Lava on land runs in sheets and ropes. On the seafloor it arrives as bulbous, spherical or tubular lobes, stacked into mounds, and the shape is consistent enough to have its own name. The difference is not the lava. It is how fast it leaves the vent.
Slowly, which gives the crust time to close. Pillow lavas form during eruptions with relatively low , where slow extrusion lets a thick crust form on all sides of a lobe and keeps individual pillows from coalescing into a sheet. Inside, they are fed through a of interconnected channels.
Pillow lavas are bulbous, spherical or tubular lobes of lava, and they form during eruptions with relatively low effusion rates. Slow extrusion gives enough time for a thick crust to form on all sides of a pillow lobe, which prevents individual pillows from coalescing into a sheet.
Inside, they are plumbed. The pillows are fed via a distributary system of interconnected channels, and they are not typically hollow, tending to solidify all the way through. Their sides often carry or scrape marks formed during extrusion.
Flows are built by accumulation. As a pillow flow forms, the newest pillows erupt from the top of the stack and travel outward a limited distance before freezing. That tends to produce steep-sided mounds or ridges, which can grow to tens of meters thick.
The field relationships show where this happens. Within the 1998 eruption area at Axial Volcano, pillow lavas occur only along the edges of the flow, where lobate lava fronts were thin and stalled out. Castle Vent sits on an older that formed before that eruption.
wat. Slow extrusion lets each lobe skin over, so the flow stacks into pillows instead of spreading as a sheet.
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The shape of a cooled lava lobe records the rate it was erupted at, so the seafloor is a record of eruption style and not only of eruption.
How do we know?
NOAA’s Pacific Marine Environmental Laboratory sets out the formation, the internal plumbing and the field relationships observed at Axial Volcano, including where pillows occur within the 1998 flow.
What is still unknown
This is a reference description of a landform and the observations behind it, not a measurement study of any one eruption.
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Where to next
How much of the deep seafloor can make its own light?seafloor