Archives / 2018 / June

Happy blog birthday!

Happy blog birthday!

If the title and image didn’t tip you off: the EGU Geodynamics blog is celebrating its first anniversary! Almost exactly 1 year ago (okay, so it’s one year and one day, because I wanted to stick to the Wednesday upload schedule), the EGU GD blog was launched! Yay! Applause! Good thing we’re not insanely vain about or proud of this and going to milk this event with a blog post. Oh wait…
Prepare for a lovely blog post where we will be celebrating ourselves (mainly), our guest authors (we’d be lost without them), and our faithful readers (you! unless it’s your first time here… in which case: welcome!).

Since the start of our blog, we have been trying to provide you with a weekly dosis of geodynamics or general-academic-life posts every Wednesday. This didn’t always go according to plan, as we had a few hiccups – most notably in February 2018, when we had an all time low of zero posts. Oops. I will use the fact that I was organising and attending a conference as an excuse. Other – less serious – hiccups occurred at other times when we very sneakily uploaded on a Thursday or a Friday instead of the promised Wednesday. Notwithstanding these hiccups, we still managed to write 58 (!!, excluding this one) posts! That’s even more than one post a week on average!
*pats herself and her team on the back*

What did we write about?

Most of our blog posts were part of our regular features, such as our popular Geodynamics 101 series, which has since been adapted into a successful EGU short course in collaboration with the EGU ECS Geodynamics team. We have also discussed several Remarkable Regions and Peculiar Planets. Several new, exciting papers have been discussed in our News & Views posts and we have reported about several Conferences, such as Nethermod and the EGU GA. Other travel adventures – often with a more geological focus – have been described in our Travel Log. To make you laugh; discuss about the current academic environment; and give you tips on how to make posters, figures, and presentations, we have the popular Wit & Wisdom posts. So, just to summarise: there is a blog post for everyone.

Who (and how many) are you (= our readers)?

We have quite a large amount of readers (hoorah! it would be very sad if no one read these posts. Which might ironically end up to be the case for this post…), with on average a minimum of 100 unique visitors per blog post, but recently nearing 200 or more unique views per blog post (and that is not counting the people that just stay on the homepage of our blog and don’t actually click on the post). Our most popular blog posts include The Rainbow Colour Map (repeatedly) considered harmful with almost 2000(!) unique views, How good were the old forecasts of sea level rise? with more than 500 unique views, and Going with the toroidal mantle flow with almost 400 unique views. Our unique readers come from all corners of the world (see figure below).

Amount of users of our EGU GD blog website per country for the last year

Who are we?

We have a very enthusiastic blog team that has been working round the clock the past year to provide you with all this content! If you still don’t know who we are by now, you can check out our introduction post here. We also recently had a new addition to the blog team, who has already written his first blog post. Together, the five of us hope to keep this blog running for at least another year! However, we wouldn’t be able to provide so many regular geodynamics posts if it weren’t for the outstanding contributions by our many guest authors. They have really proved to be the backbone of this blog, so they deserve a proper shout out! While hunting for all the names of our guest authors in our blog record, I found the following 27 wonderful people who contributed one or more blog posts (because yes, these amazing guest authors sometimes came back for more and insisted on writing multiple posts!):

• Alice Adenis
• Manar Alsaif
• Suzanne Atkins
• Marie Bocher
• Clint Conrad
• Fabio Crameri
• Juliane Dannberg
• Maximilian Döhmann
• Richard Ghail
• Saskia Goes
• Kirstie Haynie
• Matthew Herman
• Charitra Jain
• Agí Királi
• Kristina Kislyakova
• Maurits Metman
• Luke Mondy
• Elvira Mulyukova
• Jessica Munch
• Lena Noack
• Vojtech Patočka
• Jyotirmoy Paul
• Adina Pusok
• Cedric Thieulot
• Anthony Osei Tutu
• Sabin Zahirovic
• Yue Zhao

And now what?

