The Earth Story

Gondwana Cratons A Craton is an ancient piece of earth’s continental crust, one that has been stable for billions of years. The Earth naturally contains elements that give off heat, such as radioactive potassium, thorium, and uranium. When the mantle melts, those elements move into the melt, extracting them from the mantle and moving them upward. Once those elements are concentrated in the crust, the heat can rapidly escape out into space. Once that heat is removed, the crust can become cold and stable, hard to deform.

Tectonic Timeline Out of the solid, rocky worlds in our solar system, only 1 has a crust broken into pieces that change position relative to each other: Earth. We call that motion plate tectonics, and over time it has redistributed and reshaped the continents and oceans again and again. But, the Earth formed extremely hot, as a giant, molten ball of rock out in space, and so there must have been a time when plate tectonics didn’t exist and a point where it started. Since no humans were around 4 billion years ago and none of the other worlds in the solar system show anything like plate tectonics on Earth, how and when tectonics began on Earth remains a geological mystery.

The Legacy of Sir John Rae

The Rae craton forms a large portion of the Northwest Canadian Shield and is comprised mostly of Archean-aged meta-plutonic rocks commonly overlain by Paleoproterozoic meta-volcanic and meta-sedimentary rocks. The Shield forms the heart of North America, spanning from Baffin Island into the continental US. It is built of several Archean (4 to 2.5 billion years old) cratons (i.e. Slave, Rae, Hearne, Superior) which were amalgamated during Paleoproterozoic (2.5 to 1.6 billion years old) collisional events. These rocks are some of the most extensively reworked continental crust on Earth and have an extremely complex metamorphic history, with some parts of the shield having seen more than eight separate tectonic events.

The Rae Craton is named for Sir John Rae (1813 – 1893), one of the first Europeans to explore the area nearly 200 years ago. Rae was born in Orkney (Scotland) and later attended medical school in Edinburgh. In 1833, he travelled to Canada aboard a Hudson’s Bay Company (HBC) ship to Moose Factory, a small outpost on the shores of Hudson’s Bay (Ontario, Canada) where he resided for 10 years. His first chance to lead an arctic expedition came in 1844 when he was selected by Sir George Simpson to survey unexplored areas of the Canadian Arctic coastline for HBC. The harsh landscape and weather made survival in the arctic challenging, and Rae stood out for his eagerness to learn traditional methods of survival from the local Inuit populations, going against the accepted code of conduct for European explorers of the time.

Between 1846 and 1854 Rae made four more Arctic expeditions to what is now Nunavut, travelling more than 10,000 miles on foot or in a small boat, and surveying a staggering 1,800 miles of previously uncharted arctic coastline. Rae and his crew surveyed many previously uncharted areas of the Arctic coast including Committee Bay, Simpson Peninsula, Melville Peninsula, and Pelly Bay. Rae’s most famous discovery came in 1854, when he realized King William Land was not a peninsula but an Island, discovering the Rae Strait and the last link in the famed Northwest Passage.

Controversy surrounding Rae began in 1848, when it became clear that the infamous Northwest Passage expedition lead by Sir John Franklin would not return. In their search for the missing crew, Rae and Sir John Richardson made several overland expeditions. They found little evidence of Franklins crew; only a few pieces of lumber and several artifacts in the possession of the local Inuit. It was the Inuit who told Rae that Franklin’s desperate crew had resorted to cannibalism. The inclusion of this in his 1854 reports lead to his defamation and discreditation, lead by Lady Jane Franklin and Charles Dickens to preserve Franklins name. Later surveys, including that in 2014 which located Franklin’s ship the Erebus, confirmed Rae’s findings.

Rae is regarded as one of the greatest arctic explorers of all time; a testament to both the great lengths of coastline he mapped and his respect for Inuit knowledge and traditions.

References https://bit.ly/2x5QbI1 https://bit.ly/31wtzhz https://bit.ly/2KRwM5C https://bit.ly/2MPvLgO https://arcticreturn.com/john-rae/

Photos D. Melanson (Melville Penninsula). Used with permission. https://bit.ly/2MQr4DG

A deeper look

Lurking beneath all the more recent layers of rocks that drape our continents are their roots, the oldest of which are the remnants of cratons, the first nuclei of the light buoyant surface rocks that float atop the underlying mantle. When they pop into visibility at the surface, the most amazing patterns reveal themselves. These rocks have been repeatedly heated, melted and compressed as the continents moved back and forth, colliding and separating into supercontinents in their Wilson cycles.

