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Tomography and plate tectonics

Tomography and plate tectonics

The Geodynamics 101 series serves to showcase the diversity of research topics and methods in the geodynamics community in an understandable manner. We welcome all researchers – PhD students to Professors – to introduce their area of expertise in a lighthearted, entertaining manner and touch upon some of the outstanding questions and problems related to their fields. For our first ‘Geodynamics 101’ post for 2019, Assistant Prof. Jonny Wu from the University of Houston explains how to delve into the subduction record via seismic tomography and presents some fascinating 3D workflow images with which to test an identified oceanic slab. 

Jonny Wu, U. Houston

Tomography… wait, isn’t that what happens in your CAT scan? Although the general public might associate tomography with medical imaging, Earth scientists are well aware that ‘seismic tomography’ has enabled us to peer deeper, and with more clarity, into the Earth’s interior (Fig. 1). What are some of the ways we can download and display tomography to inform our scientific discoveries? Why has seismic tomography been a valuable tool for plate reconstructions? And what are some new approaches for incorporating seismic tomography within plate tectonic models?

Figure 1: Tomographic transect across the East Asian mantle under the Eurasian-South China Sea margin, the Philippine Sea and the western Pacific from Wu and Suppe (2018). The displayed tomography is the MITP08 global P-wave model (Li et al., 2008).

Downloading and displaying seismic tomography

Seismic tomography is a technique for imaging the Earth’s interior in 3-D using seismic waves. For complete beginners, IRIS (Incorporated Research Institutions for Seismology) has an excellent introduction that compares seismic tomography to medical CT scans.

A dizzying number of new, high quality seismic tomographic models are being published every year. For example, the IRIS EMC-EarthModels catalogue  currently contains 64 diverse tomographic models that cover most of the Earth, from global to regional scales. From my personal count, at least seven of these models have been added in the past half year – about one new model a month. Aside from the IRIS catalog, a plethora of other tomographic models are also publicly-available from journal data suppositories, personal webpages, or by an e-mail request to the author.

Downloading a tomographic model is just the first step. If one does not have access to custom workflows and scripts to display tomography, consider visiting an online tomography viewer. I have listed a few of these websites at the end of this blog post. Of these websites, a personal favourite of mine is the Hades Underworld Explorer built by Douwe van Hinsbergen and colleagues at Utrecht University, which uses a familiar Google Maps user interface. By simply dragging a left and right pin on the map, a user can display a global tomographic section in real time. The displayed tomographic section can be displayed in either a polar or Cartesian view and exported to a .svg file. Another tool I have found useful are tomographic ‘vote maps’, which provide indications of lower mantle slab imaging robustness by comparison of multiple tomographic models (Shephard et al., 2017). Vote maps can be downloaded from the original paper above or from the SubMachine website (Hosseini et al. (2018); see more in the website list below).

Using tomography for plate tectonic reconstructions

Tomography has played an increasing role in plate tectonic studies over the past decades. A major reason is because classical plate tectonic inputs (e.g. seafloor magnetic anomalies, palaeomagnetism, magmatism, geology) are independent from the seismological inputs for tomographic images. This means that tomography can be used to augment or test classic plate reconstructions in a relatively independent fashion. For example, classical plate tectonic models can be tested by searching tomography for slab-like anomalies below or near predicted subduction zone locations. These ‘conventional’ plate modelling workflows have challenges at convergent margins, however, when the geological record has been significantly destroyed from subduction. In these cases, the plate modeller is forced to describe details of past plate kinematics using an overly sparse geological record.

Figure 2: Tomographic plate modeling workflow proposed by Wu et al. (2016). The final plate model in c) is fully-kinematic and makes testable geological predictions for magmatic histories, terrane paleolatitudes and other geology (e.g. collisions) that can be compared against the remnant geology in d), which are relatively independent.

A ‘tomographic plate modelling’ workflow (Fig. 2) was proposed by Wu et al. (2016) that essentially reversed the conventional plate modelling workflow. In this method, slabs are mapped from tomography and unfolded (i.e. retro-deformed) (Fig. 2a). The unfolded slabs are then populated into a seafloor spreading-based global plate model. Plate motions are assigned in a hierarchical fashion depending on available kinematic constraints (Fig. 2b). The plate modelling will result in either a single unique plate reconstruction, or several families of possible plate models (Fig. 2c). The final plate models (Fig. 2c) are fully-kinematic and make testable geological predictions for magmatic histories, palaeolatitudes and other geological events (e.g. collisions). These predictions can then be systematically compared against remnant geology (Fig. 2d), which are independent from the tomographic inputs (Fig. 2a).

