In general Molten rock (magma) that pours or oozes onto the Earth's surface is called lava. The higher a lava's silica content, the more viscous it becomes. For example, low-silica basalt lava can form fast- moving (10-30 miles per hour), narrow lava streams or spread out in broad sheets up to several miles wide. In contrast, higher-silica andesite and dacite lava flows tend to be thick, move slowly, and travel short distances from a vent.
Basaltic lava Erupt primarily from fissure systems and shield volcanoes (eg., Hawaii). These fluid lava flows can be subdivided into two types, based primarily on the nature of lava flow surfaces: Pahoehoe Lava -- (pa-hoy-hoy) Hawaiian for “smooth, ropy lava”. Pahoehoe Lava -- (pa-hoy-hoy) Hawaiian for “smooth, ropy lava”. Aa lava -- Hawaiian for “rocky lava”. Aa lava -- Hawaiian for “rocky lava”. The lava is identical in both pahoehoe and aa lava flows, the difference comes from the amount of lava erupted and the speed of cooling.
Basaltic lava Pahoehoe Lava -- Hawaiian for “smooth, ropy lava”. Surfaces are smooth, billowy, or ropy. Formed from less viscous lava that advances slowly. Forms as the outer layer of the lava cools, then is dragged and folded as the flow continues to move beneath the surface. Lobes
Aa lava -- Hawaiian for “rocky lava”. Surfaces are fragmented, rough, and spiny, with a "cindery" appearance. Forms when lava initially flows rapidly resulting in heat loss and an increase in viscosity.As the flow continues to move, the surface layer is broken into jagged pieces. Basaltic lava
Pahoehoe is common near the source of a basaltic flow, where the lava is hottest, and aa is normally found farther from the source, where the lava has cooled off significantly. Basaltic lava
Intermediate basaltic lava flow with characters of both Aa and Pahoehoe on Etna – Sept. 2004 Photograph GS -
Basaltic lava Important note: Pahoehoe flows cannot turn into aa flows, and vice versa. For example, pahoehoe flows cannot evolve from lava already erupted with an aa surface. The new lava has to come from deep within the parent flow and has a separate crystallization history.
Pillow lava -- when lava is erupted under water or ice. As lava is extruded, the water (or ice) quickly chills the outer layer. Molten lava beneath the chilled surface eventually breaks through the skin and the process is repeated, resulting in a pile of lava pillows Basaltic lava Convex upward tops Cusped bottoms filling space Cross section of pillow basalts
Basaltic lava tubes These require Pahoehoe lava which has the correct thickness to form a crust/tube from the inside of which the still-molten lava then pours out. Roof collapses frequently allow access to these caves.
Basaltic lava Slow build up of fluid lava flows produces shield volcanoes as lava pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that a a warrior's shield. Achilles with shield
Basaltic lava Mauna Loa on the island of Hawaii
Andesitic lava Andesite generally produces blocky lava that advances only short distances down the flanks of a volcano. Resembles aa flows, but more regular shaped polyhedral chunks rather than jagged, vesicular, scoriaceous deposits. Bagana volcano, Bougainville Is, P.N.G. 26-April-1988
Tephra and pyroclasts Tephra (Greek, for ash) is a generic term for unconsolidated, air-fall material produced by a volcanic eruption regardless of composition or fragment size. “Pyroclasts are the individual crystals, crystal fragments, glass and rock fragments generated by disruption as a direct result of volcanic action.” (Schmid, 1981) A pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type.
Pyroclasts Ash -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments. Ash -- Very fine-grained fragments (< 2 mm), generally dominated by broken glass shards, but with variable amounts of broken crystal and lithic (rock) fragments.
Pyroclasts Lapilli -- Pea (cinders)- to walnut-size pyroclasts (2 to 64 mm). In water-rich eruptions, the accretion of wet ash may form rounded spheres known as accretionary lapilli.
Pyroclasts Blocks and Bombs -- Fragments >64 mm. Bombs are ejected as incandescent lava fragments which were semi-molten when airborne, thus inheriting streamlined, aerodynamic shapes. Blocks are ejected as solid fragments with angular shapes.
