Hawaiian Volcanoes Field Course 2010

By Erica Mitchell, Megan Young, and Jorge Perez 

Day 3 proved to be very eventful – we visited the Lua Manu lava field, which had a plethora of shelly pahoehoe lava flows, tree castings, and lava trees.  After that stop, we continued on our way to the Hilina Pali cliffs.  From the overlook we could see the steep cliffs lining the south coast of Big Island that were formed by the processes of Kilauea crater's growth.  Lastly, we viewed some cross-sections of Pahoehoe and A'a flows, and did a fault mapping exercise.

Lua Manu Lava Field

After returning from the farmer's market and eating delicious local fruits, we visited the Lua Manu lava field, where we saw great examples of shelly pahoehoe.  We had to use leather gloves and take care not to fall on or through the rough, sharp terrain.  This kind of pahoehoe flow is sheetlike, with a shiny, ropy exterior and sometimes an open cavity underneath.  A peculiar feature of this particular lava flow are the deep cyllindrical holes. They are tree castings, which are created when lava flows around a living tree.  These tree castings differ from those seen on Day 2 because they are caused by lava flow, not cow patties and tephra expelled by a fire fountain.  When these trees are surrounded by active lava, the lava touching the tree quenches (cools and solidifies) because of the high thermal conductivity of the water in the tree.  If the trees are not completely burnt out then the hole is left with charcoal, but if it burns completely, then solely a hole is left.  The absence of vesicles in the lava
and the shiny texture show that the lava was cooled too quickly for bubbles to coalesce.

Another feature highly prevalent in these fields were lava trees.  These are similar to tree castings, but they protrude from the landscape, most of them being about 2.5m tall.  They were formed when flow engulfed the trees, quenched, then subsided. The mushrooming and flaring out at the top of these trees show where the cooled top layer stuck to the top of the tree.  An interesting feature of these trees that can form is a bridge. (photo)  This bridge was formed when the top of the flow was cooled and the trees were close enough to support the cooled flow without it collapsing.  The agglutinate on the top of the bridge support this hypothesis because it was exposed to the air, and the drips on the ceiling of the bridge were formed when the top layer of the lava cooled and the bottom layer was still very hot, so the lava dripped down because of the thermal instability of this boundary.  The flows we observed flowed down the slope gradient into a crater and created a lava lake.

Hilina Pali

Our next stop was an overlook with a view of Hilina Pali, a set of steep cliffs facing the ocean.  The cliffs were created by the growth of the Kilauea crater.  Magma growth under Kilauea caused it to grow radially, but it could not move landward because massive Mauna Loa served as a buttress.  Kilauea's growth was limited to the south side of the crater, which caused intrusions of magma and rifting, or crustal thinning and faulting.  As the cliffs were pushed southward, they became over-steepened and collapsed into the ocean.  This process continues to build the Hilina Pali we see today.

Lava Flow Cross Section

 Our third stop was next to a highway where we could observe cross sections of lava flows. First we looked at the top of the flow, where we saw both Pahoehoe and A'a flow. The pahoehoe flows can be described as ropey, lobe-like, and sometimes smooth, while the A'a flows are chunky, jumbled up, and rough.  This difference is attributed to the fact that pahoehoe flow is less viscous and hotter than a'a flow.  Pahoehoe flow can  sometimes turn in to an a'a flow when its viscosity increases and the 'skin' on top can break and end up not flowing easily.  A'a flow is often characterized by a breccia layer on the outside
and a hotter core on the inside.  Its movement is quite rotational and tends to overtake jumbly rocks as it advances.  However, this intricate top layer is only a small fraction of the total lava flow.

As lava flows it cools and hardens into a shell at the top where it touches air and the bottom where it touches the ground.  This forms a protective insulating layer that keeps the middle of the flow warm, fluid, and bubbly.  As the flow is supplied with new lava, it squeezes between the crusts, inflates the whole layer, and flows downhill.  Sometimes, the lava drains out and leaves gaping holes with drips on the ceiling.  At this stop, it is possible to see multiple flows layered on top of each other.

Rifting and Mapping

 At the end of the day we were sent on a kind of scavenger hunt.  We broke into teams and walked transects along one of the main faults due to the rifting from Kilauea.  We found deep cracks cutting across an old lava flow.  We measured the location of the cracks with a handheld GPS, their direction with a Brunton compass, and their width with a tape measure.  Then, we plotted them on Google Earth.