Happy # insert relevant holiday here # to you all

I'm off camping for the next week. So here are some picture from Augrabies National Park, South Africa. The river guilty of carving the canyon is the Orange River :) These were taken in 2006 when we went on the University of Cape Town's epic Honours field trip. These two photos were taken by Nic Laidler and appear here with his permission.

This video is from January this year (2011) when Southern Africa had an unusually large amount of rain.

And last but not least, have a fantastic festive season and a wondrous new year!

Aussenkehr

Last year September (2010) I was part of a group hiking the Fish River Canyon. On the way to the canyon, driving north from Cape Town, three of us stopped off in Aussenkehr, just North of the Orange River. It is an area I have only ever driven past and a cursory Google search was unenlightening so I didn't really know what to expect. All I can say is 'what a nice surprise!'. The crags at Aussenkehr are part of the Karoo Large Igneous Province (LIP) with similar ages and geochemistry (Richardson, 1979 and 1984; Reid and Rex, 1994; Marsh, et al. 1997). The Karoo LIP covered and/or intruded most of Southern Africa and part of Antarctica, the Ferrar-Province, current estimates for the size of the LIP are around 3 000 000km² (Jourdan, et al. 2007). Outcrops of the Karoo LIP can be found in Botswana, Malawi, Mozambique, Namibia, South Africa, Zamibia and Zimbabwe with unconfirmed occurrences in Angola (Jourdan, et al. 2007).

Location of Aussenkehr; the 'A' marks the spot. Image from
Google Maps

Much of the present surface expression of the Karoo LIP in Namibia is restricted to sills and dykes. The Aussenkehr rocks are part of the Tandjiesberg dolerite (diabase) sill complex, which has at least 55km of outcrop and is between 80 and 110m thick. The sill consists of two units, the upper and lower sill (Richardson, 1979 and 1984; Reid and Rex, 1994). The columnar jointing in the dolerite cliffs make for some fantastic climbing. I need to go back to do some actual climbing, at the time I was to distracted by the scale of the dyke, clambering over the rocks and hitting things with my geopic. 

My partner, taking full advantage of the columnar joints

The vertical extent of the sill was rather impressive, car for scale

The geology/geochemistry

The lower sill, relative to the upper sill, is enriched in incompatible* elements, e.g. Rb, Zr and Y and depleted in compatible** elements such as MgO, indicating that the lower sill has a slightly more evolved*** composition than the upper sill. Trends and ratios between these elements indicate that the two sills could be related by fractional crystallisation (Reid and Rex, 1994).

*Elements which are not easily incorporated into the crystal structure of the major rock forming minerals, in the case of a mafic magma these minerals are olivine, pyroxene and plagioclase feldspar.

**Elements which are either major constituents or easily incorporated into the crystal structure of major rock forming minerals.

***As minerals are crystallised out of a magma they remove elements from the magma. This results in the composition of the magma changing. The concentration of incompatible elements increases while the compatible element concentrations decrease. The remaining liquid has a more evolved composition.

The upper sill has a fine-grained chilled margin caused by the initial contact with the country rock. This is usual for large igneous bodies intruding much colder rock. From the edge to the centre of the sill the grain-size increases as the magma had more time to cool and form larger crystals. The sill groundmass consists of plagioclase, pyroxene and opaque minerals (e.g. iron oxides and/or sulphides depending on whether the magma is oxidised or not) and the microphenocrysts present are plagioclase, augite and olivine (petrography; Richardson, 1979). Richardson (1979) identified changes in magma chemistry and phenocryst concentrations and composition from the edge to the centre of the sill. Often chemical and mineral zoning in sills is caused by minerals settling to the bottom of the magma due to gravity. However, the pattern of chemical change and lack of mineral layering normally associated with gravitational settling, in upper sill, argues against this as a mechanism for the chemical changes (Richardson, 1979). The internal chemical variation observed is also inconsistent with fractional crystallisation, assimilation and multiple intrusions of magma. The chemical zoning is continuous along the 20km length sampled by Richardson (1979) and is attributed it to the migration of phenocrysts to the centre of the sill, flow differentiation. Similar differentiation has been observed in Hawaii in the Makaopuhi lava lake (Moore and Evans, 1967). The lower sill has not been described in much detail or at least I couldn't find the journal article describing it.