Each mountain building event leaves its impression in the folding and melting of the rocks, forming a palimpsest of what geologists dryly term thermal and deformation events. Not a very poetic way, I know, to describe such tremendous forces that shaped the surface of long gone landscapes, but there you have it.

The photo shows boudinage, from the French for blood sausage. The semi molten rocks reacted to stress by straining, sliding around in a plastic manner known as ductile. Large chunks of darker low silica volcanic rock have been broken apart and intermingled with others, revealing something of the pattern of the forces. These rocks were melting while this was happening, and the white veins are the resulting new born granite. As the dark chunks separated, lower pressures resulted on one side (abive the dark mass in the photo), and the grey gneiss flowed into the area, producing the folded structure. The white veins are even more intricately folded, revealing further moments of pressure while the rocks were soft.

The photo was snapped in the Prince Charles mountains of Antarctica, 1 400 km long range whose main surface expression are nunataks (mountain tops poking out of an ice sheet). They are one of the few areas with extensive rock exposure in the whole continent, where a block of rock measuring 600x300km has sunk downwards between two faults forming a graben, a bit like Africa's great rift valley.

The event that shaped these rocks is thought to have occurred around a billion years back, with a second event around 500 million years ago with at least three recognised deformation events.

Loz

Image credit: Adrian F Corvino/Outcropedia

http://www.antarctica.gov.au/living-and-working/stations/mawson/environment/prince-charles-mountains http://www.antarctica.gov.au/living-and-working/stations/mawson/environment/prince-charles-mountains/northern-prince-charles-mountains

Bass Formation

In the Grand Canyon, the oldest rocks are highly metamorphosed schists and gneisses – considered parts of the continental basement. These rocks are only reached in the inner gorge of the Colorado River, where the river has eroded through the entire sedimentary sequence of the western United States. The top of those metamorphic units is a called a nonconformity – it is a rough, erosional surface, created when the ancient metamorphic rocks were exposed at the surface for millions of years. The metamorphic rocks of the Grand Canyon formed about 2-1.7 billion years ago, then 10% of the age of the planet passed before anything else was recorded at this site. Atop those igneous and metamorphic rocks sits a small set of tilted sedimentary rocks known as the Bass Formation or the Bass Limestone. This is the first fairly pristine sedimentary rock we find while walking up the Grand Canyon. The features in the first photo are mostly stromatolites; layers of limestone and other sediments believed to be created as mats of bacteria grew on the floors of shallow, warm oceans. The second photo was taken at the Phantom Ranch Boat Launch deep in the Canyon’s inner gorge. Look around the people – all the rocks are pretty massive, there’s no obvious layering anywhere near the people. The only place where you see layered rocks is atop the ridge in the distance – those rocks are the Bass formation. All of the lower rocks are the igneous and metamorphic rocks of the inner gorge. The Bass Formation is the lowermost member of what is known as the Grand Canyon Supergroup. The Supergroup is a series of sedimentary rocks formed in the late Precambrian, exposed at the bottom of the canyon as a sequence of rocks that have been tilted and dip off to the northeast.

The Bass is a sequence of sedimentary rocks. It contains many layers of dolomite that probably originally formed in the ocean as limestone and then were altered to dolostone after they were buried, with thin layers of sandstone, siltstone, and the occasional coarse grained conglomerate. The stromatolites are fossil bacteria colonies that formed in the ocean This sedimentary sequence indicates that water levels were changing – from rivers that deposited the conglomerates to shallow ocean waters that formed the dolomite. This unit marks the first step in a transgressive sequence – water levels were rising to cover the exposed Vishnu Schist basement rocks, and those rising waters produced this rock unit.

Geologists look for layers of volcanic ash in sedimentary rocks like this one because we are easily able to produce age dates by measuring isotope ratios in those layers. An isotope measurement on this rock gave an age of 1254 million years old. After the big tectonic events that formed the crust of the western US, that’s when the next stage – the sedimentary stage – began.

