INTERPLAY OF GEOLOGY AND ARCHAEOLOGY ON HAWAII
In 2016 a treasure trove of ancient Hawaiian petroglyphs carved into solid rock emerged from the sand on the island of Oahu in the Hawaiian chain ( See “Centuries old Hawaiian Petroglyphs Emerge….”, by S. Kuta, in: Smithsonian Magazine August 1, 2025).
Oahu is the “fourth” island north of the “big island” or Hawaii, in the archipelago of that name. Hawaii archipelago (or the state of Hawaii) is comprised six major islands. It was on Oahu in 2016 that a tourist visiting the western shore of the island near Waianae Beach, Pokai Bay, and the Pilila’au Army Recreation Center noticed a series partly exposed petroglyphs carved into what is the island’s underlying hard volcanic rock (probably extrusive igneous rock: basalt). The timing of the visit, after a relatively strong storm, was fortunate since the unusually powerful storm waves had washed away beach sand which had previously covered the feature.
It is theorized that perhaps 500 or more years ago the indigenous Polynesian peoples of Hawaii who settled on the islands about 1000 AD must have carved the geometric patterns, and stick figures of humans into the ancient volcanic basalt flows of the island of Oahu. After the 2016 discovery and report the carvings were soon recovered by beach sand and have remained lost since then. Only to reappear again in 2025 as a result of larger than usual storm waves.
Ocean waves can both deposit and erode beach sand. There is a seasonality to beach dynamics and processes. Powerful waves (often generated in the winter months) erode beach sand and tend to carry it offshore where it is deposited as features called “sand bars” which grow larger and higher during these periods. Seasonal changes, smaller lower summer waves, alter the energy of the waves and these weaker less erosive waves tend to erode sand from the bars off-shore and redeposit deposit them onto the beach. Waianae Beach was affected by these processes.
The entire carved stone panel (according to Smithsonian Magazine 2025) is about 115 feet long, and eight feet wide. It includes 26 distinct petroglyphs, 18 of which are human figures (some which include hands and fingers). Some figures cover the full width of the panel. Native Hawaiians suggest that the panel may symbolize the rising and setting of the sun. The location where the glyphs occur, on the western shore of the island, would have provided ancient observers (as well as those today) with an excellent view of the setting sun.
What is left out about this interesting story is the question of why has this 500 year old panel of carved glyphs appeared only now? Its location on a beach seems an unlikely place for such a feature which no doubt its creators wished for it to be seen and perhaps venerated.
It is unlikely that the ancients would have waited for storm waves to expose the formerly covered rock to give them opportunity to laboriously carve patterns into hard stone. The suggestion that global warming is the cause of rising sea levels of course is on the tips of everyone’s tongue..but this story is more interesting than the old, rising sea level as a result of global warming —now becoming a shibboleth. Though sea level is rising as a result of global warming—it is not the most interesting part of this Hawaii exposition
Except for the big island of Hawaii, all the other islands of the archipelago are all sinking.
The reason for this that the massive plume of magma which had been rising from the deep earth within the mantle for millions of years pushes up the sea floor or “swells” the sea floor upward ( this is termed “hotspot uplift”). The flow of this hot dense material upward causing the so-called “Hawaiian Swell” to rise almost a mile (actually 0.9 miles or 1.5 km) upward. The swelling occurs over an 800 mile wide elongate area which trends to the NW and which extends about (2,700 km) @1,700 miles in that direction. The Hawaiian chain of islands form over the hotspot, rise upward over the Hawaiian Swell, then continue on their northwesterly course as they sink back down eventually sinking to below sea level becoming under water sea mounts rather than above sea level islands.
The hot magma reaching the Earth’s crust exits as fluid lava, spreads to form volcanoes such as Mona Loa and Mona Kea on Hawaii. Mona Kea rises on the big island of Hawaii to almost 14,000 feet above sea level. But from its actual base on the ocean floor Mona Kea rises to @33,500 feet. If the sea were to evaporate Mona Kea would stand as a massive shield-cone volcano higher than Mt Everest at 29,000 ft.
This swelling of the Earth crust as it rides over the Hawaiian magma hot spot plume produces the volcanic flows which formed the Hawaiian islands. But, while the magma plume is stationary— the Earth’s crust is not. The crust is moving in a generally northwest (actually more NNW) direction at a rate of about 9.7cm per year toward the Aleutian Islands.
As the crust rides over the Hawaiian Swell it rises up almost a mile (see above) and then slowly sinks down to its original level over a distance of about 1,700 miles.
Since Oahu is about 200 miles from the high point of the Swell below Hawaii (Big Island) or @ 200/1700 or about 12% of the distance away from the maximum 1 mile uplift. Oahu is sinking and was much higher above sea level than it is now. The petroglyph site must have been located on the slope of a hill overlooking the sea…not at sea level when it was created.
Modern measurements indicate Oahu is sinking at a rate of about an inch a year (2.54 cm/yr) in some places. Over 500 years it may have subsided about 500 inches, (or 500/12=42) or about 42 feet!
If it was indeed 500 years ago that the glyphs were carved, based on the rate of known subsidence and movement of the crust, Oahu’s sea level has risen considerably over the half a millennium the glyphs are claimed to be in existence.
Based on the slope of the sea bottom (unknown at this writing) at Pokai Bay beach such a change would engender very significant alterations in environment and in terrane. An earlier Oahu’s sea shore and high tide line was much father off shore. As subsidence occurred rising surf and storm surges could have eroded headlands to produce large amounts of sand and weathered rock. These would be transported by long shore currents and would tend to cover the glyphs that were in more recent times close to sea level.
The ancient Hawaiians who created these features were acting nor on a surface of rock that was alternately covered and exposed by wave action, not on a sandy beach, but on high ground well inland. It is the fact that the island of Oahu is subsiding, and sea waves are encroaching on land which was in the past well above sea level.
The Oahu Petroglyphs are an interesting case in which geology and archaeology interact.
No comments:
Post a Comment