  • generally, lavas with more silica contain more gas

  • gasses come out of solution as magma approaches the surface and pressures decrease

  • water in the magma combines with sulfur to form sulfuric acid as aerosols

  • in low viscosity magmas, gasses can rise faster than the magma moves resulting in large quantities of gas being released before much maga is released

Gas bubbles frozen in lava produce vesicles. Sunset Crater, Arizona





Volcano Structure & Eruptive Style