Tonga: East Coast of 'Eua 26 - Volcanic boulders bound by coral

bdeh, on October 20, said:

Great composition Ian. Greetings Berend

Ian Stehbens, on October 23, said:

Dear Berend,

It was the geology that excited me. 'Eua is an uplifted limestone/coral island, but I expected that it may have been uplifted far enough to reveal the underlying volcanics. And here is the evidence, differently from what I expected: volcanic boulders cemented together in conglomerate by a coral matrix.

Worth recording, don't you think?

Ian

bdeh, on October 23, said:

I think too Ian. Greetings Berend

Ian Stehbens, on October 24, said:

EXPLORING THE RUGGED EAST COAST OF 'EUA

Part 26

The horizontal beds of limestone that have been uplifted to create 'Eua are revealed in the three steps of the eastern escarpment.

At the Bowl of Cliffs at the southern end of this coast, it appears that the uplift has brought into view the underlying volcanic geology. And at some spots in central 'Eua I believe I have recognized some dykes extruding. So one aspect of my enthusiasm for being down on Lokupo Beach was to look for further evidence of the volcanic history that pre-dates the coral of which the island is composed.

The most obvious evidence on Lokupo Beach is the collection of huge blocks of conglomerate. The conglomerate appear to the eye to be very recent. There is no metamorphosis evident at all: just a pudding of very large plums of volcanic rock cemented together by coral growth and limestone concretion.

The plums are all very rounded therefore having been tumbled and yet they are of every size. Had they been derived from some river, they are most likely to have been sorted, graded. But here in the mid-Pacific there are no rivers and it is unlikely that there ever were any ancient rivers also. But the mixture of sizes indicates, as one may assume anyway, that this is beach material that has been cemented together.

As I sheltered beneath this huge blocks of plum pudding, I marvelled at how clearly the geologic story could be read. And the thought that among the big rocks further up the coast the others were roasting crabs for lunch made it tempting to move on and stopping my wondering. But Kepu was patient.

Some of the plum pudding rock is still in place on the reef platform. It is present as ribbons in parallel, alternating with intervening ribbons of coral. Ribbon reefs of coral form in very strong tidal currents as between Australia and Papua New Guinea, where the rotation of the earth means that the tides of the Pacific must flow west through into the Indian Ocean. In such narrow and shallow stretches as Torres Strait, the flow is treacherously strong and the extension of the coral colonies is therefore linear.

The other situation in which the ribbon formation is found is in the surf zone on a fringing reef. Here the waves breaking sends the swash forward and then the returning backwash flows back to the sea. This forward backward flows are differentiated by any obstacle. The result is coral ribbons separated by gutters in which the forward-backward flows of the broken surf are concentrated. From the headlands of 'Eua, where one looks down onto the fringing reef this pattern is very plain to see.

On the southern section of Lokupo Beach there is a section where the ribbons of reef are separated by ribbons of cemented boulders. These boulders happen to be volcanic. Nearby a small lava flow is evident. The lowest levels of coralline limestone on 'Eua pre-date the lava flow for it has flowed between limestone beds. Then later the lava has been broken into blocks and rounded into boulders by the powerful wave action, worked into the linear spaces in the platform of coral, and there cemented together by new coral and other marine concretion. With further tectonic uplift and storm wave attack, sections of this concrete have been pushed and rolled up on to the existing reef platform, or dragged back into the sea and off the edge of the platform reef into deeper water.

If I was reading some open natural history book, this was speed reading of the most recent chapters.

Kepu didn't understand all this. Tongan geographical and earth science education is very limited. He knows how to catch octopus on the fringing reefs. His ancestors have bravely caught one whale annually to feed and provide oil for whole communities. To live on a uplifted coral island is to be aware of the sea, tsunamis and earthquakes, but not to understand such phenomena.

Tongans have a history of remarkable interaction with the sea. Their ancient navigations, long distance maritime migrations, warrior raids and the cycles of seasons and survival take an outsider a long time to learn and to fully appreciate. Their ancient traditions of mythology recognizes that the demi-god, Maui, fished the islands out of the sea. And the fonua appreciates the world as a complex set of relationships between land, sea, sky and people. This fonua, for Christianity in Tonga, was created by God ['Otua'] to exist in harmony.

So as I walked the rugged east coast of 'Eua with these three Tongan men, I was most impressed by their bushcraft, there strength, their understanding of the environment in terms of their day to day survival, their deep respect for others, and their generosity of living within a community - existing only while they are in relationship with others, and while they see themselves as part of the complex web of creation or fonua. When it came to ecology on a global scale, or science of reef formation, the theories and processes of plate tectonics, or reading the open geological textbook, translation was not possible.

And my wondering, meant that by the time we returned to the camp between the sheltering rocks, Ofa and Lisiate were sound asleep, the fire was ash and beside it a pile of empty crab shells.

《阿鵬》B@ngk☺k, on October 26, said:

Unusual and unseen rock formation! Can't wait to view the enlarged version. Thanks for sharing, Ian.

Ian Stehbens, 12 days ago, said:

Dear Worapong,

First an apology for not responding earlier. And thanks for your journey with me to Tonga. I'll be home in Australia soon. Ian