Mantle support of topography – a swell idea

Posted on 12 November, 2012 by Metageologist

Why are some bits of the earth higher than others? Finding mountains near plate boundaries is easy to explain – various forms of plate collision cause the crust to thicken and the surface to rise. What about Southern Africa?

Reaching a high point of 3473m, most of Southern Africa is a high plateau. It is not tectonically active – the sedimentary layers in the photo are still horizontal – but they are more than a kilometre higher than they ‘ought’ to be. Why?

African swells and superswells

In recent GSL paper, Stephen Jones, Bryan Lovell and Alistair Crosby argue that the topography of Southern Africa can be explained by mantle convection pushing the crust upwards – that flow of the mantle has a direct influence on surface topography. They go onto argue that this phenomena also controls patterns seen in ancient sedimentary basins.

A spot of theory first. On the scale of 100s of kilometres topography is controlled by the properties of the lithosphere – the Tibetan Plateau can be explained this way. But over scales of 1000s of kilometres the lithosphere isn’t strong enough. On this scale, variations in height can only be explained either because the lithosphere is thicker, or because it is being pushed up from below.

Our authors look at Southern Africa and Antarctica, areas far from convergent plate boundaries. Studying the patterns of topography and gravity, they argue that the main geographic features can be explained by dynamic support from mantle convection. So the Drakensberg mountains (pictured above) are high because they sit above a portion of the mantle that is rising, pushing the plate (and the surface) up. The low-lying Congo basin, further north sits above a descending part of the mantle.

They paint a picture where “thousand-kilometre scale dynamically supported swell topography is the norm for continental regions that are not influenced by subduction zones“, suggesting that areas such as eastern Australia, peninsular India, central USA, east Africa, eastern USA, the Siberian Shield and eastern Siberia are all affected by similar long ‘topographic swells‘. The African swells they study in detail are around 2000km in diameter and between 1.4 to 2.7km in height.

This is a tremendously important idea, suggesting that the broad patterns of the landscape can be explained by slow movements of rock deep under the surface. Patterns of convection within the upper mantle subtly change the shape of the earth we stand on. More, there is talk of an Southern Africa superswell, over 10,000km in size. A long wavelength warping of the earth’s surface that the other swells sit on and which could be partly supported in the lower mantle.

One of the beautifully elegant things about earth sciences is the way we discover that apparently unrelated aspects of the earth are linked. Discovering such a linkage provides insights into both things simultaneously. So, discovering that topography is related to mantle convection gives you new ways of understanding both. For example looking at sedimentary sequences sitting on these African swells allows us to work out how long the swell has existed for (40 million years). This in turn is an insight into the timescales of convection within the mantle hundreds of kilometres under the surface.

To the UK

The concept of dynamic topography is not new to this paper, (there was a conference about it last year), it adds importantly to the debate by quantifying both modern day swells in Africa and ancient swells in sedimentary basins.

The UK, Ireland and associated oceanic basins make up a stable piece of crust with a long sedimentary record. There is a long and successful history of hydrocarbon exploration in the area, so there is lots of data to draw on. Our authors describe four swells from this area in the last 200Ma, active in the Jurassic, early Cretaceous, Eocene and present day (based on Iceland).

Figure 4 from Jones et al 2012. Reproduced under Geol Soc fair use policy

The evidence presented suggests that these ancient swells are similar in dimensions to present day African ones and formed over similar timescales, growing in 5-10Ma and decaying over 20-30Ma.

A fail for Vail?

The effect of these swells on sedimentary sequences is dramatic – a kilometre of uplift turns a sedimentary basin into an area of erosion. Dynamic support is not just of interest to students of the mantle, but also to lovers of sedimentary basins or those who seek the oil and gas found therein.

