Chapter 5 - Volcanoes and Volcanic Hazards

• Mount St. Helens has the largest volcanic eruption in North America.

  • The 2900 m (9500 ft) summit was lowered by more than 400 meters (1350 ft)
  • The blast blew out the entire flank of the volcano.
  • Mudflows carried ash, trees, and water-saturated rock debris.
  • Ash was propelled more than 18 km (11 miles) into the stratosphere.
  • Measurable deposits were reported as far away as Oklahoma and Minnesota.
  • Ash fallout in the immediate vicinity exceeded 2 meters (6 ft) in depth.
  • Midnight-like darkness at noon.
• Hawaii Kilaeua generate quiet outpourings of fluid lavas

  • Even non explosive fountains of incandescent lava spray hundreds of meters into the ait, but most lava pours from the vent and flow downslope.
  • Active recent phase began in 1983
  • Had more than 50 eruptive phases
• Mafic rocks have high percentage of dark silicate minerals and calcium-rich plagioclase feldspar. They are also dark in color.
• Felsic rocks have light colored silicate minerals, quartz and potash feldspar.
• Mafic magmas have lowest silica content, gas content, and erupt at the highest temperatures.

  Felsic magmas have highest silica content, gas content and erupt at the lowest temperatures.
• Most magma is generated in the upper mantle by partial melting of solid rock.

  • Mostly generate basaltic magma, rises on surface, outflows of basaltic lavas.
  • or seafloor spreading, basaltic magma erupts, forms ocean floor, results to volcanism.
• Effusive and Explosive Eruptions

  • Effusive - "pouring out"
  • 2 factors: viscosity and gas content
  • Viscosity - measure of a fluid's mobility, more viscous, greater resistance to flow.

  1. Factors affecting viscosity

  • temperature and silica content
• Effusive and Explosive Eruptions

  1. Factors affecting viscosity

  • temperature and silica content
  • more silica, greater viscosity
  • silicate structures began to link together into long chains which makes magma more rigid
  • Felsic lavas - most viscous, travel at slow speeds, form short and thick flows.
  • Mafic lavas - least viscous, travel at 150 km before solidifying, more fluid.
  • Intermediate lavas - have flow rates between the two.
  • The hotter the magma, the less viscous it is
  • As lava cools and congeals, viscosity increases and flow eventually stops.
• Effusive and Explosive Eruptions

  2. Role of Gases

  • gas content of magma are related to its composition.
  • mafic magmas with fluid and low gas content: 0.5% weight
  • felsic magmas that are viscous and high gas content: 8% weight

  Explosive eruptions

  • rhyolitic magmas
  • magma blown into fragments with supersonic speed.
  • associated with eruption columns.
  • Eruption columns - consist of volcanic ash and gases, can rise 40 km into the atmosphere, hot ash running down exceeding 100 km.
• Lava flows

  • vast majority of Earth's lava, more than 90% is mafic (basaltic) magma.
  • 1% makes up the rhyolitic magma and the rest accounts of intermediate magmas.
  • most of mafic lavas erupt on the floor via a process called submarine volcanism.
  • rhyolitic magmas extrude volcanic ash than lava.
• • Aa and Pahoehoe flows
  • generated by fluid basaltic magmas
  • Aa - have surfaces of rough jugged blocks with sharp edges and spiny projections.
  • Pahoehoe - exhibit smooth surfaces that resemble twisted braids of ropes.
  • Pahoehoe flows are hotter and more fluid than Aa flows.
  • Pahoehoe lavas can change to Aa flows.
  • Pahoehoe flows often develop cave-tunnels called lava tubes.
  • Lava tubes sevre as conduits for carrying lava from an active vent to the flow's edge. Lava tubes form in the interior of a lava flow, where temperature remains high long after exposed surface cools and hardens.
• 2. Pillow Lavas

  • when outpourings of lava occur on ocean floor, the flow's outer skin quickly solidifies to form volcanic glass.
  • interior lava is able to move by breaking through the solid surface.
  • this process occur as molten basalt is extruded.
  • the result is lava composed of numerous tube structures called pillow lavas.
  • useful in reconstructing geological history becuase their presence indicates that the lava flow formed below the surface of a water body.
• 3. Block lavas

  • upper surface consist of massive, detached blocks.
  • similar to Aa lava flows with slightly curved, smooth surfaces rather than rough, sharp, and spiky surfaces.
• Gases

  • Volatiles
  • magma contain varying amounts of these gases.
  • These gases are held in the molten rock by confining pressure.
  • gaseous portion of most magma bodies range from less than 1% to about 8% of the total weight (most of its form in water vapor)
  • from abundant to least abundant released into the atmosphere from volcanoes: water vapor (H2O), CO2, SO2, Hydrogen sulfide (H2S), CO and H2.
  • SO2 combine with atmospheric gases to form toxic sulfuric acid.
• Pyroclastic Materials

