Lava erupts through fissures on the East Rift Zone of Kilauea. During an eruption lava is ejected upwards forming a curtain of fire. Lava also moves away from the fissure, flowing in the down hill direction. Paho'eho'e flows are shown on the right.
In many places as shown in this photograph, small volcanic cones form along the fissure, built from spattering lava or cinders ejected from vents.
Lava also erupts from fissures at underwater rift zones. The underwater eruptions also build volcanic cones along the fissures. There is an important difference, however, between eruptions in air and underwater: The surface of a lava flow cools much more rapidly underwater than it does in air. Rapid cooling in seawater creates a skin on the surface of the lava flow that insulates the interior of the flow from further heat loss. Pillow lavas are a product of this rapid cooling. The photograph shows lava forming a pillow underwater.
Pillow lavas form only underwater. A pillow forms when lava squeezes out like toothpaste through the thin skin of a flow; the surface of the new pillow quickly solidifies. Pillows are sack- or pillow-shaped in cross section and range up to 1 m (3 ft) in diameter. A cartoon of pillow lavas stacked on top of each other and viewed from the side is shown to the left.
A photograph of pillow basalts on the Puna Ridge is shown here. It was taken by Dan Fornari during a dive in a two-person submarine to the top of the ridge at a depth of about 1800 m (5400 feet).
Since pillow lavas form only underwater, what would you conclude if you saw them on land?
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