It’s been long understood that patterns of sedimentation are controlled by tectonic factors (creating holes with extensional or foreland basins) and by sea-level changes. In the 1970s, the work of a group at Exxon led by Peter Vail transformed the study of sedimentary basins. Their new techniques of sequence stratigraphy showed that patterns of sedimentation are best explained by different cycles of changing water depth (relative sea-level). The level of water within a typical sedimentary basin is constantly changing on both high and low frequencies.

Note that this relative sea-level change can be caused either by moving the surface of the earth or moving the surface of the water. Vail ‘chose’ to move the surface of the water and sought to explain local patterns in terms of global “eustatic” sea-level change. During periods of glaciation there is a plausible mechanism for doing this and the model is successful (for example, the Carboniferous of the UK). A longstanding problem is how to create cycles of sea-level if there are no ice-caps to vary in size and periodically change the volume of sea-water.

In his 2010 Presidential Address for the Geological Society of London, Bryan Lovell challenges the Vail hypothesis asserting that “mantle convection provides an alternative, regional, mechanism to eustatic control for explaining medium-frequency to high-frequency sea-level cycles“. Relative sea-level changes can be explained by moving the surface of the earth in a particular area. Over a mantle swell, basins will be shallower or stop being basins even if global sea-level remains constant.

Figure 2 from Lovell 2010. Reproduced under Geol Soc fair use policy

Lovell proposes a mechanism to explain the different frequencies of cycles. Mantle swells, created by convection cells, work over the 10-40 Ma timescale. He proposes that ‘hot blobs’ (grey patches above) move within the wider cells. The blobs cause their own smaller scale (and faster moving) ‘mini-swells’ that explain the higher frequency patterns seen in sediments.

What I like best about the idea of dynamic mantle support of topography is that it is testable. By starting to quantify the time and length scales of mantle swells it becomes possible to recognise them reliably in the stratigraphic record. Changes due to global sea-level variation will be correlatable across many basins whereas mantle swells will not.

Its another way of thinking about the earth, too. As someone living on a tectonically quiescent patch of the earth I’m used to thinking of nothing much happening here until the Atlantic starts to close again. It turns out that “there is no such thing as a stable continental platform” and that a change in the way rocks are flowing deep below could move Britain up or down by a kilometre within as little as 10 million years.

References

Picture of Drakensburgs via Palojono on Flickr under Creative Commons.

Jones, S., Lovell, B., & Crosby, A. (2012). Comparison of modern and geological observations of dynamic support from mantle convection Journal of the Geological Society, 169 (6), 745-758 DOI: 10.1144/jgs2011-118

Lovell, B. (2010). A pulse in the planet: regional control of high-frequency changes in relative sea level by mantle convection Journal of the Geological Society, 167 (4), 637-648 DOI: 10.1144/0016-76492009-127

Charnwood forest – misty traces of ancient landscapes

Posted on 31 October, 2012 by Metageologist

Precambrian rocks are fairly uncommon in England so I jumped at the chance to visit some with the friendly folk of Reading Geological Society. They were found in Charnwood Forest.

The pattern of rocks in England and Wales is broadly one of younging to the south east. A journey from London to Anglesey takes you backwards through time from the ‘Tertiary’ to the PreCambrian visting all the intervening geological periods. Charnwood Forest, near Leicester in the English Midlands is a reminder that this is a simplification. Only 50 miles to the north, in the Peak District, there are many kilometres thickness of Carboniferous sediments. Older rocks are far below. In Charnwood however, there are no Carboniferous rocks – later Triassic sediments sit directly on top of Precambrian rocks.

I haven’t gone all Instagram on you – it was a misty morning at Morley Quarry. The smooth rock face at the bottom is made of Proterozoic (latest Precambrian) volcanic sediments and the reddish lumpy layer is of Triassic age. The line of unconformity is a gap of over 300 million years.

On a geological map, the older rocks appear as little blobs within a red sea of Triassic sediments. This is a sign that an ancient landscape is preserved in this area. We often think of unconformities as planar structures, but this one formed on land and preserves the bumps and jiggles of an ancient landscape. Later folds and faults complicate matters, but the pattern of rocks now is still affected by the ancient landscape.

