Submarine and Sub Glacial Eruptions | Volcanoes

After reading this article you will learn about the submarine and sub-glacial eruptions from volcano.

Submarine Eruptions – Pillow Lava:

If lava flows into water or is erupted under water a special structure known as pillow lava is commonly formed. The lava chills rapidly to form a glassy but plastic skin around still liquid lava and rolls along in the manner of plastic bags full of liquid.

Soon solidification occurs and the individual more or less round solid blocks of rock settle heaping up one on another. The round or sausage shaped blocks are called pillows. They have rounded tops, but their base fits into the shape of the underlying surface.

They have the characteristic feature with the glassy, tachylitic skin and radial cracks. Most pillow lava is formed in the sea, but some may be formed in fresh water. The pillows are commonly 1 m to 1.25 m in diameter. Occasionally, however, flattened pillows may be as much as 4 m long.


In most pillow lavas, the interspaces are filled with fragments spalled off from the pillows or with sediments of the sea floor. The spaces may also be filled by infiltration of sea-bottom siliceous or calcareous ooze or other sediment of marine origin.

Sub-Glacial Eruptions:

When a sub-glacial volcano erupts, the surrounding ice melts forming large quantity of water. If the heat is insufficient to melt the entire thickness of ice present above the volcano, the water trapped will sweep out along the base and the glacier surface will collapse into large sinks, often several kilometres in diameter.

If the whole ice cap around the volcano melts, a lake is formed. In time the enormous accumulation of water will break through the ice dam confining it and consequently, water and icebergs in unbelievably huge volume will sweep over the country side destroying everything in their path. These floods are known as Jokulhlaups.


These floods are extremely violent. The maximum discharge of water during the eruptions of volcano Grimsvotn in Vatnajokull in southern Iceland in 1934 and again in 1938, each of which lasted for about one week was about 50,000 cum/sec (less powerful eruptions have taken place in later years).

In 1918, the Katla Volcano in Myrdalsjokull also in Southern Iceland in an eruption which lasted only two days discharged 200,000 cum/sec. Very huge quantity of mud, stones, sand and debris were carried by the floods and washed into the sea. The whole contour of the southern coastal section of Iceland has been altered by these deposits.

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