June 7, 2001
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Diving deeply into Kilauea's early history
Kilauea wasn't always like it is today. Two hundred thousand years and the change from a seamount to an island can do a lot to a volcano. The trick is to determine what those changes were.
You can't do it on land. No erosion has bitten deeply enough into Kilauea to expose its pre-island days. You have to look in deep bore holes or on the sea floor to see back to Kilauea's early history. Even then it isn't easy.
Recently a breakthrough was made. For the first time, the deep sea floor south of Kilauea and east of the submarine volcano, Lo`ihi, was explored and sampled by submersibles. The dives, some manned and some with a remotely operated vehicle, took place in 1998 and 1999, supported by the Japan Marine Science and Technology Center (JAMSTEC). The sea-floor observations and samples have been analyzed and interpreted in a startling scientific paper recently published by Pete Lipman (former HVO staff member), his USGS colleague Tom Sisson, and two Japanese scientists, Tadahide Ui and Jiro Naka in the December issue of the research journal, Geology.
The deepest dive went 5 km (3 miles) down to the old sea floor on which the island is built. Most of the dives were along a steep scarp or underwater pali 20-40 km (12-24 miles) offshore. The scarp faces away from the island at depths between about 3 km and 4.5 km (1.8 and 2.7 miles). Before the dives, the scarp was hypothesized to contain lava flows erupted from Mauna Loa and Kilauea, perhaps carried away from the island by a large underwater slump.
Not true! Every rock sampled from the scarp, and all views of the scarp, show that it consists of sandstone and broken rocks, not lava flows! Imagine a pali 1,500 m (5,000 feet) high in Hawai`i consisting only of sand and broken rocks. Nothing like that exists above sea level anywhere in the islands.
What's going on?
That's what Lipman and colleagues wondered. So, they determined the chemical compositions of the sand and broken rocks and found many of them to be unlike anything found above sea level on either Kilauea or Mauna Loa. Many of the samples are exceptionally rich in sodium and potassium (alkalies). One of the rocks is even so alkalic that it contains the mineral mica. Some of the chemical compositions resemble those found on Lo`ihi, but the range of compositions is much greater. The data provide solid evidence that Kilauea started out by erupting diverse alkalic rocks.
They also found evidence for a range of eruption depths by looking at the amount of sulfur in glassy samples. High amounts (more than 800 ppm) indicate eruption underwater; low amounts mean that the sulfur boiled off and suggest very shallow or on-land eruptions.
Putting all this together, Lipman and colleagues suggest that early Kilauea was a large submarine volcano, at times growing to or above sea level. It was made of alkalic lava flows and was subject to numerous slope failures, such as landslides. The slides broke up the lava flows and, with marine currents, carried the debris onto the flanks of the volcano and beyond.
Eventually the compositions changed to the more common basalt (tholeiite) found today, and pillowed basalt flows encroached on, and covered, the flanks of the early volcano. Such flows occur on a large bench between the top of the scarp and the island. The old alkalic volcano can be inferred today only from its eroded debris exposed in the scarp; nearer the island, it is covered completely by the tholeiitic flows.
The early history of Kilauea starts to come into focus. For greater clarity, though, it needs a lot more sampling, geophysical profiling, and thinking. But the general picture is there, thanks to the JAMSTEC dives and the work by Lipman and colleagues.
Eruptive activity of Kilauea Volcano persisted at the Pu`u `O`o vent during the past week. Lava moves away from the vent area toward the ocean in a network of tubes and descends Pulama pali in two separate areas. The eastern flow near the Royal Gardens subdivision continues to be active and supplies the ocean entry east of Kupapa`u. Another flow travels down the pali about 1.5 km (0.9 mi) to the west of the boundary of Hawai`i Volcanoes National Park. Lava from this western flow is now ponding in the coastal flats and not going into the ocean. The ocean entry observed west of Kamokuna last week stopped during the weekend.
One earthquake was reported felt during the week ending on June 7. A resident of Hawaiian Ocean View Estates subdivision felt an earthquake at 8:16 p.m. on June 1. The magnitude-3.8 earthquake was located 6 km (3.6 mi) northeast of Pahala at a depth of 10.95 km (6.6 mi).
Updated: June 12, 2001 (pnf)