-JBB

Image credit: NPS https://www.flickr.com/photos/grand_canyon_nps/6705376193/in/photostream/ https://www.flickr.com/photos/grand_canyon_nps/8229636485/

Reference: https://bit.ly/2REaVCr By the way, we’re going to be in the Grand Canyon Supergroup for a while.

Previous articles: https://www.facebook.com/photo.php?fbid=71718732167564 https://www.facebook.com/photo.php?fbid=717596974968016 https://www.facebook.com/photo.php?fbid=718487278212319 https://www.facebook.com/TheEarthStory/posts/718917208169326 https://www.facebook.com/TheEarthStory/posts/719035941490786 https://www.facebook.com/TheEarthStory/posts/719534524774261 https://www.facebook.com/photo.php?fbid=720485404679173

The core of Northern Africa

I found this image while searching yesterday and thought it was a particularly insightful view into the assembly of one of the continents today. Africa as we know it today is mostly surrounded by spreading centers. It isn’t colliding with anything, it isn’t running into other continents, and there is no active subduction beneath Africa. However, it wasn’t always that way, and this simple image tells a portion of that story, at least for the northern ½ of the continent.

Africa was part of the supercontinent Gondwanaland along with India, Australia, Antarctica, and South America. Each of those continents we recognize today was itself an almagamation, created from much more ancient building blocks.

These building blocks are what geoscientists call “Cratons”. They are slices of ancient continental crust, many of them established more than 2 billion years ago. In-between them are the active zones or “mobile belts”, the areas that are deformed when the cratons collide.

2 of the main cratons shown in this image can be traced all the way back to the Archaean, before the rise of oxygen in Earth’s atmosphere. The oldest rocks exposed in the West African craton are 3.5 billion years old and the oldest rocks in the Congo craton are 2.9 billion years old. These cratons started off as something like modern day island arcs, and grew outwards as more things collided with them. Each of them shows more than a billion years of volcanism and metamorphism, but both of them had been established as blocks of somewhat stable, continent-like crust by about 2 billion years ago.

The Saharan block is called a Metacraton because it is thought to have been active at its core much more recently than the other cratons. Much of that area is covered by the Sahara Desert and the thick Nubian Sandstone and by igneous rocks that are about 750-550 million years old. This area could be a recently assembled craton or it could have been a piece of older crust where the assembly of Gondwanaland caused volcanism that overprinted many of the older rocks.

In-between these 3 main blocks are the mobile belts. Sometimes a thin sliver that broke off of a different continent gets trapped in these areas by running into the edge of a growing craton – the Tuareg block and the Oubanguides Belt are examples of these as both contain Precambrian and Archaean rocks but they are stuck in the middle of provinces that were deformed much more recently.

The areas in-between these blocks were deformed and overprinted during what is called the Pan-African Orogeny, from about 750-550 million years ago. These cratons collided at that time to make up the supercontinent Gondwanaland, which then lasted for 300 million years.

There are other cratons in Africa and in ancient Gondwanaland that aren’t shown on this image – just happens to be the one I found. The Kaapvaal craton in South Africa, for example, contains rocks that are at least 3.7 billion years old, some of the oldest continental rocks on Earth.

-JBB

Image credit: http://bit.ly/2wFF4TA

Reference: http://geosphere.gsapubs.org/content/5/1/23.full

Video tour of Hamersley, the Pilbara Craton, Australia - some of the oldest continental crust exposed on Earth.

Scoured

This rock is one of many granitic rocks, often metamorphosed, found on the southwestern coast of Sweden. These rocks are exposed at Holländaröd, Lysekil Municipality. The rocks are Proterozoic aged; isotope dating the rocks shows that they’re about 1.6 billion years old. They formed by subduction along the edge of what is today the Baltic shield; a piece of continental crust that today represents most of Scandinavia and part of the core of Europe. Glaciers that formed over Scandinavia repeatedly during the last million years, including during the last ice age that ended about 10,000 years ago, scoured the lines on the rocks.

