The Earth Story

Snapped, offset rocks.

When one continent grinds slowly into another and the rocks in between (whether marine or terrestrial) are slowly crushed and thrust upwards, the resulting tectonic forces create huge networks of faults as the rocks crack, along with subsidiary mountain ranges as the stresses redistribute though the landmasses.

The main ranges are created by giant thrust faults, when huge slabs of rock (called nappes after the French for tablecloth) detach from the underlying layers along a weaker layer of rock and push up over the continents. At roughly right angles to these, subsidiary faults redistribute stress through the surrounding plate as it heaves and groans up a new mountain range and its surrounding landscape pattern. Try shoving your tablecloth across the table as if your hand was a continent and you will quickly see how the stress patterns distribute, even though cloth is a very different material to rock. The Piquiang fault is located in Xinjian province, China, on the fringes of the mighty Tien Shan, and is picked out wonderfully by the varying colours and origins of the lightly tilted sedimentary rocks smiling up at the camera. Far to the south, India is crashing into Asia, and a whole clump of mountain chains across the continent continue to grow. The Himalaya are the best known, but many other chains are growing further inland, including the Hindu Kush, Altai, and Pamirs, petering out into rolling hills and ridges such as those of Burma, Vietnam and northern Thailand.

The fault trends northwest, and is a strike slip one like the San Andreas, where blocks of rock are sliding past each other as Asia shudders. Running for some 70km, at the point where it reveals itself so clearly in the photo, it is crossing some colourful sedimentary rocks that pick it out perfectly through their beautiful 3 km offset. The bottom layers are greyish cream limestones, deposited in shallow Cambrian and Ordovician seas (542-443 million years or Ma). The sea subsided in the Silurian (443-420Ma), becoming a deeper marine oxygen poor environment, represented by the darker grey and green layers of sandstone in the middle of the sandwich. Their colour is due to unoxidised iron, that tells us the chemistry of the depositional environment.

In the following Devonian period (419-359Ma) the land rose above the waves again. The grains in the red sandstones sitting above the marine sediments are covered in a fine patina of rust giving the rocks their colour. They were originally deposited in rivers just as land plants were beginning to take over the shaping of the landscape. Other rocks were deposited on top, but have now been removed by erosion.

Their current geological adventure involves being gently tilted upwards into parallel brightly coloured ridges some 1200 metres high in the Keping Shan thrust belt. Like many mountain chains, these peaks have had multiple rises and falls. They first formed some 300 million years ago when the Tarim block (a chunk of Australia) came crashing in. During the Mesozoic era (252 to 66 million years ago), several chains of volcanic islands collided with Asia, remobilising the faults and pushing new peaks upwards each time, only to be eroded in their turn.

And 80 million years back, the current ongoing collision started, bringing about the third incarnation of this mountain range, as new stresses push through the same old deep geological weak points in the underlying continental structures.

Loz Image credit: NASA.

Further photos of the region from space: http://earthobservatory.nasa.gov/IOTD/view.php?id=82853

The first 3d pterosaur eggs

Fossil finds in China over the past decades have revolutionised out understanding of flying dinosaurs and early birds, but an exceptional discovery of five intact eggs surrounded by dozens of their adult progenitors is providing a wonderful new glimpse into a little understood corner of deep time. The genus was previously unknown, and flitted through the early Cretaceous air during the reign of their dinosaur cousins some 120 million years ago. Up until these, only four confirmed (and badly crushed) pterosaur eggs had ever been found. Hamipterus tianshanensis turned up in the Turpan-Hami basin in Sinkiang, deep in the deserts of northwestern China, abutting the Tien Shan and Pamir ranges. It is one of the world's bleaker places, just south of the dreaded Taklamakan. The surface of the site has only been scraped in the 9 years since excavations started, and many more interesting specimens are expected to appear. The pterosaurs perished in a savage storm way back in deep time, and were rapidly buried in the resulting sediments. The state of preservation of the skeletons proves that they were not carried far after death.

The eggs were soft and bendable with two shells, one hard on the outside, with a membrane within, similar to the eggs of some living snakes. Their parents seem to have buried them on a sandy lake shore, possibly in order to keep them hydrated. The fact that over 40 adults (so far) were buried together in the same storm implies that they hung around in colonies like many modern birds do. The proximity of the gees indicates that they were living near their nesting site, possibly implying parental nurture. Differences in head crests implies that both sexes were present on that long ago stormy day. The adults varied in size from 25cm to 12 metres, pretty hefty in pterosaur terms.

