World of geology
Tony Waltham (UK)
This article accompanies a book review of Tony Waltham’s book, The World of Geology. The text is broadly taken from the book itself. The world of geology is the world as we know it, that we see and that we live on. It is all about the evolution of the Earth’s crust, the nearly rigid layer less than 100km thick that is the outer shell of our evolving planet. This crust is broken into a few dozen large and small tectonic plates, which move around at rates of a few centimetres a year. Originally known as continental drift when it was first recognised in 1912, this geological activity has been referred to as plate tectonics since its processes began to be properly understood during the 1960s.
A large part of the Earth’s crust is the oceanic floor. The basaltic rock of the slowly moving oceanic plates is continuously being created along plate boundaries that are divergent, and destroyed along those that are convergent. These are the major processes of plate tectonics that keep Planet Earth evolving and alive. The oceanic basalts are similar to those in some types of volcano, but otherwise they remain largely unseen beneath the cold, dark and minimally explored waters of the ocean depths.
The second part of the Earth’s crust is the incomplete upper layer, largely of granitic composition, that forms the continents. Along with the submerged edges known as the continental shelves, these occupy about one third of our planet’s surface. Continental crust cannot be destroyed by subduction along the convergent plate boundaries because it floats on the heavier basalt. Instead, it is continually evolving and reshaping itself by being crumpled, squeezed, distorted, heated, locally melted, eroded and re-deposited. Most of this activity occurs along convergent plate boundaries, and accounts for the complexity and variety of the rocks that are seen across and beneath the Earth’s surface.
Squeezed and crumpled continental rocks form the great mountain chains – the Himalayas, Alps, Andes and more – all developed along convergent plate boundaries. The bulk of these chains consists largely of metamorphic rocks, changed from whatever their origins by heat and pressure in the zones where plates collide. Some of their material is melted at depths and becomes igneous rock, cooling down as huge granite masses or erupting hot from volcanoes; they are matched by the volcanoes and igneous rocks that create the oceanic plates along the divergent boundaries. Rocks of the third super-family are known as sedimentary, and these formed at the Earth’s surface, mainly from swathes of sediment that is composed of debris eroded from any available rock outcrops. Igneous, sedimentary and metamorphic rocks continually evolve from one to another in the great Cycle of Rocks that keeps our planet active, alive and inhabitable.
Almost a sideline within that Cycle, erosion shapes the landscapes of our world. Largely the results of water action, but with significant contributions by ice, wind and gravity, the major processes of erosion are powered by the uplift of the mountain chains. Rivers and glaciers flow from the high ground, and their movement accounts for most of the world’s erosion. Large-scale development of our landscapes is one more component of plate tectonics.
The background of our world is the geology. Superimposed on it are the processes of life. Animal life within the oceans produces shell and skeletal material that is a major component of limestones formed on the seabed before being uplifted to form new land. Plant life largely feeds the animal life, but some of it, in the right deltaic environments, is preserved to form coal. Hence the world of biology is interwoven to be a part of the world of geology. The many cyclic processes of geology fit together, with the result that Planet Earth can evolve continually and survive. This is fortunate, because non-cyclic processes tend to be terminal, and, if they were dominant, would soon render the planet uninhabitable.
Geology has an important fourth dimension: time. Ever since Earth became a recognisable planet about 4,500 million years ago, continuous geological processes have seen it evolve into the complex world that now exists. The oldest known rock dates back 4,000 million years. All the other rocks have been transformed, melted, metamorphosed, eroded or re-deposited in subsequent phases of plate tectonic activity. Most rocks now to be seen in the ground are between 1,000 million and 10 million years old, though some far older rocks do survive and new volcanic rocks are being formed at all times.
Broadly speaking, the rocks are old and the landscapes are young. Erosion shapes a landscape by working away at rocks formed long before and in totally different environments. Timescales do vary. Some parts of interior Australia have changed little in 100 million years, yet the Himalayas have developed into a giant mountain range entirely within about the last ten million years; marine limestone at the top of Mount Everest is just one consequence of the uplift generated by plate convergence. Most landscapes have evolved within the last few million years. Outside the tropics, many of their details can be traced back only to the last of the world’s great glaciations around 15,000 years ago. Even younger, the latest component of geology is the work of mankind. Large-scale mining, major construction projects and vast agricultural schemes are significant components of the planet’s landscapes, and most of these developments have been within the last hundred years or so. Man now counts as a significant geological process.
It all adds up to the vast and ramifying story that is geology. It may be difficult to comprehend the immensity of geological time, because it reaches back so far. But time lies at the heart of geology. It accounts for the many layers of infinitely slow or terrifyingly rapid processes that have formed the rocks and landscapes that are the world as we see it.
The following sections consist of a series of geology vignettes illustrating some of the ideas that have just been introduced above.
Novarupta
In June 1912, Katmai volcano produced a huge ignimbrite flow and its summit collapsed into a caldera, on a scale that is unmatched within recorded history. These cataclysmic events were witnessed by no-one, and indeed nobody could have survived them at close quarters. Perhaps fortunately, Katmai is located in the remote and sparsely inhabited wilderness of the Alaska Peninsula, 450km southwest of Anchorage. A tall ash-cloud was almost incidental to the huge 1912 eruption and this was seen from afar. It covered the few coastal villages, 20km away, with up to three metres of airfall ash, and also dropped 30cm of ash onto the town of Kodiak, 160km away. The eruption had three main bursts of activity and lasted for a total of about 60 hours.