-JBB

Image credit: https://commons.wikimedia.org/wiki/File:Smooth_granite_cliff_at_Holländaröd.jpg

References: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/18/081/18081845.pdf https://foreninger.uio.no/ngf/ngt/pdfs/NGT_74_2_114-126.pdf http://www-markinfo.slu.se/eng/soildes/berggr/bgkarta.html

Australia’s remote Pilbara Region is filled with barren landscapes, deserts, grasslands, and some of the oldest Precambrian aged rocks on Earth - the low hills you see are often deeply eroded pieces of ancient, exposed continental crust. Explore this landscape in this clip.

Sudbury impact crater in Canada.

The oval shape in the image below is the last surviving evidence of a angled blow that hit the hard metamorphic rocks of the Canadian craton around in the Proterozoic era. It has been subjected to nearly two billion years of erosion, three mountain building events and a much later smaller impact, and hosts mineral deposits unique in their mode of formation: impact melting and segregation.

The impactor was estimated at 10-15 Km across, about the size of Mt Everest. It left behind a much larger crater than is visible now, estimated to have been 250Km across, and fired ejecta that have been found as far away as Minnesota and Michigan in the USA, over 800Km away. They consist of marine sediments that have been shattered by the resulting earthquakes that are overlain by a layer accretionary lapilli, spheres of ash cemented by condensed vapour that landed on a primeval sea and sank to the bottom. The crater is the second largest on Earth, after the Vredefort impact structure in South Africa.

Back in the 60's and 70's, its origin in an asteroidal cataclysm was disputed, but further analysis has revealed the full panoply of impact evidence, including shatter cones, shocked quartz, an underlying layer of smashed rock (called a breccia) and geodesic sphere shaped carbon molecules called buckyballs that contain heliium with an extraterrestrial isotopic signature. The buckyballs originated in space and survived the impact and subsequent adventures the rocks lived through during their geological history.

This structure also hosts one of the world's largest nickel copper deposits. When the bolide smacked into the (then) volcanic rocks of proto Canada a large lava lake formed. This lake slowly solidified, in a similar pattern to large igneous intrusions such as South Africa's Bushveld complex (as a sandwich of gabbro, the intrusive equivalent of basalt in between layers of norite, olivine rich rock and granophyre, which represent the last remaining silica rich fraction that floated to the top). During this cooling and crystallisation process, metals segregated out, including nickel, copper, gold and platinum group elements. These elements are now mined around the crater's rim from the layers in which they segregated, making Sudbury one of the world's largest sources of nickel and copper.

The impact also created a complex system of hydrothermal vents, which may have provided nutrients and a good environment for early life, made up of communities of extremophile bacteria.

Loz

Image credit: NASA

http://www.thelivingmoon.com/43ancients/02files/Earth_Images_09.html#Sudbury http://stardust.jpl.nasa.gov/news/bucky.html http://www.mngs.umn.edu/meteoriteimpact.pdf

Mantle degassing in the Rockies.

As you bathe in Colorado's hot springs, your legs may be being tickled by bubbles containing mantle gas, seeping through the whole thickness of the continent to the surface. Their presence far from plate boundaries, where most of the interaction between mantle and surface occurs, has been a surprise to tectonicists.

These mantle gases are found mixed with groundwater, and their rise through the crust is linked to basement penetrating faults, providing a direct connection for mantle and crustal fluids to mix. The fluids then emerge as hot springs or travertine marble terraces, generally linked with the same extensional tectonics that produced the faults. Mantle contributions were found to increase with seismic activity, and lower again afterwards. The team has named them xenowhiffs, foreign gases in groundwater, after the mantle xenoliths brought up in volcanic eruptions.

The results came from measuring the ratios of helium and carbon dioxide across 25 hot springs and a variety of travertine deposits throughout the Rockies. Helium has two main isotopes, called He3 (light) and He4 (heavy). The heavy isotope is created by radioactive decay of metals like uranium, that are mostly concentrated in the crust. The lighter one is a relic of the Earth's formation. Most of the crust's light helium has been lost to space during post Archaean crustal reworking, so the only He3 left on Earth is believed to lie in deep mantle reservoirs. A high proportion of He3 relative to He4 if held to indicate mantle contributions, whether found in lavas, volcanic gases or groundwater. These results were then compared with seismic tomographic maps of earthquake wave speeds under the western USA to compare mantle gases with magma reservoirs. Low wave speeds tend to indicate the presence of magma.