Just to demonstrate the difference between phenocryst and
groundmass. This is from a quartz porphyry dyke.
A phenocryst is a large/larger crystal in a fine-grained
groundmass. In this image the phenocryst is quartz.

There is an unsolved conundrum for the sills in Namibia. No feeder dykes have been observed so how did magma originating near the east coast of South Africa travel so far? Reid and Rex (1994) analysed the largest and most prominent dyke in the area, the Mehlberg dyke, to determine if it was the magma conduit for the large Tandjiesberg sill complex. Chemical composition and age ruled out this option. The dyke is 133Ma and has a higher silica content than the sill (Reid and Rex, 1994). The connecting sills and dykes between the Karoo rocks in South Africa and those in Namibia have yet to be identified or perhaps they have already been eroded away.

References

Jourdan, F., Feraud, G., Bertrand, H., Watkeys, M.K. And Renne, P.R. 2007. Distinct brief major events in the Karoo Large Igneous Province clarified by new 40Ar/39Ar ages on the Lesotho basalts. Lithos, pp 195-209

Marsh, J.S., Hooper, P.R., Rehacek, J., Duncan, R.A., Duncan, A.R., 1997. Stratigraphy and age of Karoo basalts of Lesotho and implications for correlation within the Karoo igneous province. In: Mahoney, J.J., Coffin, M.F. (Eds.), Large Igneous Province: Continental, Oceanic and Planetary Flood Volcanism. Geophysical Monograph series, Washinton D.C., pp. 247–272

Moore, J. and Evans, B. 1967, The role of olivine in the crystallization of prehistoric Makaopuhi lava lake, Hawaii: Contributions to Mineralogy and Petrology , pp. 202-223

Reid, D.L. and Rex, D.C. 1994. Cretaceous dykes associated with the opening of the South Atlantic: the Mehlberg dyke, north Richtersveld. South African Journal of Geology, pp 135-145

Richardson,S.H. 1979. Chemical differentiation induced by flow differentiationin an extensive Karoo dolerite sheet, Southern Namibia. Geochem.Cosmochim. Acta, pp 1433-1441

Richardson, S.H. 1984. Sr, Nd and O isotope variation in an extensive Karoo dolerite sheet, southern Namibia. Special Publication, Geological Society of South Africa, pp 289-294

If anyone is interested in reading about geochemistry and how it all works this online text book by William White isn't bad.

Some more photos from the crags

Cool village on the bank of the Orange River. The Spar shop was reasonably stocked and having
forgotten my hairbrush in Cape Town it was most useful!

Friday geology photos: Fish River Canyon

The Fish River Canyon is one of Namibia's many wonders. The canyon is marketed as the second biggest in the world after the Grand Canyon. At 550m maximum depth and 160km long it is definitely a baby compared to the Grand Canyon but the Fish, as it is affectionately known, is a fantastic place in its own right. The first photo is from the view point at Hobas, which is the starting point of the Fish River hiking trail. The trail is 65 to 90km depending on whether you stick to the river course or cut across the meanders. The latter is far more fun, cool rocks and the added bonus of a break from the sand. The canyon is open to hikers from May to September each year. Flash flooding and high summer temperatures make it dangerous to hike from October to April. The photos below were taken when I hiked the canyon in September last year (2010).

The geology in brief

Doming related to the African Superswell is thought to have resulted in the Fish incising its bed. There are beautiful exposures of the nonconformity between the ~550-750Ma sediments of the Nama group and the +1000Ma Gordonia Subprovince, migmitites and granite gneisses. These rocks are cut by two sets of dolerite (diabase) dykes one associated with break-up of Rhodina, which abuts against the Nama sediments. The second is related to the break-up of Gondwana. These are the youngest rocks in the canyon.