For example the highest point in the area, Beacon Hill is made up of Proterozoic sediments. It was a hill in Triassic times as well – younger sediments sit in valleys below that are both ancient and modern. There are signs of the Carboniferous landscape here as well. As you might guess from the name, in nearby Coalville there are substantial Carboniferous deposits, sitting between the Proterozoic and the Triassic. The Charnwood forest area during the Carboniferous was likely land close by a water-filled basin filling up with sediment (and coal). All these traces allow us to peer through the mists of time at these ancient landscapes.

To the rocks. Beacon Hill contains good outcrops of the old fine-grained volcanic rocks.

A typical outcrop shows fine beds. The rock is now tilted and hardened by subsequent events. It shows a vertical cleavage as well, formed during the final closure of Iapetus.

The layering is mostly even, but does contain small folds, as you see below.

We also visited Charnwood lodge, which contains coarser volcanic sediments, deposited closer to the site of eruption.

Originally described as volcanic bombs, the fragments here are angular and deposited by debris flows.

Blocks are of andesitic composition, here with some trace of flow banding.

The rocks contain a pronounced cleavage, as you see above.

Cast of the holotype of Charnia masoni, courtesy of Smith609 at en.wikipedia

Although they are Precambrian, some rocks in Charnwood Forest contain fossils, subtle outlines of soft-bodied animals that look like plants. In 1956 a local schoolgirl called Tina Negus found the fossil pictured above. Sadly when she spoke to her school geography teacher, he followed ‘common knowledge’ and flatly denied the possibility that she had found a fossil in these ancient rocks, so nothing came of her discovery. A year later, Roger Mason a local schoolboy, found the same structure. Luckily he spoke to Trevor Ford, a geologist, who recognised it for what it was, a fossil, very rare and similar to others found in Ediacara in Australia. He wrote it up as Charnia masoni. Roger Mason went on to be a professor of Geology, with a focus on metamorphic rocks.

There is a moral to this story – truth in Geology resides in the rocks, not in books.

Ecton – copper, limestone and folds

Posted on 13 September, 2012 by Metageologist

England’s Peak District is made almost entirely from Carboniferous sediments, in a broad anticline. On the outside edges, mid to late Carboniferous rocks are dominated by sandstone, with subsidiary mudstone and coal. The core is an area known as the White Peak where lower Carboniferous limestones form a gentle landscape. It’s a working landscape though, with a long history of mining and quarrying, as we shall see.

Farm building showing sandstone corners and limestone fill.

While the limestone was being deposited, most of Derbyshire was a shallow shelf area. The crust was being gently pulled apart and fractured, so the shelf area was surrounded by deeper basins.

Just like the Bahamas

These seas were at tropical latitudes and, on the shelf, relatively free of terrestrial sediment. These are perfect conditions for making limestone, as numerous organisms create calcium carbonate structures that build up into rock.

The typical fossil assemblage you find in these rocks includes rudose corals, brachiopods and crinoids (ancient sea-lilies). Fossil reefs abound, made of limestones packed with (in fact made entirely from) ancient life..

Massive ‘reef’ limestone, Manifold valley.

Reef limestones are typically ‘massive’ which in a geological context means lacking obvious bedding planes or other structures. The above photo shows a cave, another distinctive feature of limestone areas. This particular cave has been linked to a ‘green chapel’ that features in a famous medieval story.

Crinoid ossicle in grey bioclastic limestone

A closer look at the fossils. The round discs is a crinoid ossicle, plates of calcium carbonate that joined up to make the stems. The rock is made up almost entirely of bits of life, either plants or mostly animals, hence the technical term ‘bioclastic’. The numerous little pin-shaped pieces in this sample are (as best as I can tell) brachiopod spines (if you know better, speak up!).