The proposed 3D slab mapping workflow of Wu et al. (2016) assumed that the most robust feature of tomographic slabs is likely the slab center. The slab mapping workflow involved manual picking of a mid-slab ‘curve’ along hundreds (and sometimes thousands!) of variably oriented 2D cross-sections using software GOCAD (Figs. 3a, b). A 3-D triangulated mid-slab surface is then constructed from the mid-slab curves (Fig. 3c). Inspired by 3D seismic interpretation techniques from petroleum geoscience, the tomographic velocities can be extracted along the mid-slab surface for further tectonic analysis (Fig. 3d).


Figure 3: Slab unfolding workflow proposed by Wu et al. (2016) shown for the subducted Ryukyu slab along the northern Philippine Sea plate. The displayed tomography in a), d) and e) is from the MITP08 global P-wave model (Li et al., 2008).

For relatively undeformed upper mantle slabs, a pre-subduction slab size and shape can be estimated by unfolding the mid-slab surface to a spherical Earth model, minimizing distortions and changes to surface area (Fig. 3e). Interestingly, the slab unfolding algorithm can also be applied to shoe design, where there is a need to flatten shoe materials to build cut patterns (Bennis et al., 1991).  The three-dimensional slab mapping within GOCAD allows a self-consistent 3-D Earth model of the mapped slabs to be developed and maintained. This had advantages for East Asia (Wu et al., 2016), where many slabs have apparently subducted in close proximity to each other (Fig. 1).

Web resources for displaying tomography

Hades Underworld Explorer : http://www.atlas-of-the-underworld.org/hades-underworld-explorer/

Seismic Tomography Globe : http://dagik.org/misc/gst/user-guide/index.html

SubMachine : https://www.earth.ox.ac.uk/~smachine/cgi/index.php

 

References

Bennis, C., Vezien, J.-M., Iglesias, G., 1991. Piecewise surface flattening for non-distorted texture mapping. Proceedings of the 18th annual conference on Computer graphics and interactive techniques 25, 237-246.

Hosseini, K. , Matthews, K. J., Sigloch, K. , Shephard, G. E., Domeier, M. and Tsekhmistrenko, M., 2018. SubMachine: Web-Based tools for exploring seismic tomography and other models of Earth's deep interior. Geochemistry, Geophysics, Geosystems, 19. 

Li, C., van der Hilst, R.D., Engdahl, E.R., Burdick, S., 2008. A new global model for P wave speed variations in Earth's mantle. Geochemistry, Geophysics, Geosystems 9, Q05018.

Shephard, G.E., Matthews, K.J., Hosseini, K., Domeier, M., 2017. On the consistency of seismically imaged lower mantle slabs. Scientific Reports 7, 10976.

Wu, J., Suppe, J., 2018. Proto-South China Sea Plate Tectonics Using Subducted Slab Constraints from Tomography. Journal of Earth Science 29, 1304-1318.

Wu, J., Suppe, J., Lu, R., Kanda, R., 2016. Philippine Sea and East Asian plate tectonics since 52 Ma constrained by new subducted slab reconstruction methods. Journal of Geophysical Research: Solid Earth 121, 4670-4741

A belated happy new year!

A belated happy new year!

It was that time of the year again: holidays! Time to take a break from work, relax, and see all your friends and family again. The blog team is no different: we took a break from blogging for a little while as well, so you had to survive the holidays without us! Did you survive Christmas day without one of our blogposts? It must’ve been dreadful, I know, but that’s life! Luckily, we have some good news: we are back with some belated happy new year wishes and wintersport recommendations. We also tried to write limericks. Also also, we discuss chocolate and peppermint. Because we can. Cheers to a good blog year in 2019! 