Loz

Image credit: Maurilio Oliveira

http://www.livescience.com/46124-ancient-pterosaur-eggs-found.html http://phys.org/news/2014-06-3d-pterosaur-eggs-parents.html

The gateway of the old world

The laden camel caravans with their cargoes of gemstones, silk and spices linking ancient Egypt, Greece and Rome with the luxuries of China along the silk road passed through; Genghis Khan and his hordes flowed through it to ravage China, the Middle East and Europe; Buddhists, Nestorian Christians, Hindu ascetics and Muslims all traded and cross pollinated ideas through it...This remote site at the border between the Xinjiang region of China and Kazakhstan is truly the most critical pass in Eurasian history.

To the west, the endless steppe, ending in the Fulda gap in Germany (through which the Soviet armies were expected to invade back in my younger days). To the north, the inhospitable taiga of Siberia and Mongolia. In all the other directions, high mountains blocking the way: Pamir, Tien Shan, Hindu Kush, a litany of snow capped peaks. From Manchuria to Afghanistan it was the only true gateway in a 4,500 km wall of mountains created by the Himalayan Orogeny. Herodotus linked it with the legendary Hyperborea where the north boreal wind came from.

Sine humankind's migrations out of Africa, the Dzungarian Gate has been the main pass between the Middle Kingdom and Central Asia. And all these movements of history flowed from geology. The valley has such straight sides because it is a graben, a block of rock bounded by two normal faults that has sunk downwards between the surrounding mountainous blocks.

The block is 10km wide and 46 long, plenty of room for a Mongol horde. Two lakes sit at either end, Alakol to the west and Ebinur to the east. The surrounding mountains are 3,000m high to the north and 4,550 to the south, while the pass is at a mere 450m, giving an idea of how far that block of rock has sunk into the crust. The responsible fault is the Dzungar strike slip fault, where blocks grind past each other, locally pulling the crust apart to sink the graben.

Loz

Image credit: Karen Nyberg

http://spaceflight.nasa.gov/gallery/images/shuttle/sts-85/html/sts085-503-061.html http://www.britannica.com/EBchecked/topic/175433/Dzungarian-Gate

Limpid Lake Kaindy

Formed by an earthquake induced landslide of large limestone blocks back in 1911 that blocked a valley 2000 metres high in Kazakhstan, which the annual rains then filled. This wondrous spot is named Birch Tree Lake and goes down to a maximum depth of 30 metres at varied points along its 300 metre length. Submerged in its clear cold greenish blue waters (because of the limestone content) is a coniferous forest. Another large grove of living trees graces the vale in which it sits in its abode in the northern Tien Shan, a northwards extension of the Hindu Kush formed like the Himalayas by the collision of the Indo-Australian plate with the Eurasian one. The best time of year to visit is the autumn, where the water reflects the snow tipped peaks around it and the birch trees are all turning golden, though ice diving is also practiced during parts of the winter. The area hosts a variety of birdlife and fish, plus varied deer, Siberian ibexes, wild boars and snow foxes.

Loz

Image credit: Exclusivepix/English Russia/ Jonas Satkauskas http://bit.ly/2918lkx http://bit.ly/292ppUz http://bit.ly/29lVtlX http://bit.ly/1oGDZzv

Salt glaciers

Salt deposits result from evaporation of large areas of briny water, with repeated events in the same environment often building up considerable thicknesses. These layers are less dense than normal rock, and more buoyant as well as less resistant to the tectonic strains involved with being emplaced deep within the crust. Even without the squeezing diapirs (rising blobs) of salt are floating up like a lava lamp off the east coast of the USA, creating many of the oil traps in the Gulf of Mexico.

Add in tectonic squishing, for example by building a mountain range in a continental collision, then the salt can be pushed up though faults and erupt onto the surface, where it gently flows downhill like a glacier if the climate is arid enough to preserve them (in wet climates they swiftly dissolve away). They have also been called salt fountains or namakirs, and mark the weak points in the crust through which the salt has pushed to reach the surface.

The first photo was snapped over the Zagros Mountains of southern Iran, where the collision of Arabia with Asia is also producing tectonic havoc. Here many diapirs have breached the surface, and 5km long grey salt glaciers are flowing down the sides of the emergent salt blobs. Their dark tones are due to high clay content.