  • when volcanoes erupt energetically, they eject pulverized rocks and fragments of lava and glass from the vent.
  • also called tephra.
  • size ranges from very fine dust and sand-sized volcanic ash (less than 2 mm)
  • ash and dust particles are produced when gas-rich viscous magma erupt explosively.
• • Welded tuff - when hot ash falls, these glassy shards often fuse to form this rock.
  • Lapilli - larger pyroclasts with size ranging from 2 - 64 mm. also called cinders.
  • Blocks - larger than 64 mm; made of hardened lava and bombs.
  • Bombs - larger than 64 mm, ejected as incandescent lava. Semi-molten when ejected so they take on a streamlined shape as they hurl through the air.
  • Bombs and blocks fall near the vent
• Pyroclastic materials are classified accdg to texture:

  • Scoria - term for vesicular ejecta produced during eruption of basaltic magmas (Size of lapilli)
  • Pumice - ejected during rhyolitic explosion or when magma is andesitic. Also lighter in color and less dense than scoria.
• Anatomy of a Volcano

  • Volcanic activity begins when a fissure (crack) develops in Earth's crust as magma moves toward the surface.
  • As the gas-rich magma moves up through the fissure, its path is localized into a pipe-shaped conduit that terminates at a surface called vent.
  • The cone-shaped structure is a volcanic cone that is created by successive eruptions of lava or pyroclastic material.
  • At the summit of most volcanic cone is a funnel-shaped depression called crater.
• Anatomy of a Volcano

  • Some volcanoes have very large depressions called calderas with diameters that are greater than 1 km. May exceed 50 km but its rare.
  • Continued activity from flank eruptions may produce one or more parasitic cones.
• Shield Volcanoes

  • produced by the accumulation of fluid basaltic lavas
  • exhibit the shape of a broad slightly domed structure.
  • begin on the ocean floor as seamounts and few had grew large to form volcanic islands.
  • many oceanic islands are either a shield volcano or coalescence of two or more shield volcanoes built upon massive amount of pillow lavas.
  • e.g. Hawaiian, Canary, Icelands, Galapagos and Easter Islands.
• Shield Volcanoes

  • Mauna Loa - largest of 5 overlapping shield volcanoes that comprise the Island of Hawaii.
  • over 9 km high
  • volume is roughly 200x greater than the large composite cone of Mt. Rainier in Washington.
  • has flanks with gentle slopes of few degrees due to very hot, fluid lava that travelled "fast and far" from the vent.
• Shield Volcanoes

  • another common feature is 1 or more large, steep-walled calderas that occupy the summit.
  • calderas on shield volcanoes form when the roof above the magma chamber collapses.
  • this occur after the magma reservoir empties, either following a large eruption or as magma migrates to the flank of a volcano to feed fissure eruption.
• Shield Volcanoes

  • in the final stage of growth, shields:
  • erupt more sporadically and pyroclastic ejections are common.
  • lava emitted is more viscous
  • these eruptions steepen the summit area which often becomes capped with clusters of cinder cones.
  • this explains why Mauna Kea, a more mature volcano that had not erupted has a steeper summit than Mauna Loa.
• Cinder cones

  • built from ejected lava fragments that begin to harden in flight to produce scoria.
  • size range from fine ash to bombs that may exceed 1 meter.
  • its pyroclastic fragments tend to have basaltic composition
  • some produce extensive lava fields - these lava flows generally form in the final stages of the volcano's lifespan (when magma lost its gas content).
  • have a simple, distinct shape
• Cinder cones

  • steep sided due to its high angle of repose (steepest angle at which a pile of loose materials remains stable)
  • a slope between 30 and 40 degrees.
  • have large, deep craters
  • produced by a single, short-lived eruptive event.
  • 95% formed in less than a yr.
  • once event ceases, the magma in the "plumbing" connecting the vent to the magma source solidifies and the volcano does not erupt again.
• Paricutin

  • located 320 km west of Mexico City
  • 1943 - eruptive phase began in the cornfield owned by Dionisio Pulido
  • 2 weeks prior to 1st eruption, numerous tremors were felt.
  • February 20 - sulfurous gases began billowing from a small depression that had been in the cornfield.
  • During night, hot glowing fragments were ejected from the vent and explosive discharges threw hot fragments and ash as high as 6000 meters.
• Paricutin

  • 1st day - cone grew to 40 meters, 5th day - more than 100 meters
  • June 1944 - clinkery aa flow 10 meters thick moved over much of the village, only remnants of church exposed.
  • It will not erupt again.
• Composite Volcanoes

  • Stratovolcanoes
  • Most are located in a narrow zone that rims the Pacific Ocean called Pacific Ring of Fire. This active zone includes a chain of continental volcanoes distributed along the west coast of Americas, such as the large cones of Andes in South America and Cascade Range.
  • These are large, symmetrical structures consisting of alternating layers of explosively erupted cinders and ash interbedded with lava flows.
• Composite Volcanoes

