Monthly Archives: October 2018

Crossing the harbour bar

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A safe harbour offers a place of refuge. Those in peril (or evading taxes), running before a storm, crossing a figurative bar to welcome respite. Non-figurative harbours, the coastal kind, have traditionally provided safe haven for mariners escaping inclement weather or foes.

Harbours fill and are emptied of seawater on the tide. Sea water that enters or exits is commonly focussed through narrow inlets. Here, powerful currents are generated that carry fish, sediment, flotsam, and unwary boats. Filling on an incoming tide is like a cleansing, a renewal; outgoing tides reveal channel arteries that keep alive the bars and broad flats of mud and sand, textured Kandinsky-like.

Northern New Zealand’s west coast has 6 harbours distributed along a 300 km stretch of coast. Each is protected by large sand barriers that have built over the last 2-3 million years with sand moved inshore by successive rises and falls of sea level.

Many of New Zealand’s harbours are drowned valleys, where rising sea level (following the last glaciation) has inundated dissected landscapes. Rising seas have crept up valleys, leaving the exposed high ground to front an intricately embayed coastline, islands, and estuaries that extend their marine fingers far inland.

New Zealand’s west coast is open to large swells, generated by westerly winds across a 2000 km expanse of Tasman Sea. Sea conditions along this coast are often rough. Access to the open sea via harbour inlets, requires sailors to ‘cross the bar’ – the zone of shallow, constantly moving sand. Strong tidal currents, particularly out-going tides can increase wave heights even further, as well as making wave conditions in general very choppy. The sea condition can change rapidly. Many a boat has come to grief across these west coast bars, a mix of bad luck and poor judgement (NIWA has real-time images of current bar conditions at several locations).

The oceanographic and geological term for sand bars at the entrance to harbours and lagoons is tidal delta. Tidal deltas can form on the seaward margin, in which case they are called ebb tidal deltas (because they are downstream of the outgoing tide). Those that form inside harbours and lagoons are flood tidal deltas where sand is deposited by incoming tides.

Raglan Harbour is small but it sports a very nice example of a symmetrical ebb tidal delta. The delta extends 1.5 – 2 km from the harbour mouth. Darker hues (image below) that mark the main channel contrast nicely the shallower sand bars on either side over which waves tend to break. These marginal sand deposits are called swash bars.

Westerly swells approach the coast with relatively straight crests. As they pass over the shallow delta platform, they move at a slower speed because they interact (friction) with the sea floor. Some of the wave energy is transferred to the sea floor such that sediment is moved as ripples and dunes. Slowing waves also build in amplitude (height); this is the region where waves break. However, the same waves in the adjacent, deeper water are moving at a faster pace – trace the crests of each wave and you will see it ‘bending’ around the delta.

Most of the tidal delta remains submerged even at low tide. Parts of the swash bar that are exposed during low tide show evidence for sand movement, mostly as ripples, large and small. Sand is moved during flood and ebb tides. The shape of these sand bars changes from one tide to the next, demonstrating that this is a dynamic environment.

 

The Raglan tidal delta consists almost entirely of sand. In contrast, Raglan Harbour and its estuaries contain a high proportion of mud. So where does all that sand come from?

The tidal delta is part of a much larger system of sand transfer – supply and demand from the adjacent continental shelf to the adjacent beaches, shallow sand bars (commonly formed by rip currents) and sand dunes. Sand in the inshore region is also moved along the coast by long-shore currents and it is this sand that continually feeds the delta. The delta in turn, via its main channel, moves sand back onto the shelf, completing the cycle.

The beach south of the tidal delta continually changes its profile. At times the profile is an uninterrupted swath of black sand along most of its length (about 3 km). At other times a significant volume of sand has been removed exposing ancient boulder deposits from nearby Karioi volcano; sand removal frequently occurs during stormy weather. The sand dunes also participate in this budgeting exercise. Sand transfer from the beach (and dunes) is probably a combination of movement directly offshore by rip currents and wave undertow, and long-shore movement towards the delta. Sand replenishment and removal from the beach, and addition to the tidal delta, is part of a much larger system of sand supply and demand – nature’s sand budget.

Sand moved onto the swash bars helps to replace sand that is removed by the deep, fast-moving channel. Channel flows in narrow inlets like the one at Raglan are commonly 4-6 km/hr (1-2 m/second), which may not sound fast (try swimming against it) but is sufficient to move large volumes of sediment during each tidal cycle. There are some small sand bars in the harbour itself, but the channel is an effective flushing mechanism that prevents the estuaries and tidal flats from clogging up.

Changes in sea level have a profound impact on coastal sand systems. If sea level falls, the beach and dunes would follow the retreating shoreline, the harbour would eventually become the domain of non-tidal rivers and swamps, and the main channel would be free to meander over a broad expanse of exposed continental shelf. Tidal deltas might be more ephemeral structures, constantly on the move. This was probably the scenario during the last glaciation, when sea level was more than 100m below its present position.

Perhaps of more immediate concern is a rise in sea level (the present situation) which would erode older foreshore beach and dune deposits, and destabilise some cliff areas south of the Harbour. The Surf Club at the south end of Ngarunui Beach would need to move – yet again. The Harbour area flooded at high tide would increase, resulting in a greater volume of seawater entering and exiting the narrow inlet. To accommodate this, the inlet would need to expand, or the speed of current flow would need to increase. Changes such as these would have an immediate effect on the size and shape of the tidal delta.

