01-31-2014, 01:04 PM
quote:
The other finding is that this gigantic magma cavern feeding this continuous flow is pauhoehoe magma, indicating slow oozing for a long time ... unless some kine of blockage or breach occurs.
Magma is magma. There is no "pahoehoe magma," just as there is no a'a magma.
Below is more information regarding the variations in surface lavas.
quote:
http://volcano.oregonstate.edu/what-are-...-they-form
Peterson and Tilling (1980, p. 273) suggested two general conditions that determine whether pahoehoe or aa forms:
If lava slows, cools, and stops in direct response to the corresponding increase in viscosity only, it retains its pahoehoe form.
If lava is forced to continue flowing after a certain critical relationship between viscosity and rate of shear strain is achieved, the lava changes to aa.
Peterson and Tilling called this critical relationship the "transition threshold." They found that if the rate of shear strain is high, the transition threshold is reached at a lower viscosity than if the shear strain rate is low. The converse is also true. If the viscosity of the lava is high, a relatively low rate of shear strain may achieve the transition threshold, and the lava changes to a'a.
People often ask if there is a compositional difference between aa and pahoehoe lava. There is no systematic chemical difference between aa and pahoehoe lava. Lavas with the identical compositions can form both aa and pahoehoe. Lavas that have slight chemical differences tend to have different temperatures and viscosity's but the critical factor influencing the transition from pahoehoe to aa is the viscosity of the lava.
quote:
http://www.geo.cornell.edu/geology/Galap...Types.html
Aa forms when lava flows rapidly. Under these circumstances, there is rapid heat loss and a resulting increase in viscosity. When the solid surface crust is torn by differential flow, the underlying lava is unable to move sufficiently rapidly to heal the tear. Bits of the crust are then tumbled in and coated by still liquid lava, forming the chunks. Sometimes the crust breaks in large plates, forming a platy aa. Pahoehoe forms when lava flows more slowly. Under these circumstances, a well-developed skin can form which inhibits heat loss. When a tear in the skin does form, it is readily healed. Both magma discharge rate and the steepness of the slope over which the lava flows affect the flow rate. Thus aa lavas are associated with high discharge rates and steep slopes while pahoehoe flows are associated with lower discharge rates and gentle slopes. The steep slopes of the large western Galapagos volcanos thus generally consist of aa, making ascent very difficult (and occasionally painful!). The less common pahoehoe flows on these volcanos are erupted from vents on the gently sloping apron or the caldera floor. Flows which begin as pahoehoe can convert to aa when a steep slope is encountered.
quote:
The link below gives a better explanation of the differences in a'a and pahoehoe.
http://hvo.wr.usgs.gov/volcanowatch/arch...05_27.html
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