Iris van Zelst

I once tried to ski down a slope
as friends thought there might be hope
I was covered in snow
from my head to my toe
If they invite me again it’s a ‘nope’

So, as many of you might have guessed, winter sports (or any sports, really) are not entirely my thing. Particularly skiing did not go down well for me. However, as a true Dutch girl, I do really enjoy ice skating and can recommend it thoroughly! However, this year no winter sports at all for me: I flew towards the sun in an effort to actually destress from work (feeble attempt as I brought my laptop, but still, kudos for trying, right?). I hope everyone had a very nice holiday and relaxing break. May all your (academic) wishes come true in 2019!

I also tried cross-country skiing once. That was infinitely better than alpine skiing. It was actually fun!

Grace Shephard

In hemispheric defiance of the “wintersport” edition, I am currently back Down Under where I have replaced the (seemingly eternal) television coverage of cross-country skiing with cricket, swapped a toboggan for me ‘togs’, and exchanged a pull-over for some ‘pluggers.’ I wish all of our blog readers a very happy and safe end to the year that was, and a fabulous start to the next!

What Aussies call swimming-related attire from bit.ly/AusWords

Anne Glerum

This year I spend winter in Berlin,
Where no snow has fallen and the ice is too thin.
So I drink myself heavy,
With hot chocolate and Pfeffi,
And wait for the fresh air of spring!

In the weeks before Christmas, Christmas markets dominate the streets of Berlin. Besides delicious food, they offer mulled wine and, as I discovered this year, hot chocolate with peppermintliqueur. A green version of the liqueur is made by Pfeffi, while a colorless Berlin-made peppermintliqueur is called Berliner Luft. It’s as clear and fresh as Berlin’s air according to the manufacturer. Although the freshness of Berlin’s air is debatable, the combination of chocolate and peppermint is delicious. I wish everybody a fresh start of the New Year with loads of hapiness!

Holiday recommendations – blog break summer 2018

Holiday recommendations – blog break summer 2018

Even dedicated workaholics such as the editors of your EGU GD Blog Team sometimes deserve a break! Let me clarify that by saying ‘an intentional break’ (because uploading every Wednesday is hard!). We will be ‘on holiday’ during August, so there won’t be any new blog posts then. But don’t worry: we will be back stronger than ever in September and we already have a lot of very good blog posts in the pipeline for you. To start the holidays properly and to get you in the holiday spirit as well, the EGU GD Blog Team shares their geodynamical holiday recommendations with you. Enjoy & relax!

Iris van Zelst – Edinburgh

Hutton’s Section with a very young me (in 2012) for scale

Go. To. Edinburgh. Seriously: Edinburgh is the place to be for anyone who has an affinity with the Earth sciences. In this beautiful, historic city, James Hutton – the founder of modern geology, who originated the idea of uniformitarianism – lived and died. Everywhere in the city you can find little reminders indicating this iconic scientist lived there. You could, for example, visit his grave, and hike to his geological section on Edinburgh’s Salisbury Crags. There are also little plaques spread around the city that mark significant James Hutton places and events. The city itself is also steeped in a mix of geology and history: Edinburgh Castle, situated on the impressive volcanic Castle Rock, boasts an 1100-year-old history and towers over the city. Directly across from the castle, connected by the charming Royal Mile is Holyrood Palace, where you can soak up even more history – Mary Queen of Scots lived here for a while. Nearby, there is Holyrood Park where you can find the group of hills that hosts Hutton’s Section and a 350 million year old volcano named Arthur’s Seat. Climb it when the weather is nice and you will have the most amazing view of Edinburgh. The whole park is perfect for day hikes and picknicks.
Even if you (or your travel buddy) are not that into Earth Sciences (or history), Edinburgh has plenty of other attractions. It is the perfect place for book and literature lovers with the large International Book Festival every August and a very rich literary history with iconic writers such as Walter Scott (Ivanhoe), Robert Louis Stevenson (Strange Case of Dr Jekyll and Mr Hyde; Treasure Island), Arthur Conan Doyle (Sherlock Holmes), and – more recently – J. K. Rowling (Harry Potter). Theatre fans will also love Edinburgh, particularly during August when it hosts the Edinburgh Festival Fringe – the largest arts festival in the world.
I totally should’ve booked a trip to Edinburgh this year… Learn from my mistakes and enjoy it in my stead!

The view of Edinburgh when you’re standing on top of Arthur’s Seat: a more than 300 million year old volcano. Pretty epic.
Picture by me in 2012 (also: proof that the weather can be good in Scotland!)