Over a quarter of the helium in the groundwater, and a whopping three quarters of the CO2 was found to come from a mantle source, transported by fluids that had moved though more than 50Km of continental crust. Due to the complex tectonic history, ascribing exact sources for these gases is hard. Some may come from the recent extension related magmatism in the basin and range province, some may be due to the subduction of the Farallon plate and the Laramide Orogeny, which hydrated the mantle, lowering its melting point, resulting in magma that is visible on seismic tomograms. This mountain building event also heated and thinned the continental keel. Another possible contribution may be the consequences of part of the subducting Farallon slab breaking off and sinking into the deeper mantle as its minerals pass through the eclogite pressure transition and metamorphose to denser forms.

The pervasive presence of mantle gases in groundwater throughout the western USA has several implications. Carbon sequestration plans may not mitigate atmospheric CO2 as efficiently as we hope, because if mantle gases can pass through the whole thickness of the continent via faults and seepage, and the whole west is seismically active, then no formation may be truly safe from leakage events if used to store man made CO2. The ratios suggest that the mantle gases are taking less than three million years to pass through the crust. The relative contributions of fault and seepage based transport will have to be estimated, though with careful assessment procedures sequestration should work, since gases have been injected into reservoirs with favourable geometry without signs of significant leakage. It must also be emphasised that the amount of CO2 degassing from the mantle is minute relative to anthropogenic emissions, so like volcanoes, this process cannot be blamed for climate change.

A second problem is related to contamination of water resources, since these mantle gases bring dissolved metals such as arsenic and uranium with them, that then end up in the groundwater. Some of the springs had higher levels of arsenic than permitted for human or agricultural use. As the easily mined aquifers of the west, the Oglala in particular, become depleted, people are going to be seeking water elsewhere, and the risk that this metal rich water will be used is high. This will either require costly separation of the metals, or accepting the type of serious health problems that plague Bangladesh, ever since NGO's dug wells into an arsenic bearing aquifer back in the 70's and 80's.

Tectonically, these results imply that the entire mantle beneath the western USA is degassing heterogeneously. Similar results have been found near the San Andreas fault and other places in California and Nevada. By contrast, springs above the thick craton of the Canadian Shield show no similar geochemical connection between mantle and surface. This type of research offers vital clues to the nature of the complex interactions between asthenosphere, mantle and crust, in a tectonically complex area of extension, accreted terranes and recent orogenies.

Loz

Image credit for Pinkerton Hot Spring: E.R. Pape, via state geothermal data project.

http://www.geosociety.org/gsatoday/archive/15/12/pdf/i1052-5173-15-12-4.pdf

Original paper, paywall access: http://gsabulletin.gsapubs.org/content/121/7-8/1034.abstract

Clearwater Lakes impact craters, Canada.

In a recent post on Manicougan impact crater (http://tinyurl.com/d4osf4n), we discussed the possibility that it might have been one of several near simultaneous strikes worldwide. They are thought to result from the breakdown of an asteroid passing within the Earth's Roche limit, when gravity wells disrupt approaching bodies. Alternately it could have been an asteroid with satellites, since several have now been found by astronomers. Here we share a certified double impact crater, also from Canada (near Hudson Bay), and dating from 290 million years ago (in the early Permian).

The lakes are 32 and 22 km wide, and like Manicougan very well preserved. One might wonder whether some divine agency preferentially selected Canada for heavy bombardment, but their apparent abundance in that country is an artefact of geology. The rocks that make up the Canadian Craton (or shield) are very resistant Archaean (and younger) gneisses, so impacts there get weathered more slowly than in other lithologies. In this image the tectonic lineation of the gneiss is clearly visible running from upper left to bottom right of the image. This results from multiple compression events imposing a grain on the rock. Most of the older preserved impacts are found in these ancient continental nuclei, since they are all composed of these tough high grade metamorphic rocks.