Nobody visited the volcano until 1916, when the Griggs Expedition, from Harvard University, sailed along the coast and then walked into Katmai. They found the yawning caldera that had replaced Katmai’s summit, and assumed that this had been the source of the 15 cubic kilometres of pumice-rich ignimbrite, most of which filled the valley of the Ukak River. This drains the western side of the volcano and is now known as the Valley of Ten Thousand Smokes. Griggs also found the obviously fresh and aptly named Novarupta but thought that it was merely a minor vent. Not until 40 years later did careful mapping reveal that this was the source of the huge ignimbrite flows that filled the Valley of Ten Thousand Smokes.
A shared magma chamber allowed Katmai to collapse when the magma frothed out of Novarupta, some ten kilometres away. Since its formative eruption, Novarupta has been plugged by glassy rhyolite in the shape of a lava dome nearly 400m across. This sits inside a small, asymmetrical ring of late-stage tephra, and both are surrounded by fractures defining their own miniature caldera. In this view, the high volcanic cone is Mount Griggs, an entirely separate volcano that has not erupted in nearly 4,000 years. The skyline on the extreme right in Fig. 1 includes part of the rim of the Katmai caldera. Its jagged profile is all that remains of the tall volcanic cone that was there until that eventful day in 1912.
Gold at the end of the rainbow
Klondike remains one of the most evocative words in any language, even when it is really the name of a minor tributary to the Yukon River in north-western Canada. Its fame dates from the winter of 1897, when it was the target of the world’s greatest-ever gold rush, with thousands of stampeders fighting their way through the unforgiving Arctic terrain. The days of scooping pans full of gold from the alluvium of the Klondike River and its tributary, Bonanza Creek, are now long gone, but were outlasted by a very substantial goldmining industry. Huge floating dredges followed behind the shovels and sluices of the early miners, until 1966, when they too ceased operations.
Nowadays, the gold is won from terrace gravels on the flanks of the two main valleys and the main extraction method is hydraulicking. Powerful monitor pumps blast jets of water onto exposed faces of soil and rock. These wash away the few metres of organic muck cover and then eat into the gold-bearing gravels, while also thawing any surviving permafrost. The loose sediment is washed into a basin, from where it is loaded into sluices, and these are highly efficient at separating out the gold particles, which range down to dust size. The monitors’ water jets are so successful because they also eat into the bedrock of gold-bearing, altered schist, while leaving untouched the hard, unaltered, unmineralised schist beneath.
Most of the Klondike gold is not a classical placer orebody, because the gold was deposited by hydrothermal fluids in both the lower layers of the gravels and also the upper few metres of bedrock. Only some of the gold was reworked by erosion from its original source and re-deposited in the classic alluvial placers that were the target of the original gold rush. This explains why there is no mother-lode within the Klondike catchment; hardly any gold lies in the scatter of hydrothermal veins that lace the bedrock schist. It also explains why a small community of miners continues to extract gold from shallow workings in gravel and schist along the bitterly cold valleys of the Klondike.
Plate divergence at Myvatn
Well-known as one of the few places where a divergent, constructive plate boundary can be seen on a land surface, Iceland boasts various locations where it is claimed, albeit fairly loosely, that a person can stand with one foot on the Eurasian Plate and the other on the North American Plate. The Myvatn area, in north-eastern Iceland, is one of the best divergence sites and its finest feature is the Grjótagjá fissure out in the old lava fields to the east of the lake.
This is a strikingly beautiful open fissure that is constantly increasing in width. Grjótagjá lies along one edge of a small down-faulted block known as a graben. This is itself just one of the multiple dilation features along a zone that is some few kilometres wide and can be identified as the currently active line of the plate boundary. The fissure width is due partly to the plate divergence and partly to rotation of the block of lavas on the right, outwards and downwards into the graben.
The water table at this geothermal hot-spot is about ten metres below surface level, and deep pools within the fissure are fed by hot water that is circulating slowly from depth through dilation fractures in the basalt. When the photograph in Fig. 3 was taken, the pool temperature was around 60°C, but it has since become cooler by nearly 20°C, so that nowadays the fissure does not normally steam in such dramatic style. In the background, the pyroclastic-rich volcanic cone of Hliðarfjallis is no longer active, but the Krafla volcano lies only a little farther to the north along the plate boundary.
Though Grjótagjá may grow to greater widths in future years, its position on the plate boundary means that, ultimately, it is doomed to be obliterated. It is quite likely to be filled with new magma derived from below, to become a dyke, or one element within a dyke swarm. Alternatively, it will be buried beneath basalt lavas emanating from some nearby fissure eruption. But until a new eruptive phase generates either of those situations, Grjótagjá will remain a classic geological site that displays divergent plate tectonics in action.