Luca Dal Zilio – Aeolian Islands

My recommendation? I vote for the Aeolian Islands! Smouldering volcanoes, bubbling mud baths and steaming fumaroles make these tiny islands north of Sicily a truly hot destination. This is the best place to practice the joys of “dolce far niente“: eat, sleep, and play. The Aeolian Arc is a volcanic structure, about 200 km long, located on the internal margin of the Calabrian-Peloritan Arc. The arc is formed by seven subaerial volcanic edifices (Alicudi, Filicudi, Salina, Lipari, Vulcano, Panarea, and Stromboli) and by several volcanic seamounts which roughly surround the Marsili Basin. The subduction-related volcanic activity showed the same eastward migration going from the Oligo-Miocene Sardinian Arc to the Pliocene Anchise-Ponza Arc and, at last, to the Pleistocene Aeolian Arc. My favourite island, Stromboli, is one of the few volcanoes on earth displaying continuous eruptive activity over a period longer than a few years or decades. I like Stromboli because it conforms perfectly to one’s childhood idea of a volcano, with its symmetrical, smoking silhouette rising from the sea. Most of this activity is of a very moderate size, consisting of brief and small bursts of glowing lava fragments to heights of rarely more than 150 m above the vents. Occasionally, there are periods of stronger, more continuous activity, with fountaining lasting several hours, violent ejection of blocks and large bombs, and, still more rarely, lava outflow. I can’t quite explain what made it so special to me. It may be because Stromboli itself is an island, and all the time during the hike I enjoyed splendid sea views (with a beer in my hand). It may be the all encompassing experience, where I could see, hear and literally feel the lava explosions. It was simply fantastic.

Credit: Flickr

Anne Glerum – Montenegro

In case you don’t make it to Montenegro/Serbia this summer, it’s fun in winter too. And yes, it’s fun in spring too – there’s snow, mountains and a younger me on a tiny sled. Photo courtesy of Cyriel de Grijs

My geo-holiday-destination: Montenegro!
A summer without beach-time is not a summer to me (already got one beach-day in this year, phew). Being Dutch, a proper holiday also requires some proper mountains – or hills at least. And no trip is complete without cultural and culinary highlights to explore.
Montenegro is a country that ticks all the boxes. Situated along the Adriatic Sea it hosts a score of picture-perfect beaches; quiet or taken over by the jet-set, intimate coves or long stretches of white sand, take your pick.
Further inland, you reach the Dinarides orogenic chain, the product of 150 My of contractional tectonics and later collapse during the Miocene. Traversing the chain into neighboring Serbia will lead you past complete ophiolite sequences, syn-orognic magma intrusions and major detachment zones of the extensional orogenic collapse.
Visit the centuries old fortified coastal cities of Budva or Kotor or one of the many churches and frescoed monasteries spread around the countryside. For more bodily sustenance, enjoy the fresh fish dishes, rich meats or the regional cheeses and yoghurts. Seasonal fruits are eaten for dessert or, even better, turned into wine and rakija. Ehm, why I am not going there again this year – this time in summer?

Not-so-sunny spring view from St. John’s fortress onto Kotor along the Bay of Kotor. Photo courtesy of Cyriel de Grijs

Diogo Lourenço – CIDER Summer School

This year, my favourite geodynamical destination is CIDER 2018! It’s far from holidays… but it’s really cool! For the last three weeks (one week to go), we have been intensely learning about heterogeneity in the Earth, and trying to understand it in an interdisciplinary perspective with contributions from geochemistry, geodynamics, and seismology. Quite an intense schedule and a lot of information to process, but I think we are all learning a lot, and hopefully in the future we will use more constraints coming from other fields into our own work. Oh, and did I mention that it is happening in Santa Barbara? Great Californian weather, beautiful coastal landscapes, barbecues by the beach, and swimming in the ocean, all sprinkled with scientific discussions! Quite the geodynamical destination, no?

