Download Sample material for Geography Test I

Sample material for Geography Test I
Volcanism is the set of geographical process resulting in the expulsion of molten
rock, as lava, at or near the Earth’s surface. The most notable product of volcanism
are volcanoes, the landforms created when lava and hot particles escape from the
Earth’s interior through openings or vent in the Earth’s surface and then cool and
solidify around the vents. As such volcanicity relates itself with both the exogenic
and endogenic environments. I include all the processes and mechanism that relates
them to the origin of magma, gases, vapour, their ascent and ultimate development
of various plutonic and surface features.
Exogenic volcanicity
The exogenic volcanicity includes the wide range of
typology.
The periodicity of occurrence has been taken as the
basis to classify volcanoes
Dormant volcano
It is the volcano that has not erupted recently but is
considered likely to do so in the future. The presence of
relatively fresh (within 1000yrs old) volcanic rocks in a
volcano suggests its capability of repeat performance.
Mt. Baker near Washington-British Columbia border is
the prominent example.
Volcanism
Exogenic
Fissure type
Central type
Hot springs
Solftara
Geyesers
Mud flow
Endogenic
Batholith
Laccolith
Lappolith
Phacolith
Duke
Sill
Extinct volcano
It is the volcano that has not erupted for a very long time and is considered unlikely
to do so in future. One indication is the extensive erosion that erodes the core since
the last eruption. A true extinct volcano is no longer fueled by a magma source.
Emperor seamount chain is the example.
Volcanic eruptions range from the quiet oozing of molten basalt from Kilavea
volcano in Hawaiian island to the catadysmic explosion of Krakatoa in Indonesia.
The quiet fissure type
This type of volcano occurs along fracture, fault where slow upwelling results in lava
spread over ground. The lava flow depends on the nature of Magma. The entire
prominent volcanic plateau excels the example of large territorial coverage by
Magma flow. Deccan plateau excel the example of large territorial coverage by
Magma flow. Exaples are Deccan plateau India, Colorado plateau (USA) Brazilian
shield, Abassinian Plateau (Ethiopia) Arabia plateau (S. Arabia) etc.
Geysers are the hot springs from which column of hot water and steam is
explosively discharged at intervals.
Spring Marks continuous discharge of the same.
These are prominently identified with Iceland (Geyser) New Zealand and USA(Yellow
stone National Park).
Solftara fumarole is jet of gases issuing from a small vent. Gas temperatures in
fumaroles are extremely high (up to 320 0C) Most of the gas (99%) is water. Thus
solftara is emitted superheated steam.
Volcanic mudflow/Lahar is the water mix down-slope
movement of pyroclastic material. Lahar may contain a
range of particle sizes from finest ash to enormous
boulders. It is often produces on snowcapped volcano
and hot pyroclast melts large volume of snow or ice.
Columbian Lahar along Andean slope 1985 is the most
devastating example.
The central type of ejection includes several commonly
referred components of volcano. The mobility of molten rock
material from below surface is via the vent and ejection mouth
crater. The most identified category has been further sub
classified into four on the basis of intensity and frequency of
occurrence.
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The Hawaiian type
The Vesuvian type
The Strombolian type
The Pelean type
The Hawaiian type is referred to sub aerial basaltic lava flow with precise absence
of gaseous constituents. The gaseous constituents being absent, the ejection is
quietest. The movement of basaltic magma down slope as burning strings is named
as Pele’s hair.
The Pahoehoe and AhAha are the two typologies of Magma Movements with
temperature as high as 1175 0C.
Vesuvian/Pilinian is intense type. Lava is viscous and blocks the vent to increase
pressure. Even the core gets shattered in angular fragments. Ash laden cumulus
clouds are common consequence.
Strombolian is moderate intensity ejection. Pumice, bombs magma and gases are
being ejected. These are quiet rhythmic ejections.
Pelean is the most disastrous, linked with most viscous Rhyolithic Magma or
Andestic magma Nuee ardente or glowing clouds develop as quick condensation
develops. Krakatoa (Indonesia) Martinique (Lesser Antilles) are the sites where
such ejections have been experienced.
The intrusive volcanicity
The intrusive volcanicity is identified with highly
viscous magma. The various types of features
that develop due t solidification of Magma include
most of the intrusions carried on by Magma to
the existing cracks or pores.
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Batholith is large Mass of granite. As the hot Magma rises it melts and incorporates
the older rocks lying above it.
Laccolith is dome like mass, which overlaps the sedimentary rocks. Sill sheet of
igneous rocks that beds itself horizontally by the rising magma develops vertical
intrusion.
Lappolith is the saucer like mass.
The ridge and trough development in the bedding plane avails phacolith.
Plate Tectonics and Volacanism
Volcanism is mostly associated with weaker
zones of the Earth’s surface. The plate tectonics
theory explains volcanicity mechanisms almost
completely. The plate boundaries are the weak
zone that marks the activity and determines both
nature and intensity of volcanic eruptions. The
plate boundaries are classified into three broad
groups and volcanicity associates itself with all
three. Most of the fissure ejections links itself
with divergening or constructive plate margins.
The sub-aqueous basaltic ejections result in the constant upwelling and development
of Mid Ridges. The development of quiet fissure type ejection is in obvious reference
linked with absence of pressure, as gaseous constituents are negligible. Iceland,
Hawaiian island and Mid Atlantic ridge are the examples.
On the continental plate divergence the intensity of ejection increases, as clear
passage for gaseous constituent is not there. Along the Great Rift Valley, Mt.
Mulanje, Mt. Rungwe, Mt. Kenya, Mt. Kellimajaro are representative examples.
The divergence of African and Somali plates marks the prevalence of Mechanism.
Converging plate boundary or Destructive are the crudest spread. The subduction
of the oceanic plate avails volcanic activity. Cirrcum-Pacific Belt stretching from West
North American coast to East Asian Archipelago also includes the Erebus Mountain of
Antarctica. Most of the high volcanic core and Mountains are found in this belt. Mt.
Rainear, Mt. Shasta (USA) Mt. Cotapaxi (Ecuador) Mt. Fujishan (Japan) Mt.
Mayon (Philippines) are the prominent examples. The belt converges with East
Indies archipelago as mid continental belt. The entire category of volcanoes of Alpine
Mountain chain and Mediterranean Sea are included. Mt. Vesuvius, Mt. Etna, Mt.
Demavand (Iran) Mt. Poopa (Myanmar) are the prominent cones.