Bioclastic limestone with ?brachiopod spines

Slab of limestone showing crinoid debris Wooton Mill.

Here’s a worn slab of reef limestone, showing abundant crinoid stems, still in one piece.

These pictures were taken on a recent trip to Derbyshire, where I didn’t spot a decent brachiopod. Conveniently just the other day I spotted one in the gravel of my drive. There are many massive quarries in the Derbyshire limestones and some of it ends up in as builder’s gravel.

Brachiopod in grey limestone piece of gravel from my drive

Copper-bottomed

There is a long history of mining in the Peak District. There are many ore bodies, typically found in veins associated with faulting. Blue John, an beautiful banded form of fluorite was mined for ornamental purposes. Less glamorously, lead, zinc and copper have been mined since Roman times. The mineralisation was deposited by hot fluids that formed as the sediments became buried.

Around Ecton the mining was mainly for Copper. Ecton Copper Mines, were most active from early C17th until 1891. Peak production was 4000 tons in 1786. A major use of copper at this time was covering the bottom on ships in the Royal Navy. Sheets of copper (or zinc copper alloy) were used to protect the wooden ships from the attentions of marine life, particularly Teredo worms (‘shipworm’). The Navy’s ships were cutting edge technology at the time, projecting British power across the globe. They were sometimes used for scientific work too, for example His Majesty’s Ship Beagle that carried Darwin on his famous voyage.

Traces of the mining remain. Found at the bottom of Ecton Hill this is mostly like a sough (pronounced “suff”) a channel to drain the mine shafts higher in the hill.

Evidence of mining, Ecton

Ecton also contains this odd rock.

Lithified scree

This is calcrete, an angular limestone cemented by groundwater CaCO3 during the Pleistocene. Basically its old scree that got stuck together.

large block of lithified screen, Ecton

There’s quite a lot of it.

Deep waters

The Ecton limestone is a thinly bedded limestone, with minor shale layers. It represents deeper water sedimentation, very different from the shallow reef sediments. Equivalent rocks further from the shelf edge are simple mudstones. Further north they form the Upper Bowland Shale, which is a source rock for conventional Irish Sea gas-fields and a potential source of shale gas, extracted by the controversial process called fracking.

Here, very near a shelf, limestone dominates.

Ecton Limestone. Layers of mud and limestone, plus later chert bands

The shiny black beds are chert – silica layers that formed after deposition. Here’s a single bed of limestone. It is ‘graded’, with coarse debris at the bottom and fine at the top and so likely deposited by a turbidity current. Some of the bits of dead animal forming in the shallow water on the shelf got carried into deeper water by a single major event.

Single graded bed of Ecton limestone

Here’s a closer look at a coarse layer. The fine grained limestone has been partially replaced by chert, which makes the white fossils stand out nicely. There are various sections through crinoid ossicles, which here are completely broken up, as you would expect since they have been transported downhill from where they grew.

Detail of chert band showing crinoid stems

Folding in Derbyshire limestones is rare. Apes Tor in Ecton is unusual in showing tight folds. This is most likely because they are layered and easier to fold compared with the nearby massive limestones. Sadly when I was there the outcrops were heavily vegetated. If you look at the next photo and immediately spot the anticline, you are probably a structural geologist.

Detail of folding Apes Tor, Ecton

Ludchurch – sandstone, landslips and a beheading game

Posted on 4 September, 2012 by Metageologist

The ‘Dark Peak’, the land to the south and east of Macclesfield rising up above the Cheshire plain, is a wild place. We are in England though, and even here in the North, things are only mildly wild. This is no wilderness, we are only 25 miles from Manchester, once the ‘workshop of the world’. The area is criss-crossed with (exciting narrow) roads and dotted with farms, but is not totally domesticated or defined by human control. Some 600 years ago its primary purpose was for hunting, as the place name Wildboarclough reminds us. I recently visited a wild place here which is geologically interesting but also spooky, with echoes of medieval violence.

