Day 10:  September 15, 2004

Reporting:

Anna Wallace, Shannon Webber, Karen Weitemeyer

After about an hour and a half hike along the coast, we reached a tephra cone, which had eroded to form a bay with green sand. Here we did a stratigraphy exercise with our groups to describe the units, the layering, bedding, proportions, and composition. An hour later we reconvened to discuss our findings. As a class we discussed the units starting at the base.
 
UNIT 1: Tuff Breccia: This is a heterogeneous aphyric, plagiopheric, and reddened unit. It is at least 3 m thick and contains 2-20cm blocks imbedded in the breccia. It is more resistant to weathering and has a gradational change into the next unit.

UNIT 2: Vitric Lapilli Tuff: Grey thinly bedded, parallel and finely laminated layers, with lapilli tuffs. The pyroclasts contain and abundant amount of olivine crystals. This unit shows bomb sags and contains sorting much like the Keanakako’i ash, a poorly sorted ash with lapilli and bombs. On the eastern side of the cone there is a coarsening of grains. The top of this unit contains accretionary lapilli.
 
UNIT 3: A’a Basalt, showing the textbook marking of an a’a then a massive section and then an a’a above, and thin pahouhou. Contained olivine phenocrysts.
 
UNIT 4: Pahala Ash: orange color and very reworked and well sorted appears to be a massive ash. In depressions the ash layer is thick and in highs it is thinner. The upper portion of this layer becomes a chocolate color, which contains root casts as well as some sparse vegetation. Pahala Ash is also seen directly on top of Unit 2 due to A’a lava flow not reaching the top of Unit 2.
 
We interpreted this stratigraphy to denote a tuff cone caused by a vent on the eastern side of the bay. As a result of water, magma and hot rock interaction. The Carbon-14 age at the base of the a’a basalt is dated at 28,000 years BP, while the Pahala Ash above is dated to 10,000 –20,000 years BP. Therefore the tuff cone has to be older then 28,000 years BP. We notice the trade winds have significantly influenced the deposition of sediments from the cone, as there is an obvious thickening of the pyroclastic unit to the South West. The cone is old and no longer active it is sinking into the ocean (has already subsided ~100m), which is causing the erosion of the cone and the green sand beach. This is where we spent an hour and a half having lunch and enjoying the beach.
 
Return to Vans at Southern Point @ 14:31:00 L
Brief Stop in Pahalau’u for Ice cream and Drinks. @ 15:00

We hit the trailhead at 16:00 and went for a 20 minute walking through and a’a and then dropping down about 10m into a pahouhou and mixed Keanakako’i Ash to the footprint display. Here we learned about the 1790 Kilauea’s summit explosion, whose toxic gases reportedly killed 80 men, women, and children. These unfortunate souls were crossing the Ka’u desert with the Keoua chief when they were killed. The explosion also filled the sky with ash and debris, where they mixed with moisture. The trade winds carried this pyroclastic density current across the Ka’u desert, and it they coated the land. The victims, their discoverers, and later travelers left footprints in the moist, accetionary lapilli rich bed, which later solidified to preserved their footprints.

Here we discovered, away from the trail, a beautifully preserved set of footprints in the accretionary lapilli.