The Earth Story

Thornton Force Thornton force is a 14m waterfall on the River Twiss; located in the Ingleton Waterfalls group in Yorkshire, UK. The waterfall drops from horizontal limestone (deposited 330 million years ago) onto dipping sandstone deposited 500 million years ago. This means while the water falls 14m over the fall, it passes an age gap of 170 million years.

Beautiful Bouma This annotated rock shows 4 of the classic layers of the Bouma Sequence, a type of sedimentary rock produced by a deposit called a turbidite. Turbidites or turbidity currents are produced in submarine landslides. In canyons or places where lots of sediment is deposited as in river deltas, occasionally sediment offshore becomes oversteepened and collapses. The sediment will then pour down the steep continental shelf and re-deposit at lower elevations.

The Grand Banks Earthquake 1929

Many people haven’t heard about the Grand Banks Earthquake and it is hardly surprising. The earthquake itself did very little damage and the tsunami produced pales in comparison to those of Indonesia or Japan. However, the Grand Banks Earthquake is very important to geologists because it led to the discovery of a brand new phenomenon: the turbidity current.

I want you to think of a landslide. Now imagine that landslide was underwater. This is effectively what a turbidity current is, a load of sediment rich water hurtling down the continental slope towards the basin floor. The current is driven by gravity as the sediment within it is far denser than the surrounding water and the turbid nature of the flow allows it to propagate over large distances.

In 1929 no-one had ever heard about a turbidity current mainly because no-one realised they existed. This is hardly surprising considering they occur so far off the coast and happen to be underwater. At 5:02pm on November 18th a 7.2 magnitude earthquake occurred 250km of the coast of Newfoundland with the effects being felt as far away as New York. Luckily damage was limited to Cape Breton Island where a few chimney stacks fell over and some roads became blocked by landslides.

It wasn’t until 2.5 hours later that a tsunami up to 13 metres high struck the eastern seaboard killing 28 people and laying waste to over 40 villages in southern Newfoundland (newspaper reports all mention the loss of 280,000lbs of salt cod which I’m sure was heavily mourned too). The wave even reached the coast of Portugal, albeit several hours later.

The tsunami was so powerful that it lifted some buildings clean off their foundations, depositing them some distance from where they had first stood. There was even a general merchandise store that is said to have been moved 60 metres inland and unceremoniously deposited in a meadow. When the owner reclaimed his store he found that despite its epic voyage all of the stock was undamaged and had remained stacked on the shelves!

When scientists began pouring over the data they noticed several strange reports of broken transatlantic cables. These cables had been placed on the seabed in the Grand Banks area, and appeared to be in a similar position to the earthquake epicentre. Not only that, but the cables closest to the epicentre had broken first while those further away didn’t break till later. Overall 23 cables had been broken over a 12 hour period giving an average speed of movement of 55km/hr.

It took scientists over 20 years of modelling, hypothesising and generally scratching their heads before they managed to work out what had happened. What they discovered was that the earthquake itself was not responsible for the Tsunami. It had merely made sediment on the continental shelf unstable, causing it to slide downhill. This process, named a turbidity current, had not only broken the cables but had displaced enough water to generate a tsunami!

So why should we care about turbidity currents? Well firstly they can generate tsunamis and destroy any form of seabed based cable unfortunate enough to be in their way. Secondly they are fantastic hydrocarbon reservoirs and are exploited in the Forties Field in the North Sea to the Wilcox Formation in offshore Gulf of Mexico.

References:

http://bit.ly/1H1aNw7

http://bit.ly/1QFvHbf

Further Reading:

http://bit.ly/1csXJaS

http://1.usa.gov/1G2VA1F

Image credit:

Houses - photograph by H. M. Mosdell from the collection of W. M. Chisholm

Boat – Provincial Archives, Government of Newfoundland and Labrador (PANL)

Turbidites!

These rock layers may not look that distinctive, but to geologists these tell a really cool story. These are turbidites, the remnants of debris flows off the shore of an ancient ocean.

Turbidites form when sediment piles up just off of a shoreline, often carried to the area by a river. Eventually, even underwater, big enough piles of sediment will collapse and avalanche downslope. Sometimes they do so under their own weight, sometimes an earthquake will set them off.

The avalanche of debris produces a recognizable pattern to geologists. The heaviest particles, the biggest grains, settle out at the bottom of the debris flow, and the sequence “fines upward”, meaning the grain sizes get smaller.