View from Hobas look out point

Migmatite :)

Noncomformity between the Nama sediments (top) and the Gordonia migmitites (bottom)
The dark band is one of the older dolerite dykes

Desert horses. They are the descendants of escapees once owned by the German colonisers.

 

Grunert, N., (2000) Namibia - fascination of geology: A travel handbook. Klaus Hess Publishers, pp 176

Photo week continued: St Lucia

Following Evelyn, from Geoneys, fantastic idea of a week of geology photos here is my second instalment.

St Lucia is one of the windward islands in the lesser Antilles, West Indies. I have fond memories of this beautiful island as not only is it the location of my sister-in-common-law and her husband's wedding, it was also my first trip to an active volcano.  Until my visit to the Soufrière Volcanic Centre I had only ever traipsed around the innards of long extinct volcanoes. To visit a place where the last eruption was a mere 20 000 years ago with a hydrothermal system still bubbling away was a real treat. Hydrothermal systems are of particular interest to me as I have spent the last four years studying their fossil remnants. To see what my 144 to 133Ma rocks may have looked like was exciting. Volcanism in the Lesser Antilles is related to the subduction of the North American Plate beneath the Caribbean Plate. The rocks of St Lucia are mafic to intermediate ranging from basalt to andesite. Below is a series of photos from the Soufrière Volcanic Centre showing the (probably) 40 000 year old crater, town of  Soufrière, Sulfer Springs hydrothermal vents and the Diamond falls.

The town of Soufriere nestled in the Qualibou/Soufriere caldera. The peaks behind the town are the
Gros and Petit Piton

Sulphur Springs from a distance

Sulphur Springs bubbling away. The dark colour is caused by iron and sulphur reacting

Little mud eruptions

Diamond Falls, the Botanical Gardens. The water is mineral rich and the colours
are caused by these minerals precipitating out of solution onto the rock face

Grand Piton, I never got close enough to look at the rocks but one source I've found says the pitons are
volcanic cones.

Reference:

Geology of Soufrier

Week of Photographs

Jumping on the week of field photos bandwagon (albeit three days late) here are three photos from around Namibia. Enjoy :)

Desert pavement and Welwitschia, NW Namibia
roughly 10km from Brandberg

Sossusvlei, dried water channels, Southern
Namibia

Fish River Canyon, Southern Namibia. The Rocks above the unconformity are
from the 750-550Ma Nama Supergroup. Below the unconformity
are rocks from the +1000Ma Gordonia subprovince

I shall describe the geology of these areas in future posts. My posting schedule will probably be a cool mid week photo with description and a nice long post on the weekend.

Auckland Volcanic Field

   New Zealand's North Island is underrated as a geological wonder. Yes the South Island has mountains and glaciers and faults and, and, and... But the North Island has volcanoes (Figure 1) including the ‘supervolcano’ under Lake Taupo. Having spent all of my life living in geologically stable places I am rather excited that Auckland, where I'll be for part of December, is only a few hours drive from the Taupo Volcanic Zone. This week I discovered that I won’t actually have to drive very far to reach one of the North Islands many volcanoes. Auckland, it turns out, is sitting in a volcanic field (Figure 2). I shall be able to roll out of bed into the ‘6km ring of death’. How exciting! Scaremongering aside what is the Auckland Volcanic Field all about? 

Figure 1: Locations of the many volcanoes on
New Zealand's North Island

   The Auckland Volcanic Field is related to a hotspot located beneath the North Island. Excess heat from the hot spot caused small volumes (<0.4 km³) of magma to form which rose to the surface resulting in phreatomagmatic and magmatic eruptions [1-3, 5-7]. The field has an areal extent of approximately 360 km² [1-3, 5-8] and to date between 3 to 4 km³ of material, mostly trachybasalt and basanite, has been erupted. Approximately 60% of this material is associated with Rangitoto, the youngest volcano (600-700yrs). It is also one of the few to have erupted more than once [1-8]. Onset of the volcanism is unclear. Radiocarbon and thermoluminescence give maximum ages of 141 ka whereas K-Ar ages are variable, affected by excess Ar, with a maximum age of 250 ka [2]. However, it has been noted that the frequency and magnitude of the Auckland eruptions have increased over time [2, 3, 6, 8 & 9].