Just had to cross the street from the KITP building where the conference is happening to take this photo…

Grace Shephard – Svalbard

Geoscientists are no strangers to travelling to exotic places and many of us take the opportunity to turn a work-related trip into potential holiday scouting. My suggested destination is most probably the northernmost point you can quite easily travel to on this planet – Svalbard.
Svalbard is an Arctic archipelago located around between 74-81°N latitude. It is sometimes confused with Spitsbergen, which is actually the name of the largest island where the main settlements, including Longyearbyen and Barentsburg, are situated. The islands are part of Norwegian sovereignty, though with some interesting taxation and military restrictions (the Svalbard Treaty of 1920 makes for some pretty interesting reading). Svalbard is host to a stream of tourists and scientific researchers year-round, and this week I will travel back to Longyearbyen as a lecturer for an Arctic tectonics, volcanism and geodynamics course at the University Centre in Svalbard (UNIS).
Geologically speaking, Svalbard makes for a very interesting destination. It offers a diverse range of rock ages and types; having experienced orogenic deformation events, widespread magmatism, and extensive sedimentary and glacial processes.
If you’re after a more usual tourist package amongst the draw cards are of course iconic polar bears (though please keep your distance), stumpy reindeer, arctic foxes, whales, birds and special flora. There are many glaciers – in fact around 60% of Svalbard is covered in ice – as well as fjords and mountains, former coal mining settlements… the list goes on. You are even spoilt for choice between midnight sun or midday darkness, depending on the time of year, so prioritise your activities wisely. Plus, did I mention those miles and miles of unvegetated, uninterrupted rock exposures to keep any geology enthusiast happy?… if you’re lucky you might come across some incredible fossil sites.

Itinerary recommendation, tried and tested: Whale watching and fjord cruising to a Russian mining ghost town (Pyramiden) followed by an important sampling of the world’s northernmost brewery.

Remarkable Regions – The Kenya Rift

Remarkable Regions – The Kenya Rift

Every 8 weeks we turn our attention to a Remarkable Region that deserves a spot in the scientific limelight. After looking at several convergent plate boundaries, this week the focus lies on part of a nascent divergent plate boundary: the Kenya Rift. The post is by postdoctoral researcher Anne Glerum of GFZ Potsdam.

Of course an active continental rift is worthy of the title “Remarkable Region”. And naturally I consider my own research area highly interesting. But after seeing it up-close and personal on a recent 10-day trip organized by the University of Potsdam, Roma Tre and the University of Nairobi (stay tuned for the travel log, or read that of the University of Potsdam), I must say, the Kenya Rift is a truly beautiful and fascinating region.

Figure 1. Topography (Amante and Eakins 2009) and kinematic plate boundaries (Sarah D. Stamps based on Bird 2003) of the East African Rift System (EARS). Plate boundary colors schematically indicate the western and eastern branches of the EARS.

Constituting one segment of the 5000 km long East African Rift System (EARS, Fig. 1), the Kenya Rift is host to an amazing landscape, wildlife and people, all of which somehow tie back to continental rifting processes. Although the youngest rifting phase in Kenya commenced in the Miocene, the east African region as a whole has been shaped by rifting episodes since Permian times (Bosworth and Morley 1994). The present active rift system runs from the Afar region in the north all the way south to Mozambique and is split into a western and an eastern branch that run around the Archean Tanzanian Craton (Chorowitz 2005, see Fig. 1). Generally speaking, the western branch is more seismically active, but deprived of magmatism, compared to the eastern branch, of which the Kenya Rift is part (Chorowitz 2005). Three processes characterize the EARS (Burke 1996) as well as the Kenya Rift specifically: normal faulting, volcanism and uplift.

Uplift

The Tanzanian Craton together with the enveloping western and eastern EARS branches constitutes the broad, uplifted area coined the East African Plateau (~1200 m elevation, Strecker 1991; Simiyu and Keller 1997, Fig. 2). The onset of uplift of this plateau can be constrained to the Early Miocene with the help of one of the longest phonolitic lava flows on Earth (> 300 km, Wichura et al. 2010; 2011) and a whale that stranded inland 17 Ma (and was only recently found again after going missing for 30 years, Wichura et al. 2015). Plume-lithosphere interaction is thought responsible for the uplift (e.g. Wichura et al. 2010), although there is disagreement about the continuity of the low seismic velocity anomalies seen in the east African upper mantle and whether they are connected to the lower mantle. For example Ebinger and Sleep (1998), Hansen et al. (2012), Sun et al. (2017) and Torres Acosta et al. (2015) advocate for one East African superplume, while Pik et al. (2006) distinguish separate lower and upper mantle plumes and Davis and Sack (2002) and Halldórsson et al. (2014) consider a lower mantle plume splitting in the upper mantle.