The name comes from the exceptional clarity of their waters. A ring of islands reveals an impact ripple in the rock, and this raised central area is filled with impact melt. The two bolides simultaneous impact must have been an impressive last sight. It remains unknown whether they were an orbiting pair, or one asteroid that was broken up as it approached our world. While rare on earth, multiple related craters are more common on geologically dead planets, since these are not resurfaced by plate tectonic and erosional processes.

Loz

Image credit: NASA.

http://www.universetoday.com/19616/earths-10-most-impressive-impact-craters/

http://www.lpi.usra.edu/publications/slidesets/craters/slide_29.html

http://webecoist.momtastic.com/2010/01/12/heavy-hitters-earths-most-amazing-meteor-craters/

http://passc.net/EarthImpactDatabase/clearwaterwest.html

Earth’s early impacts

Today, asteroids large enough to cause global effects only hit the planet Earth once during a period of tens or hundreds of millions of years. However, when the solar system was younger, there was much more debris around and it was likely stirred up by occasional big motions of the giant planets. Although geologic processes have removed many of the craters from these early impacts, scientists are gradually finding evidence like these rocks for their existence.

These rocks come from one of many drill cores taken in Western Australia. Australia preserves some of the Earth’s oldest crust in an area called the Pilbara Craton. This craton is filled with large granitic plutons and metamorphosed sedimentary rocks stretched around them, a remnant of the formation of Earth’s earliest continental crust. The sedimentary rocks are over 3 billion years old and therefore also preserve chemical remnants of the environment when life was first evolving and diversifying. To investigate these processes, scientists have taken a series of drill cores through the sediments that give them intact stratigraphic sequences through these Archean aged rocks.

This rock comes from one of those drill cores. Its age is constrained by dating of other rocks in this drill core to be 3.46 billion years old. The small grains you see are spherules, small bits of molten rock splashed out by an impact. They are found in a drill core through different layers of chert, a silica-rich rock that is common in Archean sediments; the different colors come from different parts of the surrounding unit.

To establish that these were produced when an asteroid hit Earth, a team of researchers led by a scientist at Geoscience Australia characterized their textures and chemistry. They found elevated levels of platinum group elements, things like platinum, palladium, gold, and iridium, in addition to other elements like nickel and sulfur that are rare on Earth (they’re locked in the core) but abundant in meteorites. They also found angular particles (impact debris) and occasionally spherules that were broken and injected with quartz, illustrating the violence of their formation.

This spherule layer is the 17th impact spherule layer known from either South Africa or Australia. Those impacts occurred over hundreds of millions of years, so life would have had time to recover from them, but discovering so many of these layers shows that Earth was truly much more of a shooting gallery in the Archaean and surviving this environment would have been difficult for many species.

Interpreting this history is also complicated for geologists; because the rocks are metamorphosed and difficult to date precisely, some of those impacts could be correlated between the continents, but that’s hard to tell. For example, a few meters above the main spherule-bearing layer there is a second layer containing spherules. Because the rocks are metamorphosed, the scientists can’t tell if that layer actually represents a second impact or instead just a sedimentary reworking of spherules from the lower layer.

-JBB

Image credit and reference: Glickson et al., 2016 (Precambrian Research) http://bit.ly/25aGpBm

References: http://www.psu.edu/dept/spacegrant/ABDP/ http://bit.ly/23VUQmW

Hampi Boulders

This site is found in India, in the central/southern portion of the peninsula. The rocks are truly ancient; they are metamorphosed granites that are part of the Dharwar Craton. This craton is a stable piece of crust assembled about 2.7 billion years ago after a series of collisions and outbreaks of volcanism. Similar gneisses outcrop in a number of places in the central part of India.

This granitic outcrop has hosted a variety of cities during the past millennia; four dynasties ruled a kingdom called Vijayanagar, with its capitol city found here. After the fall of that kingdom, the area lay neglected, leaving a variety of structures and artifacts from hundreds of years ago still in tact in the area.

-JBB

Image credit: http://bit.ly/1S1u8a0

References: http://hampi.in/history-of-vijayanagara http://bit.ly/1V9oyoI http://bit.ly/1qOUEcH