Structures of Hawaiian Lava Flows
by Jill Ralston and Kyle Raverty
Molten Lava Entering the Sea - Lava is the term for molten rock at the Earth's surface (molten rock beneath the Earth's surface is magma). Lava temperatures may reach more than 1400 degrees C (about 2500 degrees F), but most Hawaiian lavas are between 1,000 and 1,100 degrees C.
A'a refers to the rough, blocky, clinkery surface of some lava flows. Some people think it is named for the cries of persons attempting to walk barfoot across its jagged surface. Ouch!
Pahoehoe refers to the smooth, ropy, billowy surface of some lava flows. These flows are formed from relatively fluid lava which is at higher temperatures and has more gas dissolve in it than in a'a flows.
Lava flows behave in many ways like flowing water - they seek the lowest elevation, driven by the force of gravity. Here we see pahoehoe lava which has flowed down a crevice in an earlier flow.
Columnar joints form when a lava flow stagnates and cools. This may happen in a lava lake, and can also happen when shallow injections of magma occur near the earth's surface. The cooling lava shrinks or contracts and forms cracks perpendicular to the cooling surface. These cracks are called joints, and they form columns that are often six-sided.
Lava Trees form when pahoehoe flows around trees. The lava backs up on the up-flow side of the tree, and flows around the tree, forming a lower surface on the down-flow side. This phenomenon results in a directional indicator that may permit one to determine the path of flow of the lava.
As the flow builds up, the tree dies, and may even be consumed by fire inside the lava tree, which is left standing as a stark reminder to the devastaion caused by volcanism.