Category Archives: Volcanism

Ropes, pillows and tubes; modern analogues for ancient volcanic structures

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Analogies are the stuff of science. In geology, we frequently employ modern analogies of physical, chemical, or biological processes to help us interpret events that took place in the distant past. We cannot observe directly geological events beyond our own collective memory. Instead, we must infer what might have taken place based on evidence that is recorded in rocks, fossils, chemical compounds, and the various signals that the earth transmits (such as acoustic or electrical signals).  Analogies are not exact replicas of things or events, although they may come quite close. Their primary function is to guide us in our attempts to interpret the past.  As such, they are part of our rational discourse with deep time. Analogies are at the heart of the concept of Uniformity espoused by our 18th and 19th century geological heroes, James Hutton and Charles Lyell; they are the foundation for the common dictum “the present is the key to the past”, coined by Archibald Geikie, an early 20th century Scottish geologist.

Even though lots of people have written about this, I figure one more example that illustrates the methodology won’t hurt. Forty years ago, I worked on some very old rocks on Belcher Islands, Hudson Bay, that included volcanic deposits. Looking at the photos (35mm slides), I still marvel at the geology, the fact that something almost 2 billion years old is so well preserved, makes it look like the volcano just erupted.

Here are three ancient structures that were constructed by flowing basalt lava. Each can be compared with modern volcanic structures and processes that we can observe directly.  We can interpret the ancient structures according to the similarities and differences between the modern analogues and the ancient versions. The examples are from strata known as the Flaherty Formation, a succession of volcanic rocks exposed on Belcher Islands, Hudson Bay. Continue reading

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Islands with attitude; the devastation wrought by collapse of oceanic volcanoes

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Krakatoa, 1883, and the seas shivered. The eruption, one of the largest in recorded history, delivered tsunamis that swept away entire villages around Indonesia and its neighbours; little more than the flotsam and jetsam of nature’s fickleness.  Five years later, in the same general neighbourhood, nature was at it again.

Ritter Island, barely a speck on most maps, is a volcanic edifice rooted to the floor of Bismarck Sea between Papua New Guinea and New Britain. In 1888, most of the island slid beneath the waves, creating avalanches of rocky debris.  Eye-witness accounts tell of multiple tsunamis over a 3-hour period, and waves at least 8m high with run-ups to 15m above sea level.  Ritter Island is an active volcano, but at that time it was not erupting in any major way.  The island landslide is probably the largest in recent history – more than 4 cubic kilometres of volcanic rock were dislodged and redeposited along the seafloor. Slope failures like this are called volcanic sector collapses. Continue reading

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Lahars; train-wreck geology

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Christmas morning in New Zealand is synonymous with mid-summer barbecues at the beach, deservedly lazy times, perhaps a bit of over-indulgence. That morning, in 1953, Kiwis were expecting to awaken to news of the Royal tour; the newly crowned Queen was doing the rounds of towns and countryside, perfecting that royal wave to flag-waving folk lining the streets. Instead, they awoke to the news of a train disaster near Mt. Ruapehu, one of three active volcanoes in central North Island; a railway bridge on Whangaehu River, near Tangiwai, had been washed out on Christmas Eve.  Train carriages were strewn along the river banks, 151 people were killed.  The culprit was a geological phenomenon known as a lahar. Continue reading

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Overture to a cave; the spectacle of jointing in ancient basalt lava flows

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Saturday August 8, 1829, Felix Mendelssohn and his traveling companion Karl Klingemann, took a boat trip to Fingal’s Cave, the entrance to a world beneath Staffa, an inconspicuous dot on the edge of the Atlantic. Staffa is part of the Hebrides Archipelago, west Scotland. Celtic legend called it Uahm Binn, ‘The Cave of Melody’, that in story was part of a bridge extending to the iconic Giant’s Causeway in County Antrim (Northern Ireland).  The Celtic name is apt; Atlantic swells echoing countless songs.  Klingemann later wrote “Fingal’s Cave…its many pillars making it look like the inside of an immense organ, black and resounding, and absolutely without purpose, and quite alone, the wide grey sea within and without.”. Continue reading

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Class 5; Geology for Kayakers, Kaituna River

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#1 Kaituna River, New Zealand

  • Sam moving into Trout Pool Falls

KaKaituna mapituna River is a glorious water-course that has a bit of everything for kayakers and rafters, from beginner Class 1 and 2 rapids to serious class 5 waterfalls.  It is probably one of the more popular kayaking and rafting rivers in North Island, New Zealand, in part because it is so easy to get to, and close to the Rotorua centre of tourism.

The Kaituna is an outflow of Lake Rotoiti, its headwaters next to the Highway at Okere Falls (just down the Road from Okere Falls Café); it exits at the coast in Bay of Plenty.

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