 A long time ago I collected rocks. You can probably imagine that. Amateur geologist and all that. And growing up in Oamaru I learned how the East Coast of the South Island had had three distinct areas of volcanic activity in the past: Port Chalmers Dunedin, Cape Wanbrow Oamaru, and Banks Peninsula Christchurch. Interesting that these volcanic outcrops created good places for harbours (I should probably include Moeraki as well by the way).
The earthquake sequence and location at Christchurch has got me thinking. Like a lot of people. I look at Banks Peninsula's geology - how different it is from the Canterbury Plains - and wonder if something more fundamental is happening than movement along a few relatively quiet (until recently) fault-lines.
What got me thinking this way were these headlines:
Port Hills half a metre taller after Christchurch earthquake
Reported by Stuff - 2nd March 2011
Christchurch's Port Hills are nearly half a metre taller in places as a result of the colossal forces unleashed by last week's earthquake.
Satellite analysis by GNS Science shows the top of the roughly east-west buried fault responsible for the February 22 magnitude-6.3 quake lies between one kilometre and 2km below the southern edge of the Avon-Heathcote Estuary.
Land on either side of the fault has slipped horizontally as well as vertically, causing the Port Hills to rise by about 40 centimetres and land just south of the Avon-Heathcote Estuary to shift to the west by a few tens of centimetres. I formed an intuitive view that the volcanic mass of Banks Peninsula was in same way separating itself, or at least moving independently, from the rest of the Canterbury Plains which is the cradle for much of urban Christchurch.
So I did a bit of research. First a bit of background from an Encyclopaedia of New Zealand 1996:Banks Peninsula is situated in about the middle of the east coast of the South Island on the margin of the Canterbury Plains.
It is approximately 450 sq. miles in area and its highest point is Herbert Peak, 3,014 ft. It comprises two extinct volcanoes which were active less than half a million years ago. Their craters have subsequently been enlarged to many times their original size by stream erosion; they were then invaded by the sea during the postglacial world-wide rise in sea level beginning about 15,000 years ago.
They now form the harbours of Lyttelton and Akaroa. Originally Banks Peninsula was an island, but it became tied to the Canterbury Plains at some late stage in geological history when the growing alluvial plain reached its base.... Throughout the European history of Christchurch there has been debate as to exactly when the volcanoes happened, and whether the harbours were formed by volcanic action, glacial action or weathering. |
 This map shows the geology of Banks Peninsula, and highlights the sequence of volcanoes that formed it over time. The Geology of Banks Peninsula, By R Speight, which was read in October 6 1942, at the Canterbury Branch of the Royal Society of NZ, cites investigative work done previously by WM Davis – a visiting geologist.
His view was that the formation of the cliffs and landform of Banks Peninsula is by erosion, not by the formation of the volcanic cones. He asserted that the valleys were cut “when the land was much higher…” He notes that “in very many cases on the East Coast clffs descend into very deep water…” Davis researched wells dug in the area, and was particularly interested in a well dug in Heathcote Valley – in that case solid rock was reached at a depth of 708 feet. Speight writes, “This seems to indicate that the former Heathcote Valley was eroded to that depth, and that the land once stood over 700 feet higher…”
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 This diagram is a geologic cross section of Banks Peninsula and where it intersects with the Canterbury Plains. In Volcaniclastic Rocks of the Orton-Bradley Formation, Banks Peninsula, a thesis submitted in fulfilment of the requirements of the Degree of Master of Science in Geology in the University of Canterbury by Richard Sutton in 1993, we read:
The composition of the Banks Peninsula volcanics correlates well with the hotspot or plume type of intraplate volcanism, usually when a plate moves over a fixed hotspot or plume within the mantle, a linear chain of progressively younger volcanoes forms, such as occurs at Hawaii. This does not appear to have occurred in this case, though here is some evidence of migration of volcanism from Lyttelton to Mt. Herbert to Akaroa....
Banks Peninsula consists of several large coalescing, composite volcanoes, composed predominantly of alkali rocks. Originally an island, the volcanoes were joined to the mainland by the progradation of the Canterbury Plains by deposition of loess and alluvium derived from the erosion of the Southern Alps. Banks Peninsula shows little evidence of deformation in the region. The volcanoes were formed on a pre-existing basement high, formed by the horst and graben structures in the Torlesse rocks, underlying the Canterbury Plains (Sewell et al. 1988). |
 So what does all that mean? Well what it basically says to me is that the volcanoes that form Banks Peninsula have formed on top of the layer of rock that underlies the Canterbury Plains. The volcanic action spewed up volcanic rock through cracks or fissures in that layer, and formed a pile of volcanic rock now known as Banks Peninsula. This underlying layer of hard rock is part of the crust of the earth that floats above the earth's mantle and molten magma that lies beneath, as depicted in this graphic.
Putting all this together, the scenario is that after the volcanic activity that formed Banks Island, there was tremendous downward pressure exerted on the Torlesse rock layer below. Just simply the force of gravity on this new mass of rock - area 450 square miles, average height (now) around 300 metres. This force over time has forced the layer below, which is floating on molten magma, downward. Like a boat floats deeper when more people jump into it. (This accounts for the observations of geologist Davis - above). The underlying rock layer sank till some sort of steady state was reached over time. Or maybe it got jammed.
And now we are experiencing a reverse ripple from below. Perhaps the downward movement was too far, too fast, got stuck, geologically speaking, maybe there are additional pressures at work, but what appeared to be in balance is now finding a new level. The underlying rock layer is lifting, floating up.
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When you research something like this on Google, it is interesting what comes up. I end with this discovery. In the Journal of Environmental & Engineering Geoscience; November 1995; v. 1; no. 4; p. 427-488, we find a paper:
Geology of Christchurch, New Zealand, by L. J. Brown, R. D. Beetham, B. R. Paterson, and J. H. Weeber, of the Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand. Note that this research was published in 1995, 16 years ago. In this we find the following statements:
Christchurch is situated on the east coast of the South Island of New Zealand in the south Pacific Ocean. The city is located at the coast of the Canterbury Plains adjacent to an extinct volcanic complex forming Banks Peninsula. The site of Christchurch was mainly swamp, behind beach dune sand, and estuaries and lagoons, which have now been drained.....
Geological constraints of concern to Christchurch include flooding, variable foundation conditions, slope instability on the Port Hills, and coastal erosion. The Waimakariri River with its catchment in the southern Alps, regularly flooded Christchurch prior to stopbank construction and river realignment, which began shortly after the city was established in 1850. Variable foundation conditions as a consequence of a high water table and lateral changes from river floodplain, swamp, and estuarine-lagoonal environments, impose constraints on building design and construction....
The geology, tectonic setting, and active seismicity of the Christchurch area indicate that future large earthquakes will occur which will have major impact on the city. Earthquakes are expected to produce liquefaction, landsliding, ground cracking, and tsunami. Planning and design to mitigate the consequences of these phenomena are an essential prerequisite for preparedness....
The identification and quantifying of geological hazards, and the implementation of regulation and planning designed to discourage irresponsible land use, should continue in the future as the geological knowledge and database is expanded....
Which is all news to me. Though it clearly wasn't to some people in 1996.