Figure 2. Topography (Amante and Eakins 2009) and fault traces (GEM) of the central EARS. Triangles indicate off-rift volcanoes, dotted grey lines the three segments of the Kenya Rift.

Magmatism and volcanism

The northward motion of Africa over this hot mantle anomaly has been thought the cause of a north-to-south younging trend in the age of the ensuing EARS volcanism and rifting (e.g. Ebinger and Sleep 1998; George et al. 1998; Nyblade and Brazier 2002), although more recent studies arrive at a more spatially disparate and diachronous rifting evolution (Torres Acosta et al. 2015 and references therein). In general, massive emplacement of flood-phonolites preceded the onset of rifting in Kenya around 15 Ma (Torres Acosta et al. 2015). With ongoing rifting, and localization of faulting towards the rift axis, volcanism also migrated towards the center of the rift. Since the Miocene, massive amounts of volcanics have thus been emplaced (144,000-230,000 km3, MacDonald 1994; Wichura et al. 2011). Moreover, dyking also accommodated a significant part of the extension, with 22 to 26 % of the crust in the rift valley being composed of dykes (MacDonald 2012). Not surprisingly, the highlands directly around the rift valley, the Kenya Dome (Fig. 2) formed through a combination of volcanism and uplift (Davis and Slack 2002) with elevations of up to 1900 m.

The composition of rift magmatism is bimodal, showing phonolites and trachytes on the one side and nephelinites and basalt on the other, predominantly resulting from fractional crystallization of a basaltic source. The low viscosity of these magmas allows the young volcanoes in the volcano-tectonic axis to reach significant heights (see Fig. 3; MacDonald 2012). The most impressive volcanoes are to be found outside of the rift however (Fig. 2), with Mnt. Elgon reaching 4321 m and Africa’s highest mountains Mnt. Kenya and Mnt. Kilimanjaro reaching up to 5200 m and 5964 m, respectively (Chorowitz 2005).

Figure 3. View on the crater rim of the 400 ky old Mnt. Longonot volcano in the tectono-magmatic rift axis, at 2560 m asl. Courtesy of Corinna Kallich, GFZ Potsdam.

Normal faulting

The Kenya rift itself is composed of 3 asymmetric segments, distinguished by sharp changes in their orientation (Chorowitz 2005, Fig. 2). The 2300-3000 m high Elgeyo, Mau and Nguruman escarpments result from the steep Miocene east-dipping border faults in the west, while the antithetic border faults on the eastern side formed later during the Pliocene (Strecker et al. 1990). The older border faults formed along preexisting foliation generated by the Mozambique Belt orogeny in the late Proterozoic (Shackleton 1993; Hetzel and Strecker 1994). A change in strike of this foliation from NNE in the northern and southern Kenya rifts to NW determined the change in orientation in the central Kenya rift (Strecker et al. 1990). Consequently, different generations of faults in the northern and southern rift segments run parallel, while in the central segment, the Pleistocene change in extension direction from ENE-WSW/E-W to the present-day WNW-ESE/NW-SE directed extension results in obliquely reactivated border faults and younger, en echelon arranged left-stepping NNE-striking fault zones along the rift axis (Strecker et al. 1990). Extension is transferred between the different zones by coeval normal and strike-slip faulting or dense sets of normal faults.

Figure 4. View of lake Magadi and the Nguruman escarpment. Lake Magadi is a saline, alkaline lake, commercially mined for trona. Courtesy of Corinna Kallich, GFZ Potsdam.

Human evolution

The uplift, volcanism and normal faulting together have set the stage for human and animal evolution. For example, the shift in hoofed mammals from eating predominantly woods to grazing species evidences that the large-scale uplift modified air circulation patterns resulting in aridification and savannah-expansion at the expense of forested areas (Sepulchre et al. 2006; Wichura et al. 2015). The rift basins enabled the formation of large lakes, which were subsequently compartmentalized by tectonic and volcanic morphological barriers (Fig. 4). On the short-term, lake coverage varied due to tectonically induced changes in catchment areas, drainage networks and outlets. Maslin et al. (2014) actually found a correlation between this ephemeral lake coverage and hominin diversity and dispersal. Lake highstands link with the emergence of new species and allowed the spread of hominins north and southward out of east Africa. Remarkable, or what!

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