Old Man of Hoy
Recognised by many, but seen by not quite so many, Scotland’s Old Man of Hoy is a magnificent example of a sea stack. It stands off the west coast of Hoy, the most south-westerly of the Orkney Islands, so that it is easily seen from the Scrabster to Stromness ferry (seen on its northbound journey in Fig, 4). However, the best view is from the cliff top, at the end of a delightful footpath that climbs steadily from the village of Rackwick. The Old Man is 137m tall and survives as a mere remnant of the extensive vertical cliffs that are retreating in the face of Atlantic storm waves eating at their bases. It stands less than a hundred metres from the main cliff, linked by the remains of a rock rib that is now draped in collapse debris.
The stack is formed in strong Hoy Sandstone, a unit within the Old Red Sandstone that was deposited by rivers around 370 million years ago and now forms much of the Orkney Islands. Vertical jointing and almost horizontal bedding in the sandstone ensure the stack’s survival, even when it is so tall and narrow. Critical to the stack’s longevity is a basalt lava that poured from a nearby volcano just before the Hoy sands were deposited in their mountain basin. The lava is only about five metres thick beneath the Old Man, but is now fortuitously exposed at sea level, and its strong rock forms a dark platform that takes the brunt of erosive wave action.
Within the sandstone sequence exposed up the walls of the stack, bedding planes and thin shale bands are etched into almost level ledges, and these make splendid nesting sites for birds. Orkney is justly famous for the tens of thousands of sea birds that nest on many of its spectacular sandstone cliffs. The Old Man of Hoy has relatively few avian residents, but these include northern fulmars, which are notorious for their defensive ploy of vomiting on rock climbers who pass by their nest-sites. Rock ledges are abundant on the Old Man of Hoy, but fulmars and climbers have to compete for the convenience of their level benches.
Underground ice in Lomonosovskaya
Columns and cascades of pure ice can decorate a cave even more beautifully than their counterparts in calcite or gypsum can ever achieve. Lomonosovskaya is one of a number of caves cut into the low hills of the gypsum karst of the Pinega Valley, away to the northeast of Arkhangelsk in arctic Russia. The cave’s spacious galleries and chambers appear to have been formed largely by major flows of meltwater that were generated underneath a warm-based, Quaternary ice-sheet, probably during the final, wasting stage of the Last Glaciation. A series of parallel, neighbouring cave systems was formed in this way, and only small parts of them were subsequently eroded away by further movements of the ice. Today, those same cave passages carry smaller underground streams from open sinkholes, and their flows are augmented by dripping water that percolates through from the forest floor not far above the caves.
The area becomes extremely cold during the famously long Russian winters, but the Pinega karst does lie just outside the arctic permafrost zone, so the ground freezes only down to about a metre deep and remains unfrozen below that. However, freezing-cold air blows through the main cave passages between their open entrances. Groundwater in fissures within the rock above the caves then continues to seep downwards but freezes to form ice almost as soon as it enters the cold cave air. The result is a fabulous array of long icicles, frozen cascades and ceilings frosted with giant ice crystals. At the same time, the cave rivers freeze over, creating underground skating rinks.
Nearly all these ice features are formed anew during each winter and then melt away in the following summer. Every year, the ice is fresh and sparkling, and it makes the caves appear exceptionally beautiful. It is unavoidable that a few muddy footprints are left behind by the tiny number of visitors who get to the caves in winter, but the footprints disappear each summer before a new batch of clean ice is formed during the following winter. These are the ultimate in renewable Earth resources.
Mount Popa
There are two Mount Popas in the highlands southwest of Mandalay in central Myanmar, or Burma as it used to be known. The original Mount Popa is a substantial stratovolcano formed of basaltic lavas and pyroclastic deposits, along with landslide debris from various flank failures. A minor eruption in 442 BC is remembered in local legends that are regarded as reliable, but most of the volcano’s activity was much earlier and well back into the Pleistocene. From afar, it now appears as a rather unremarkable mountain with a forested shield more than ten kilometres across, capped by a small summit cone that overlooks the scar of an ancient lateral collapse.
Near the outer edge of its western flank, another volcanic vent forms a more conspicuous landform, where its resistant core has been exhumed by the erosion of softer rocks from around it. This is the crag of Taung Kalat, which is also widely known as Mount Popa, and overlooks the small town of Popa. It is a popular tourist site, whereas few visitors even notice the true Mount Popa six kilometres away to the east. This smaller Mount Popa is a typical volcanic plug, formed of basaltic andesite. Rising some 200m above the surrounding land, its bare-rock walls are broken crags that stand close to vertical around much of its perimeter.
A remarkably level summit is nearly 100m across and is occupied by a Buddhist monastery with a fine cluster of golden stupas, a small community of guardian monks and huge numbers of macaque monkeys. The approach is a flight of more than 800 steps, which were once maintained by a solitary hermit monk. They have now been rebuilt in marble for the benefit of the many pilgrims and tourists, who have to walk up without wearing shoes. It is debatable whether the smaller Mount Popa of Taung Kalat is a parasitic vent within the larger Mount Popa or is the plug of a totally separate and far older volcano. Either way, the imposing crag is an outstanding geological feature that has become an important part of the local religion and culture.