The primary colour of this land is green. Note how even the stone walls are green, because the sandstone rock is covered with moss. The wooded hillside you see is a dip-slope – the slope follows the upper surface of a layer of durable coarse sandstone. The woods contain a magical secret place, which is where we are headed.

Our destination is best reached via Gradbach. This contains an old silk mill, where in the 19th Century water-power was used to drive machinery to weave silk. Such places (more usually involving cotton weaving) were the nursery of the industrial revolution. Water power was soon replaced by coal-fired steam power and production shifted from rivers in the middle of nowhere into big cities such as Manchester. Though far from rivers, the new buildings full of looms were still called mills. The Gradbach mill is now a Youth Hostel, giving children a taste of the wild.

Once across the river and in the woods, everything seems normal. You walk up some muddy paths and you reach a set of small crags. Carboniferous sandstone, coarse grained and with cross bedding – very typical of the area. Note the green algae staining.

Sitting within these woods, almost hidden amongst the undergrowth is an entrance to another world.

You walk down a little way and you are in a deep dark space like nowhere else I’ve ever been. The air is cool, muffled and moist. Every surface is damp and green. The sky above seems a very long way away, peeking down between two great walls of sandstone, covered in ferns, moss and liverworts. A place that is neither underground or above ground, a chasm not a cave, a rocky place full of plants. This is Ludchurch.

Geologically, this is a large landslip, of unknown age. The bedding planes in the rock, the Roaches Grit, are sloping down hill. A weak layer, probably shale, allows movement sideways and the rock parts along vertical joint planes. A chunk of rock 100s of metres long has slid a few metres downhill, creating a hole in the ground that’s filled with plants and mystery.

What did the medieval inhabitants of the land make of it? Without our modern day paths, the most obvious way to encounter Ludchurch is to ride over the top and fall to your death into it. Even now dead sheep can be found at the bottom. Over the years Ludchurch has gathered many myths and stories and inspired a great work of literature.

The Green Knight

Sir Gawain and the Green Knight is a complex story from the late 14th Century. Sir Gawain, of King Arthur’s court, gets involved in a ‘beheading game’ with a mysterious Green Knight (like you do). The climax of the story takes place in a Green Chapel. The story is written in a Cheshire dialect, now extinct but recognisable only a few generations ago. Many scholars have linked the Green Chapel directly to Ludchurch itself. That a local man writing a story of the supernatural would be inspired by Ludchurch makes perfect sense to me.

The Green Knight has been linked to the Green Man, a common carved or painted figure in old English churches. It represents a pagan vegetative deity, a figure who embodies the growth cycle of plants. Such a figure would certainly like Ludchurch, covered with plants as it is. The fact that it is made of Carboniferous rocks is appropriate too. This was a time when the air was rich in oxygen, supporting vigorous forests – a golden time for a Green Man. How appropriate then that a block on the base of Ludchurch shows a plant fossil, partly covered in moss and liverworts.

Spooky postscript

Crag containing ‘Green Chapel’

I visited Ludchurch during a week’s holiday based in the area. On an earlier day I’d visited the Manifold valley in search of interesting rocks to tell you about (watch this space).

On this other trip I photographed a crag of massive reef limestone. When later researching the link between the Green Chapel and Ludchurch, I found a website that decided that a limestone cave at Wetton Mill was a better fit for the Green Chapel. The name sounded familiar – I clicked on their photo and found a picture of the same crag from the same spot!

This is purely a coincidence of course. The nature of things is that wildly improbable coincidences happen all the time. It still made the hair on the back of my neck stand up.

A note on nomenclature. Ludchurch is more popularly known as Lud’s Church, but my father, who holidayed here in the 1940s and has always lived nearby, knows it by the non-possessive version that I’ve used. There are many suggestions as to who Lud was, which means no-one knows, but for sure it is nothing to do with the Luddites. Don’t let anyone tell you otherwise.