A typical turbidite will start at the bottom with sandy grains, maybe even larger stuff, and the grain size will decrease going upward as progressively finer grains settle out. Finally, each turbidite is topped by a layer of very fine grained clay particles that can even be a different color from the stuff below it. This sequence even has a name – the “Bouma” sequence.

Turbidites show up throughout the geologic record because they’re easily preserved. They form in areas in the ocean that aren’t likely to be eroded and they form in areas with lots of sediment that can bury and protect them afterwards. This sequence photographed here shows several turbidites - the big units are the coarse-grained sand, while the thinner layers are silt and clay rich.

-JBB

Image credit: https://flic.kr/p/UgSoRead more: https://courses.washington.edu/sicilia/pdf/JBturbidites_fans.pdf http://trg.leeds.ac.uk/

Taiwanese Typhoons Trigger Turbidites

These sedimentary layers are turbidites, the remnant of ancient debris flows underwater. Turbidites are the main way that sediment deposited in the shallow water near a continent makes its way into the deep ocean. Rivers and streams bring sediment to the ocean and deposit it near the shore, and then eventually so much sediment is deposited near the shore that it becomes unstable and slides down to the deeper part of the ocean, often in submarine channels.

Turbidites can be recognized in sedimentary layers by what is known as the Bouma sequence – a classic pattern where the coarsest grained sediment is at the bottom and the sediment gets finer to the top. In this sequence, the big brown layers are sandy, while they thinner dark layers are silt to clay. These turbidites started as flows of sand and silt moving cascading down from shallow water to deep water. The heavier sand stayed at the bottom of the flow, forming the sandy layer, while the silt and clay were suspended above the flow and only settled out into layers after the flow stopped.

Scientists at Tongji University in Shanghai just published an interesting study regarding turbidites in Taiwan. Turbidites can be triggered by many things; weather on shore, earthquakes, or just random events. Over a 3.5 year period from 2013 to 2016, an area of Taiwan that feeds into a submarine canyon known as the Gaoping Canyon received heavy rainfall from 16 typhoons. The scientists tracked turbidites in this area and found that 72% of the sediment delivered to this canyon as turbidites occurred during these storms. While this is not the case everywhere, at that particular spot, typhoons are therefore the main trigger of sediment moving into the deeper ocean.

-JBB

Image credit: https://flic.kr/p/UgSo

Reference: http://bit.ly/2vhdm1f

Slow motion laboratory simulation of a turbidity current moving across topography in a desktop box

Turbidite beach

This spot is Whakataki beach on the southeast coast of New Zealand’s North Island, about 150 kilometers from Wellington. The fascinating looking layered rocks are turbidites – submarine sediments that have been uplifted and tilted until they formed part of the beach.

About 25 million years ago, the Pacific Plate started pushing towards the microcontient today nicknamed Zealandia – a small piece of continental crust that had rifted away from Australia tens of millions of years earlier. This collision between two tectonic plates began producing the mountains found throughout New Zealand today, and as those mountains began rising they began to erode and shed sediments into the ocean.

Some of those sediments flowed down the slope of the uplifting continent as debris flows that geologists call turbidites. In the only 32 meters of sediment exposed at this site, scientists have mapped at least 360 different turbidites. A sand layer followed by a layer of finer-grained mud represents a single turbidite here. When the sediment flowed downslope, the denser sand was deposited at the bottom of a sequence and then finer-grained mud settled down on top.

These turbidites are thought to have been fairly slow moving debris flows as fast moving debris flows would carry more coarse grained sediment not found at this site. As happens in many mountain ranges, the sediments deposited when the mountains first began forming are now caught up in the ongoing faulting, a process which has now pushed these turbidites up to the surface and tilted their once nearly-flat lying layers. The turbidites are weathering into layers as the fine-grained material is eroding away more rapidly than the stronger sandy layers.

-JBB

Image credit: Philip Capper https://flic.kr/p/5eVqUT

References: http://bit.ly/2mGPs9X http://adsabs.harvard.edu/abs/1992SedG...78..111N http://juliansrockandiceblog.blogspot.com/2012/04/whakataki.html

The future of the Cascadia Fault

Lacus Lemanis- Tsunamis past and future?

