Friday, August 5, 2016

The Hawai'i That Was: Exploring Pololu Valley on an Unstable "Dead" Volcano

Do the signs add a hint of an element of danger to this hike?
The stereotypical image of Hawai'i includes many things (most of which are seen in the opening credits of Hawaii Five-O), but one of them is surely the dramatic fluted cliffs clothed in tropical vegetation. It's not hard to understand why, since most tourists visit Oahu, and most of the mountains there are quite steep. The Big Island of Hawai'i is often a surprise, then, because there just aren't that many dramatic sheer cliffs. The island is composed of young shield volcanoes like Mauna Loa, Mauna Kea, or Kilauea, and shields have gentle slopes.
This is most surely not a "gentle" slope.
There is one major exception. The older shield of Kohala on the northernmost tip of the island has a twelve mile long stretch of coastline that is composed of dramatic cliffs and deep valleys. The Pololu Valley is found at the north end, while Waipi'o Valley forms the southern end.The cliffs are rugged enough that no roads penetrate the coastal wilderness. Kohala began erupting around a million years ago, with the latest flows occurring around 120,000 years ago.
Source: U.S. Geological Survey

We would expect older volcanoes to display a great deal more erosion, but that's only part of the story at Kohala. The rest of the volcano has no similar sea cliffs. Something else happened here. New technology for accurate mapping of the sea floor offshore of the Hawaiian Islands has provided a useful clue. There is chaotic debris on the ocean floor at distances reaching as much as a hundred miles. The islands have been falling apart!
It is difficult to imagine the scale of these events. The largest debris avalanche ever witnessed by humans was the one that came off the flank of Mt. St. Helens during the eruption of May 18,1980. It traveled about 12 miles. An avalanche off the side of Mt. Shasta in California traveled about 28 miles, but no one was about to see it happen, seeing as how it is more than 300,000 years old.
When the flanks of the Hawaiian Islands collapsed, the runout was more than 120 miles at Molokai, and 80 miles below the cliffs of the Pololu Coast. The sudden displacement of sea water by the rock generated devastating mega-tsunamis. One such event left chunks of coral reef 1,300 feet above sea level on the island of Lanai.
Don't worry too much about killer tsunamis on your vacation journey. As horrific as they are, they take place at intervals measured in the tens or hundreds of thousands of years, and there are no signs that any are imminent. The Pololu avalanche took place about 120,000 years ago.
A short but steep trail leads from the end of the road to the valley floor several hundred feet below. It travels farther out across the coastal cliffs providing a better view of the beach cliffs along the coast south of Pololu.
The floor of the Pololu Valley is surprisingly flat, and one might wonder that people aren't living and farming here. They once did in large numbers, but the taro fields were destroyed by a series of more "normal" tsunamis in the twentieth century, and the fields have been abandoned. There is a rich archaeological record of farming and housing sites all along the valley.
They collected rocks too. Not for their intrinsic beauty, mind you, but to construct an impressive heiau at Pu'ukoholā on the other side of the volcano, more than twenty miles away. Heiaus are large temple sites composed of hundreds of thousands of boulders that were passed hand-to-hand between thousands of "volunteers" working for King Kamehameha in the late 1700s. The workers could face punishment for dropping a stone and breaking up the rhythm of the line.
The beach is composed of black and gray sand. There is also a system of sand dunes higher up the beach. Dunes are relatively rare on the islands, but wind blew coastal sands into a series of dunes reaching an elevation of 100 feet. Enjoy the pools, beach and dunes, because the hike up is quite steep.
Waipi'o Valley along the Kohala Coast
We weren't able to stop at the valley on the other end of the Kohala Coast, but Waipi'o Valley is very much worth a visit if you are on the Big Island. There are some stunningly high water falls in the upper valley.
Post a Comment