Figure 2: Volcanic centres of the Auckland
Volcanic Field. From Ruaumoko website

   There are about 50 eruptive centres in the Auckland Volcanic Field (Figure 2), most of which are monogenetic, one eruption cycle [1-3, 5-8]. Thirty-five of the volcanoes show evidence of phreatomagmatic eruptions, 11 of these are only tuff rings expressed as maars. Twenty-four of the 35 have three phases, an early tuff ring surrounding a later stage scoria cone which is finally breached by lava flows [1 & 5]. There are also small shields (Rangitoto), 'frozen' lava lakes and plugs in present. The volcanic structure is dependent on magma-water interaction and the duration of the eruption [5 & 6]. The majority of the Auckland volcanoes erupted on land therefore, aquifers, lakes and rivers are thought to be the dominant water source for the phreatomagmatic eruptions [1]. The country rock in the north of the field is Mesozoic greywacke and 0-1km thick Miocene flysch while in the south it is Pliocene - Pleistocene sands and silts which are a few metres thick [1]. Aquifers are mainly associated with the Pliocene - Pleistocene sediments but they do occur in the fractured parts of the Miocene flysch [1].

   The lack of exact ages and the monogenetic character of the Auckland Volcanic Field volcanoes means that it is almost impossible to determine patterns [2]. This uncertainty has resulted in various studies and simulations with the aim of mitigating casualties in a future eruption [2 - 4, 6, 8 & 9]. The article which drew my attention to the Auckland Volcanic Field is in response to Sandri, et al (2011) which assesses the risk associated with a future eruption in Auckland. In 2008 the New Zealand government ran 'Exercise Ruaumoko'. This is a similar concept to California's 'Shake Out' except Ruaumoko only involves government agencies. During the Ruaumoko simulation it was assumed that the initial base surge would have a 1-3km run-out [3, 4 & 9]. However, further research combined with modelling done by Sandri, et al (2011) suggest that the run-out is larger. The purpose of the Sandri, et al (2011) was to assess the future risk of the Auckland Volcanic Field and create a volcanic hazard map for potential new vents. The study had two aspects a long-term hazard map and a short-term simulation similar to the Ruaumoko exercise.

Mount Eden. From the Ruaumoko and GSN websites

References:

1: Allen, S.R., Bryner, V.F., Smith, I.E.M. and Ballance, P.F. 1996: Facies analysis of pyroclastic deposits within basaltic tuff-rings of the Auckland Volcanic Field, New Zealand. New Zealand Journal of Geology and Geophysics, pp 309-327

2: Cassidy, J. and Locke, C. A. 2004: Temporally linked volcanic centres in the Auckland Volcanic Field. New Zealand Journal of Geology and Geophysics, pp 287-290

3: Lindsay, J., Marzocchi, W., Jolly., G., Constantinescu, R., Selva, J and Sandri, L. 2010: Towards real-time eruption forecasting in the Auckland Volcanic Field: application of BET-EF during the New Zealand National Disaster Exercise ‘Ruaumoko’. Bull. Volcanology pp 185-204

4: Sandri, L., Jolly, G. and Lindsay, J. 2011: Combining long- and short-term probabilistic volcanic hazard assessment with cost-benefit analysis to support decision making in a volcanic crisis from the Auckland Volcanic Field, New Zealand. Bull. Volcanology pp 1-19

5: Shane, P. and Smith, I., 2000: Geochemical fingerprinting of basaltic tephra deposits in the Auckland Volcanic Field. New Zealand Journal of Geology and Geophysics, pp 569-577

6: Auckland Council webpage

7: Auckland Volcanic Field page and Rangitoto fact sheet, GSN website

8: Wikipedia

9: Exercise Ruaumoko webpage

The papers form the New Zealand Journal of Geology and Geophysics are open access

Testing

Chapman's Peak Drive unconformity between the
Graafwater formation(top) and the Cape Granite (bottom)