Monthly Archives: November 2018

Beach microcosms and river analogues

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We are regular visitors to the beach; walks with the kids-grandkids, the dog, swimming, fishing, or just sitting and cogitating. It’s easy to get lost in the timeless rush of waves, their impatient foam. My mind reels at the thought that the sea has been doing this for more than 4 billion years. It’s a bit like getting lost in the night sky. There’s so much to discover.

Beaches are geological domains – part of a continuum that extends to the deep ocean, but a part that is easily accessed.  Geological stuff happens there. My attention is always grabbed by the small streams that drain across beaches at low tide. Whenever we came across one of these my kids would scatter, lest they be regaled yet again about the fascination of miniature worlds. I admit it was a bit over the top, so it goes…

Some beach outflows come and go with the tides, others are more permanent leakage from inland drainage. Some trickle, others rush. They are all fascinating, as microcosms of grander floodplain or massive deltas. Project this microcosm to the real world of geological process, of cause and effect. In doing this, you are engaging in the scientific process of creating your own analogy, an insight into a larger universe.

The streams usually start afresh with each tidal cycle. As tides recede, stream flow begins to erode its channel, deepest at the top of the beach. The channels may be straight and narrow, or broad networks of braided sand. Continue reading

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Difficulty breathing: The Atacama salt lakes

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I had the good fortune to work in the Atacama volcanic region a few years ago. It may be the closest I get to walking in a Martian landscape (NASA tests its Mars rovers there)

The mountains of Atacama, also known as the Altiplano-Puna Plateau, is one of the driest places on earth; it is located inland from the coastal Atacama Desert. A parched landscape littered with volcanoes, valleys where the few toughened blades of grass eke out a living, and salars, the salt lakes where there is barely a ripple. The salars are a kind of focal point for local inhabitants – Vicuña that graze on spring-fed meadows, flamingos that breed on the isolated breaks of open water, and foxes that lie in wait for both. It is a harsh environment, but stunning; glaring snow-white lake salt against a backdrop of reds and browns. And overhead, crystal skies, fade to black. Continue reading

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Witness to an impact

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The dinosaurs were snuffed out in a geological instant (well not exactly, but that is a popular image).  The Chicxulub bolide, its girth 10-15 kilometres, collided with Earth 65 million years ago, leaving a 150 kilometre-wide crater and enough dust and aerosols in the upper atmosphere to darken latest Cretaceous skies for decades.

Like all planetary bodies in our Solar System, Earth has received its share of meteorite and comet impacts. We still bear the scars of some. Every day, bits of space dust and rock plummet towards us – most burn up on entering the atmosphere, but a few make it to the surface. Occasionally they even startle us with air-bursts – Tunguska in 1908,  Chelyabinsk (2013), both in Russia. But humanity has never witnessed a decent sized impact, at least in recorded history. It’s all theoretical. Continue reading

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Atlas of cool-water carbonates

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This is a companion post to the Atlas of tropical carbonates, and the Atlas of modern coral reefs

For decades, all limestones were thought of as belonging to tropical realms: coral reefs, carbonate platforms, and sabkhas. Beginning in the 1970s, examples of non-tropical carbonates were published, although not always without a bit of pushing and shoving – not all adherents to the tropical paradigm were willing to give up their dominance. In New Zealand, Cam Nelson’s (Auckland and Waikato universities) studies of the Te Kuiti Group limestones, showed clearly that the usual components of tropical carbonates were different: hermatypic corals (other than the odd solitary form) and aragonite sediment-producing calcareous algae like Penicillus and Halimeda are absent, there is a predominance of shelly biotas in the carbonate framework, and marine cements generally lack aragonite. Continue reading

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