Types of Volcanic Flows Erupted During Quiet
Eruptions:
Pillow lavas formed underwater have a variety of shapes. They could be
bulbous, or tubular and are often broken by small ‘‘Yam shape’’ protrusions.
Pillows show radial jointing and polygonal shaped cooling cracks. The
bulbous pillows are sub-spherical and occur on flattish surfaces; vertical
pipes channeling lava feed them and they grow upwards and expand laterally on top of the feeding tubes. Elongated tubular pillows are sub-cylindrical in shape because their lava has flowed down-slope. Trapdoor pillows
are formed on top of another pillow during the extrusion of lava from a
circular crack. They form a cylindrical cap on top of the original pillow. A
‘‘Haystack’’ is a volcanic cone with radial draped elongated pillow lava
tubes on its sides (Fig. 5.9). Lobated flows are flattened lobes with respect to
pillows and their interior shows flat ledges (\10 cm thick) representing
cooling levels. Such flows fill depressions and fissures. They are formed by
the rapid growth and drainage of extruded lava (see also lava pond). Flat
flows, also called sheet flows, are like lobes without any inflation and they
can be ropy, jumbled, and hackly. They are formed during the rapid extrusion of fluid lava. Sheet flows are thin, less than 30 cm thick, and they display
different shapes according to the landscape encountered. Jumbled flows are
a type of sheet flow with broken up and re-cemented glassy crusts. Ropey
flows are formed during lava extrusion in a semi-circular and confined
pathway therefore the melt laps over itself and cools in ‘‘coiled rope’’
shapes. Blocky and tabular flows consist essentially of silica-enriched lavas
that have a surface morphology and texture that is different from basaltic
pillow. Depending on their composition, they are blocky (a silica-rich dacite
flow) and/or tubular with ‘‘corkscrew’’ shaped tubes and flattened lobes.
They can be found on spreading ridges as well as in ocean basins. Their
surface morphology is characterized by a spiny and rughose thicker glassy
crust (3–20 cm thick) than what is found on pillow lava or sheet flows of
basaltic composition. The interior of these silica-rich flows is characterized
by the presence of large cavities, (up to 8 cm in diameter). They are formed
during the eruption of viscous lava, which expanded laterally and formed a
thicker chilled crust due to the lower cooling rate than that of more fluidal
basaltic lava. Scoria-like (Clinker) volcanic structure refers to massive or
fragmented debris, which have been re-cemented during the advance of a
new lava flow. They are similar to the typical ‘‘aa’’ flow found in Hawaii.
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5 Earth’s Mantle Melting and Volcanism
Eruptions:
Pillow lavas formed underwater have a variety of shapes. They could be
bulbous, or tubular and are often broken by small ‘‘Yam shape’’ protrusions.
Pillows show radial jointing and polygonal shaped cooling cracks. The
bulbous pillows are sub-spherical and occur on flattish surfaces; vertical
pipes channeling lava feed them and they grow upwards and expand laterally on top of the feeding tubes. Elongated tubular pillows are sub-cylindrical in shape because their lava has flowed down-slope. Trapdoor pillows
are formed on top of another pillow during the extrusion of lava from a
circular crack. They form a cylindrical cap on top of the original pillow. A
‘‘Haystack’’ is a volcanic cone with radial draped elongated pillow lava
tubes on its sides (Fig. 5.9). Lobated flows are flattened lobes with respect to
pillows and their interior shows flat ledges (\10 cm thick) representing
cooling levels. Such flows fill depressions and fissures. They are formed by
the rapid growth and drainage of extruded lava (see also lava pond). Flat
flows, also called sheet flows, are like lobes without any inflation and they
can be ropy, jumbled, and hackly. They are formed during the rapid extrusion of fluid lava. Sheet flows are thin, less than 30 cm thick, and they display
different shapes according to the landscape encountered. Jumbled flows are
a type of sheet flow with broken up and re-cemented glassy crusts. Ropey
flows are formed during lava extrusion in a semi-circular and confined
pathway therefore the melt laps over itself and cools in ‘‘coiled rope’’
shapes. Blocky and tabular flows consist essentially of silica-enriched lavas
that have a surface morphology and texture that is different from basaltic
pillow. Depending on their composition, they are blocky (a silica-rich dacite
flow) and/or tubular with ‘‘corkscrew’’ shaped tubes and flattened lobes.
They can be found on spreading ridges as well as in ocean basins. Their
surface morphology is characterized by a spiny and rughose thicker glassy
crust (3–20 cm thick) than what is found on pillow lava or sheet flows of
basaltic composition. The interior of these silica-rich flows is characterized
by the presence of large cavities, (up to 8 cm in diameter). They are formed
during the eruption of viscous lava, which expanded laterally and formed a
thicker chilled crust due to the lower cooling rate than that of more fluidal
basaltic lava. Scoria-like (Clinker) volcanic structure refers to massive or
fragmented debris, which have been re-cemented during the advance of a
new lava flow. They are similar to the typical ‘‘aa’’ flow found in Hawaii.
118
5 Earth’s Mantle Melting and Volcanism