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USGS Geology in the Parks

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Lava flow trail side trip:

Take a closer look at a monster vesicle

Really huge vesicle along the Bonito Lava Flow trail
The surface of this vesicle has broken so that you can see through to the other side.

Walk just around the corner from stop 1 and you'll see a really huge vesicle! While lava is still very hot and fluid, gas bubbles form and work their way toward the lava surface. If you look at an active lava flow you can watch bubbles rise to the surface, then POP! with a spray of molten droplets as gas escapes from the flow.

Often the surface of the lava flow cools and hardens first, forming a crust with still-flowing lava beneath it. In this case, gas rises but is trapped beneath the crusty roof. As more and more bubbles are trapped by the roof, they grow together to form a really big bubble like the one you see here.

Compare this mega-vesicle at Sunset Crater with the one forming just beneath the roof of this lava tube in Hawaii. Even though the lava has long-since hardened at Sunset Crater, you can easily see that the processes forming these two bubbles are very similar.

Step up to take a closer look at the roofs in these photos you'll see something that looks like stalactites found in limestone caves. These solidifies lava drips provide another reminder that the trail you are walking on was once a 1000°C-plus molten lava flow.

Mega-vesicle seen from the Bonito Lava Flow trail.
This image shows the thin, crusty roof above a lava tube. Click here to view Pu`u`O`o, an active cinder cone.
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