Pyroclastic textures Within this size classification, specific types of pyroclasts can be further defined by physical attributes A little controversial
Pyroclastic textures Lapilli-size fragments of basaltic lava may cool quickly while airborne, to form glassy teardrop-shaped lapilli called Pele's tears. Pele's tearsPele's tears During strong winds (in fire fountains), these molten fragments can be drawn out into fine filaments called Pele's hair. Pele's hairPele's hair (Pele is the Hawaiian fire goddess of volcanoes).
Pyroclastic textures Non-explosive eruptions often produce lapilli- to bomb-size fragments, called spatter which remain airborne for only a short amount of time so that are still liquid when they hit the ground surface. spatter
Pyroclastic textures Vigorous gas escape in felsic lavas produces pumice, a very light colored, frothy volcanic rock texture made of glass. pumice The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes. Pumice is so light that it actually floats on water.
Pyroclastic textures Similar gas escape in mafic lavas produces reticulite. reticulite Reticulite has a lower density than pumice, with vesicles occupying up to 98% of the total volume. Unlike pumice, however, most of the bubble walls in reticulite are broken so that it sinks in the presence of water.
Pyroclastic textures Reticulite, however, is not as common as scoria, a denser mafic rock texture containing a smaller abundance of relatively large vesicles. scoria Brittle Vesicularity: 70-85% Crystallinity: 2-50%
Pyroclastic Rocks Classification of the pyroclastic rocks. a. Based on type of material. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.
Pyroclastic deposits Pyroclastic FALL deposits Relate to magma chamber geometry, discharge rate etc. Derived from Plinian eruptions where they fall out of the eruption plume The deposit is composed of highly vesiculated dacitic to rhyolitic pyroclasts which can be distributed for hundreds of square kilometers away from the vent. Larger bombs deposited near the vent, smaller ash and lapilli and pumice further away.
Pyroclastic deposits Pyroclastic SURGE deposits Dilute mixture of gas and ash (similar to pyroclastic flows) but are mostly gas. Turbulent suspension results in wavy-, lenticular- or low angle cross bedding. Richer in crystals and lithics than pyroclastic flows and are better sorted.
Pyroclastic deposits Pyroclastic FLOW deposits Nuee Ardents (Peleean activity) -- these contain dense lava fragments derived from the collapse of a growing lava dome or dome flow Non vesiculated block fragments suspended in ash (block and ash flows) Non vesiculated block fragments suspended in ash (block and ash flows) Pumice flows (ignimbrites) -- these contain vesiculated, low-density pumice derived from the collapse of an eruption column. Vesiculated, pumice-dominated with subordinate ash Vesiculated, pumice-dominated with subordinate ash
Pyroclastic deposits Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano. Have steeper slopes than shield volcanoes, with slopes of 6 to 10 degrees low on the flanks to 30 degrees near the top.
Pyroclastic deposits Mount Ngauruhoe, New Zealand 61 eruptions since 1839 Mount Doom, Mordor
Pyroclastic deposits Tephra (or cinder) cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions. They usually consist of basaltic to andesitic material and are actually fall deposits that are built surrounding the eruptive vent. Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose unconsolidated material) and are usually between about 25 and 35 degrees.
Pyroclastic deposits SP Crater, in the San Francisco Volcanic Field
Pyroclastic deposits Volcanic domes are masses of solid rock that are formed when viscous lava is erupted slowly from a vent. Most domes are short-lived features, because they are commonly destroyed by collapse partly due to volcanic explosions and due to strains set during cooling. The speed of dome growth varies considerably, but some rise by as much as 25 m/day.
Pyroclastic deposits Mt St. Helens
Pyroclastic deposits Plinian fall deposit (1m) Lithic clast Ignimbrite (10-100m) Pumice clast Ash fall deposit Lava flow
Eruption products Basaltic lava flows and deposits Andesitic flows Rhyolitic pyroclasts, pyroclastic rocks and pyroclastic deposits Used to interpret volcanic history of a region.
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Hornito structure – Etna Sept. 2004 Wanna visit Italian volcanoes? Send me an e-mail for advice: email@example.com