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Bits of North America that were left behind

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The jigsaw puzzle of continents and oceans, the ground beneath you, the seas beyond, even the weather you enjoy or endure, are governed largely by plate tectonics. This grand mechanism creates plates along mid-ocean volcanic ridges, then proceeds to push them down the throat of subduction zones. Plates collide, tearing at each other’s crust. Volcanic hiccups, earthquakes, and crustal dismemberment are all part of a tectonic plate’s stressful life. And occasionally in this mad nihilist rush (after all, millimeters per year is pretty quick), bits are left behind.

The landscapes of north Scotland and northwest Ireland are underpinned by rocks that once belonged to North America, or at least an ancient version of it. As geological puzzles go, they are iconic; here James Hutton unraveled the problems of deep time, and Peach and Horne sliced the ancient crust into moveable slabs. The rocks are part of the Caledonian Orogen, a mountain chain that formed from tectonic plate collisions more than 400 million years ago, stretching from Scandinavia to Scotland-Ireland, east Greenland, and the Appalachians of eastern USA and Canada.

The choice of a starting point for a story like this is a bit arbitrary because continental and oceanic plates, and the plate tectonic mechanisms that propel them across the globe, date back at least one billion years, possibly earlier. For convenience, this tale begins on the ancient continent of Laurentia about a billion years ago; Laurentia was an amalgam of North America, Greenland, and (what would become) north Scotland and northwest Ireland tucked along its eastern margin [The first four figures here are modified from an excellent technical summary of this important period in Earth’s history, by David Chew and Rob Strachan, their Figure 1, in Geological Society of London, Special Paper 390, pages 45-91, 2014].

Three groups of rock that underpin the Scottish Highlands, originally formed along the eastern Laurentian margin. Lewisian gneisses. Some as old as 2.7 billion years, were part of the basement foundations of Laurentia (Panel 1 above). Two major groups of sedimentary rock were also deposited along the eastern margin – the Moine group of rocks, that beneath the Northern Highlands we now see as metamorphic rocks, originally formed as sediment shed from the ancient continent about 1000 to 870 million years ago. Dalradian metamorphic rocks that now form the Grampian Highlands also originated as sediments and volcanics from about 800 to 510 million years – metamorphism occurred much later.

For the next few million years Laurentia moved south (south of the Equator!) towards, it is hypothesized, a volcanic arc, similar perhaps to modern Ring of Fire volcanic arcs that rim the Pacific Ocean (Panel 2). Collision between Laurentia and the Grampian Arc initiated the first phase of Caledonian mountain building 475-465 million years ago (Panel 3).

Several other events were also taking place at this time. Laurentia itself was rotating anticlockwise. Two smaller continental plates appeared on the scene: Baltica (that would later become Scandinavia and north Europe), and Avalonia (whence the rest of England, Wales and south Ireland resided), both were migrating north towards Laurentia. The intervening ocean, the Iapetus, was gradually shrinking as its crust was devoured down at least three subduction zones (Panel 3).

The Iapetus eventually closed; some slivers of oceanic crust (called ophiolites) were scraped off and incorporated into the Caledonian mountain complex, but most of this once-grand ocean basin was consumed in Earth’s grand recycling depot.

Baltica and Laurentia were involved in head-on collision around 435-425 million years (Panel 4). The Moine thrust, one of the defining ‘moments’ of tectonic dislocation and metamorphism in the Caledonian, developed during this interval. In contrast Avalonia’s approach was more oblique and it appears this smallish continental fragment slid past Laurentia. Avalonia’s legacy is that south England, Wales and south Ireland were now stitched firmly to their northern cousins. This plate tectonic assemblage has withstood tempests, bolides, and glaciations for the last 400 million years; 2000 years of geopolitical ructions are insignificant in comparison.

The amalgamation of Laurentia, Baltica and Avalonia eventually became part of a much larger continental mass, a super continent called Pangea that included Africa, South America, Antarctica, Australia and Asia (and of course, New Zealand). This amalgamation was well underway 335 million years ago. Pangea began to break apart about 175 million years ago, a separation that over the next 175 million years would give us our most recent plate tectonic configuration of ocean basins and continents (Plate 5).  Break up of Pangea took place in several stages, but the event that is of interest here took place about 75-80 million years ago. [Chris Scotese has created an excellent animation of these events, set to nice music].

Atlantic Ocean had its beginnings during the early stages of Pangea break up, 175 million years ago. Atlantic Ocean’s expansion is centered along a submarine spreading ridge of volcanism (that today stretches from Iceland almost to Antarctica). The spreading ridge migrated northwards, which means that new ocean floor was also being created incrementally northwards. During the early stages of North Atlantic Ocean expansion, the British Isles were still firmly attached to the old Laurentia margin.  But by 80 million years the locus of spreading had moved west of Britain and Ireland (Plate 5), and it was at this point that the ancient roots of north Scotland and Ireland became divorced from North America and Greenland – a decree absolute.

The period of Caledonian mountain building is one of the most studied in the geological community (at least two centuries worth, and 100s of 1000s of scientific papers), much of it undertaken before plate tectonics was discovered in the mid-1960s. Nevertheless, plate tectonics theory has provided a more global context, and a more rational, mechanistic approach to solving the myriad geological complexities.

I recently visited some of these rocks in the Scottish Hebrides and Connemara – and yes, there is complexity at every level of observation. The story I have presented is simplified – perhaps woefully so. But even a simple rendition can promote understanding. I’d like to think so.

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