The other two images were taken in the northern Himalayan region, on the edge of the Tien shan chain. The collision of India with Asia is fracturing and pushing upwards great masses of rock, through whose bounding faults the salt is being squeezed up like toothpaste from a tube. The rocks here are marine sand, mud and siltstones, coloured by varying degrees of iron oxides. They are grinding slowly downhill at a rate of 5mm a year. The second photo shows the edge of one of the glaciers.

These glaciers are small and rare. You need rising salt in the underlying lithology, the diapirs to reach the surface (many stall on the way up) and an extremely arid climate to preserve them long enough to flow. Ancient ones from the Triassic have been found stacked atop each other in Germany.

Loz

Image credits: NASA

http://go.nasa.gov/1GB4jKz http://go.nasa.gov/1NMmw7E http://geology.com/stories/13/salt-glacier/

The first 3d pterosaur eggs Fossil finds in China over the past decades have revolutionised out understanding of flying dinosaurs and early birds, but an exceptional discovery of five intact eggs surrounded by dozens of their adult progenitors is providing a wonderful new glimpse into a little understood corner of deep time. The genus was previously unknown, and flitted through the early Cretaceous air during the reign of their dinosaur cousins some 120 million years ago. Up until these, only four confirmed (and badly crushed) pterosaur eggs had ever been found. Hamipterus tianshanensis turned up in the Turpan-Hami basin in Sinkiang, deep in the deserts of northwestern China, abutting the Tien Shan and Pamir ranges. It is one of the world's bleaker places, just south of the dreaded Taklamakan. The surface of the site has only been scraped in the 9 years since excavations started, and many more interesting specimens are expected to appear. The pterosaurs perished in a savage storm way back in deep time, and were rapidly buried in the resulting sediments. The state of preservation of the skeletons proves that they were not carried far after death. The eggs were soft and bendable with two shells, one hard on the outside, with a membrane within, similar to the eggs of some living snakes. Their parents seem to have buried them on a sandy lake shore, possibly in order to keep them hydrated. The fact that over 40 adults (so far) were buried together in the same storm implies that they hung around in colonies like many modern birds do. The proximity of the gees indicates that they were living near their nesting site, possibly implying parental nurture. Differences in head crests implies that both sexes were present on that long ago stormy day. The adults varied in size from 25cm to 12 metres, pretty hefty in pterosaur terms. Loz Image credit: Maurilio Oliveira http://www.livescience.com/46124-ancient-pterosaur-eggs-found.html http://phys.org/news/2014-06-3d-pterosaur-eggs-parents.html

Your Fault of the Week NASA has again provided us with an image that should find itself in all future structural geology texts: this one of the Piqiang Fault within the Tien Shan mountains of China. Structural geologists in the crowd, this is for you… For those in the “geologic know,” it’s easily recognizable as a strike-slip fault (where two rock masses are sliding against each other along a ~vertically oriented fault plane). The sense of movement on this fault is termed “left-lateral” because whichever side of the fault you’re standing on, the other side appears to be moving to the left. The San Andreas fault of California (http://tinyurl.com/phnga2f) is also a strike-slip fault, but of a completely different sort. The San Andreas marks the movement of two tectonic plates sliding against each other. The Piquiang Fault has formed as a result of two tectonic plates barging into each other head on so to speak, the Indian plate moving north against the Eurasian plate. During this motion, in addition to forming the Himalayas, rock strata become horizontally detached, and are “thrust” atop each other: not surprisingly, faults of this nature are call “thrust faults.” But – all the motions within a thrusting stack of rock do not occur at the same rate of speed. Rocks in the center of the thrust move faster than those that are sort of dragging along the margins. When this happens, the entire pile can be split, or torn apart, by series of vertical fault planes to allow for this differential speed. Each of these faults have strike-slip motion and are called, again not surprisingly, “tear faults.” The Piquiang Fault is a tear fault cutting a thrust stack. That it is left-lateral implies that it is on the left side of the thrust stack since the central parts of the thrust unit would be moving relatively faster than the external parts. Whew. Technical? Sorry, but sometimes the structural geologist in us simply breaks out… Annie R Image from NASA Earth Observatory: http://earthobservatory.nasa.gov/IOTD/view.php?id=82853&src=fb More reading: http://www.mining-journal.com/supplements/mj-kyrgyz-republic-supplement-0313/an-extremely-complex-geology?SQ_DESIGN_NAME=print_friendly http://maps.unomaha.edu/Maher/geo330/sandbox/tina3.html