On To Day 11Back To Day 9Day 10. Cones and Craters

October 5, 1998. Cones, craters, rivers of lava pouring from the top and streaming down. For most people, that is the essence of volcanism. So imagine our excitement when the sonar images revealed these very features on Puna Ridge.

Learn how to interpret side-scan sonar images.

Most of what we know about how cones and craters form comes from studying subaerial volcanoes (volcanoes on land). The sonar images are now providing evidence that these same processes may apply to submarine (underwater) volcanoes as well.

Lava channelLet’s look at how we think cones and craters like Pu’u’O’o (the site of the current eruption) form. Magma rising from the mantle, pools underneath the summit of Kilauea, and creates a magma reservoir. As the pressure builds, some of the magma moves laterally along the rift zone. These lateral extensions of magma are called dikes. We think that these dikes reach all the way to the end of Puna Ridge.

At different points along the rift zone, magma rises from the dike and erupts through the surface. If the eruption goes on long enough, layer after layer of lava builds around the eruption site. The result is the familiar cone.

CraterNow think about what would happen if the supply of magma rising up into the cone stops. Now the inside of the cone is hollow. Without the support of the magma, the top of the cone collapses and forms a crater. It’s as if you removed the filling from a pie. Without the filling to prop it up, the crust collapses.

The sonar images show what clearly appear to be numerous cones and craters that formed from past eruptions along the top of the ridge. The sonar images have also revealed other features that may be remnants of past eruptions. Lava pouring down the side of volcano creates channels. Through a series of complex processes, the outer most layers of lava in these channels hardens, creating a lava tube. The lava from the eruption flows through these lava tubes. These tubes can continue until the slope of the volcano suddenly flattens out. At that point, the tubes may break and lava pours out in all directions. This is called a secondary eruption.

Sonar image of Puna Ridge
Sonar image of a crater on Puna Ridge.

As the lava spreads out, it creates a terrace or table. It may help to consider what happens were you to angle a hose towards your driveway when the temperature was well below zero. The water would spread out along the driveway and freeze into a thick layer of ice.

One idea is that lava bursts out from the base of the table and forms a new table right below it. This process repeats itself, creating a series of steps. As lava drains out from beneath the table, the top of the table collapses and forms a crater.

Staircase cartoon
Lava flows from beneath tables and creates a series of steps.

The sonar images show structures that may be lava channels. On the south side of the ridge, an area that the sonar has not reached, the bathymetric maps show possible lava tables and craters. Once we have finished the sonar mapping, we will send ARGO II down to photograph these features and learn their true identities.

In fact, Debbie Smith wants to send ARGO II inside the craters. It should be exciting!


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