It consisted of golf ball to soccer ball sized pieces that covered an area of the ocean the size of Philadelphia. The arrangement of the pumice boulders on the seafloor, sometimes precariously stacked four blocks high, suggested that these pieces were also at first buoyant but then gently sank as their pores became saturated with water.
Other gas poor dense lavas flowed from the vents similar to lava that slowly flows out of the volcanoes of Hawaii. How is it that these different lava types form? The magma contains dissolved gases, which form bubbles as the pressure on the magma is reduced during its ascent. An explosive eruption occurs on land when these dissolved gases are released suddenly—think of the bubbles in a coke bottle spurting out when a shaken bottle is opened and the pressure is released all at once.
But underwater the magma still faces the crushing pressure of tons and tons of ocean water once it reaches the seafloor. The Havre volcano, stretching between 3, and 4, feet below sea level, experiences a pressure between 92 and times that of sea level, which scientists suspect dampened its explosiveness and shaped the various types of lava formations.
Technical Announcements. Employees in the News. Emergency Management. Survey Manual. The steam plumes that usually rise from these ocean-entry points do not surprise visitors; it seems reasonable that incandescent lavaat a temperature of about 1, degrees Celsius 2, degrees F would cause seawater to boil.
Sometimes, however, streams of lava pour into the sea with little or no steam, as if the lava were not hot at all. How can this be? In order for a volume of seawater to boil, lava must heat it to at least Obviously, there's a lot more water than incandescent lava, so if the heat is transferred slowly from lava to seawater, heated water mixes with cooler water, and the boiling point is not reached.
Circumstances that enhance the rate at which heat is transferred to near shore water will favor steam formation.
This melted rock, or "magma", may collect in a reservoir a few kilometers below the seafloor, awaiting eruption. Much of the magma eventually freezes in place there within the crust, forming the bulk of the new oceanic crust without erupting at all.
Average oceanic crust is about 10km thick, but only the upper 1 to 3 km are formed by eruption processes. When magma pressure builds up enough to force its way out to the seafloor, eruption occurs. Microscopic sample of rock in a high-pressure lab; it shows evidence of magma formation. Geologist Rajdeep Dasgupta uses hydraulic presses to crush rocks at high pressures.
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