  • has andesitic magma but sometimes eject fluid basaltic lava and felsic lava.
  • can generate explosive eruptions that eject huge quantities of pyroclastic material
  • conical shape with a steep summit and gradually sloping flanks.
  • coarser fragments ejected from the summit crater accumulate near the source and contribute to the steep slopes.
  • finer fragments are deposited as a thin layer and flattens the flanks of the cone.
• Composite Volcanoes

  • during early stages, lavas are abundant and flow greater distances from the vent.
  • as a composite volcano matures, the shorter flows that come from the central vent armor and strengthen the summit.
  • steep slopes exceed 40 degrees.
  • e.g. Mt. Mayon and Fujiyama.
• Volcanic Hazards

  Pyroclastic Flow

  • consist of hot gases infused with incandescent ash and large lava fragments.
  • also called nuee ardentes (glowing avalanches)
  • these fiery flows can race down steep volcanic slopes at speeds exceeding 100 km per hr.
  • have 2 components: (1) a low-density cloud of hot expanding gases containing fine ash particles. (2) ground hugging portion of pumice and vesicular pyroclastic material.
• Volcanic Hazards

  Pyroclastic Flow

  • Driven by gravity - pyroclastic flows are propelled by the force of gravity and move in a manner similar to slow avalanches.
  • gases reduce friction between ash and pumice fragments that is why some pyroclastic flow deposits are found many miles from their source.
  • Surges - these low density clouds seldom have sufficient force to destroy buildings in their paths; but deadly.
  • Surges may occur when powerful eruption blasts pyroclastic materials out of the side of a volcano, or generated by the collapse of tall eruption columns during an explosive event.
• Destruction of St. Pierre

  • 1902 - pyroclastic flow and surge from Mt. Pelee destroyed the town.
  • a low-density surge spread south of the river and engulfed the city.
  • 28 000 were killed
• Destruction of Pompei

  • C.E. 79 eruption of Mt. Vesuvius
  • Mt. Vesuvius had been dormant with vineyard along the slopes.
  • Less than 24 hrs - the entire city was entombed under layers of volcanic ash and pumice
  • City remained buried for 17 centuries.
  • 1st day of eruption - rain of ash and pumice accumulated at a rate of 12 to 15 cm.
  • had 2 more dozen explosive eruptions since C.E. 79; most recent is 1944.
• Volcanic Hazards

  2. Lahars

  • large composite volcanoes may generate this type of mudflows.
  • occur when volcanic debris becomes saturated with water and moves down steep volcanic slopes.
  • some lahars are generated when (1) magma nears the surface of a geologically clad volcano, causing large volumes of ice and snow to melt. (2) heavy rains saturate weathered volcanic deposits and lahar may not occur when volcano isnt erupting.
• Volcanic Hazards

  3. Tsunamis

  • most associated with displacement along a fault located on the seafloor.
  • some result from the collapse of volcanic cone.
  • e.g. 1883 eruption of the Indonesian island of Krakatau.
• Volcanic Landforms

  • Calderas - large steep sided depression with diameters exceeding 1km and somewhat circular form.
  • less than 1 km - collapse pits/craters
  • formed by: (1) collapse of the summit of a large composite volcano following an explosive eruption of silica-rich pumice and fragments. (2) collapse of the top of a shield volcano caused by subterranean drainage. (3) collapse of a large area caused by discharge of colossal volume of silica-rich pumice and ash.
  • 1 - Crater Lake calderas, 2 - Hawaiin Calderas, 3 - Yellowstone calderas.
• Crater Calderas

  • crater lake, Oregon, is located in a caldera approximately 10 km wide and 600 meters deep.
  • formed about 7000 yrs ago when composite cone, Mt. Mazama, extruded 50 to 70 cubic km of pyroclastic material.
  • with support loss, 1500 meters of summit collapsed, producing a caldera filled with water.
  • later on, volcanic activity built a small cinder cone in the area.
• Hawaiian Calderas

  • form gradually because of the loss of lava from a shallow magma chamber underlying a volcano's summit.
  • e.g. both Mauna Loa and Kilauea measure 3.3 to 4.4 km calderas and 150 meters deep.
  • walls are almost vertical and caldera looks like a vast, flat bottomed pit.
  • Kilauea's caldera formed by gradual subsidence as magma slowly drained laterally from underlying magma chamber, leaving summit unsupported.
• Yellowstone Calderas

  • Eruptions eject huge volumes of pyroclastic materials mainly in the form of ash and pumice fragments
  • 3 caldera forming episodes have occurred over the past 2.1 million years. Recent eruption (63000 yrs ago) had episodic outpourings of degassed rhyolitic and basaltic lavas.
  • In the upcoming years, a slow upheaval of the flood of caldera has produced 2 elevated regions called resurgent domes.