Lac Leman, aka Lake Geneva, is one of Europe's largest post-glacial lakes. It sits in an ancient tectonic basin, eroded into a typical glacial U-shaped valley at the edge of the Alps, separating in two the Jura range. The glaciers that carved this gash in the Jurassic limestone platform (that borders the squished metamorphic rocks of the African plate's collision with Europe) advanced and retreated during the Pleistocene glaciations, widening and deepening the valley with each iteration. The lake is shared between France and Switzerland. The waters of the river Rhone, born around Mont Blanc (Europe's highest peak) flow through without mixing, taking about 7 years to do so, and depositing an underwater delta of alpine sediment as it enters. A gold mine now exists at the head of the lake, filtering precious metals out of the Rhone's water.

It is a rich and peaceful part of the world, growing light red wines and slightly sparkling whites. On its edge sits the famous spring and spa town of Evian, source of a delicious water with reputedly curative properties. The Swiss city of Geneva has existed at the lake's exit since pre-Roman times, and is now filled with the UN's European HQ, varied international organisations and NGO's, banks, oil traders and CERN (of Higgs boson fame). It is an ideal place to live: good food, comfort, Swiss efficiency and peacefulness. The lake's geological past however has not always been as placid as one might believe, as we sit enjoying the summer sun, watching the paddle steamers puffing past towards Montreux (as in the Jazz Festival), while eating lake perch and sipping a glass of wine by its blue waters. It has seen several lake tsunamis, and may see more in the future.

Lake (and fiord) tsunamis happen when a chunk of rock from the surrounding uplands fails and crashes into the lake, displacing the water. Swiss researchers from Geneva university have recently reconstructed the events of 563 CE, when a landslide fell into the Rhone upstream of its entry point into the lake, using the geological record to explain a historical mystery. Many villages were destroyed in the resulting tsunami, which overtopped Geneva's city walls and washed away its mills and bridge, resulting in extensive flooding of the city. A new examination of the sedimentary record has shown that these events happened, just as described in the old chronicles and given us the 'how' behind this devastating wave.

A tsunami should not have occurred because the landslide occurred upstream in the river, and scientists speculated that it had in fact dammed the Rhone, the wave being unleashed when the dam failed. A 250 million cubic metre nappe of turbidite sediment (measuring 10 by 5 Km) was discovered in the middle of the lake, using sonar data and sediment cores, and dated using C14 to a period bracketing the date given in the historical records. The mechanism behind this 'impossible' tsunami is simple: the landslide sparked a turbidity current in the soft sediments of the Rhone's delta at the lake head, displacing enough water to power the wave. Modelling has shown that its height reached 8 metres at Evian, and 13 at Lausanne. Despite the dissipation of the wave's energy as it travelled, the focussing effect as the lake narrows into a funnel as it nears low lying Geneva sent it back up to 8 metres, a mere 70 minutes after starting.

This was not the only such tsunami in the past, four deeper turbidite deposits were found in the sediment cores, nor will it be the last to occur. A pan of rock by Bex at the lake's head is thought to be weakening, and may spark another catastrophe in the future. Unlike in the 6th century, over a million people now live by the lakeside, a quarter of them in the Geneva funnel. Since the lake is small, warning times are much less than in the Pacific ocean, where most places get hours to evacuate the population to higher ground. Surveys of the delta are now underway, in an attempt to determine how close to collapse it might be. The older turbidites urgently need to be dated, to get a rough idea of the frequency of these events, and a hazard map for the entire lakeshore must to be compiled.

Whenever modern and past geological hazards are compared, we keep on encountering the same problem: many more people live in these at-risk areas than ever before, and expensive infrastructure has now been built there (in Leman's case, Geneva, motorways, train tracks and the villas of the hyper-rich), making the destructive potential of any future event that much greater.

Loz

Image credit: Hardo Muller.

http://www.economist.com/news/science-and-technology/21565583-millennium-and-half-ago-geneva-was-destroyed-giant-wave-recent-research

http://www.nature.com/news/ancient-tsunami-devastated-lake-geneva-shoreline-1.11670

http://www.nytimes.com/2012/11/20/science/earth/a-tsunami-in-switzerland-lake-evidence-says-yes.html?_r=0

http://news.nationalgeographic.com/news/2012/10/121031-alps-tsunami-geneva-nature-geoscience-science/

Original paper, for those with paywall access:

http://www.nature.com/ngeo/journal/v5/n11/full/ngeo1618.html

Lab experiment simulating a turbitidy current, a mixture of sediment and water, flowing down a slope. Love how it is channelized and then spreads out, just like it does in the Ocean.