Speaking for the entire blog team, we have had a blast this year and we are very much looking forward to continue with this blog and to bridge outreach, geodynamics, and general academic life. We hope that we can more firmly establish ourselves in the geodynamics community in the coming year and hopefully we will meet and collaborate with many more (recurring) guest authors to continue making this blog a success. Thanks to everyone who has been involved in the blog in any way by either writing or reading it. Cheers to another successful year!

Magma oceans

Magma oceans

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 latest ‘Geodynamics 101’ post we will talk about magma oceans!

When you think about the Earth a long, long time ago, in its late stages of formation, which picture comes to your mind? A lot of volcanoes erupting? Lava running from cracks in rocks? Meteorites falling? Maybe dinosaurs roaming the land? Well, first of all, there was certainly no life, and the dinosaurs only showed up somewhat recently in the history of the Earth (~240 million years ago, while the Earth is ~4500 million years old). In fact, our planet was a much more extreme environment that one might think… Even if frequent, meteorites would only arrive every few thousands of years. Temperatures at the surface were extremely high, as high as 2000 K (1727 °C). Yeah, forget it, too hot even to go to the beach. That, and… there was no land, because a big part of the rocks that form the Earth were molten and our planet was covered in a deep magma ocean. An ocean you definitely don’t want to go in for a swim.

Weather prediction for the Earth, 4.5 billion years ago: really hot, the sort of hot that doesn’t allow you to work on your tan because… well, everything was molten. This photo was in fact taken in the Hawaii Volcanoes National Park, in May 1954 during the eruption of the Kilauea Volcano. Credit: photo by J. P. Eaton, May 31, 1954

What?! A magma ocean?

That’s right, a magma ocean. Think of it as a body of magma, consisting of a mixture of molten or partially-molten rock, volatiles (dissolved gas and gas bubbles) and solids (suspended crystals). The magma behaves as a liquid, which means that the quantity of solids floating in the system is not enough for the solid particles to connect with each other. When that happens, we are left with a crystal mush, where the magma flows inside a porous rock, instead of having solid particles floating in the liquid, as happens in a magma ocean.

A magma ocean is an extreme environment that is vigorously convecting. Several laboratory experiments suggest that the consistency of such systems is something like honey, with uncertainties depending on several factors such as pressure, temperature, the amount of water and the presence of crystals. Yeah, extremely hot molten rocks flowing like honey covering all the surface of the Earth and extending to thousands of kilometres depth. Very, very different from today’s Earth, hum? By definition a magma ocean needs to encompass a substantial fraction of the planet (something like more than 10%), otherwise we can call it a magma pond or a magma lake. No swimming in these as well. Also, no ducks there.

Sounds a bit like science fiction, right?

Yeah… However, the idea has been around for some centuries. In the late XVII century, Gottfried Wilhelm Leibniz proposed that Earth began as a uniform liquid, and differentiated compositionally as it cooled. Others, such as Comte de Buffon, or Lord Kelvin tried to calculate the age of Earth based on the assumption that the Earth was once completely molten and cooled to present-day (and is still cooling).

Ironically, humankind had to go to the Moon in order for the magma ocean hypothesis to evolve to its modern form. Samples returned from the Apollo missions showed that the whiter part of the crust you can see when you look up to the Moon is composed of rocks called anorthosites. In short, and without going into detail, the only mechanism for the formation of these rocks that has attained general acceptance is if they formed through flotation of crystals to the top of a magma ocean on the early Moon. Other rocks recovered from the Moon are also consistent with a fractional solidification of a magma ocean.

If the Moon went through a magma ocean stage it is likely that the Earth underwent it too. Just as the other rocky planets in the inner Solar System: Mercury, Venus and Mars. In fact, various observations point to the occurrence of magma oceans in the early evolution of rocky planets. We know this through the observation of, for example, iron meteorites, or on Earth because we know that a large iron core exists. A magma ocean facilitates the formation of the core early in the history of the planets. Iron is a heavy metal that is thought to have arrived on the early Earth mixed with silicon and silicates. A very hot magma ocean would have melted the rocks and metals and allowed the heavier liquid iron to sink down to the centre of the Earth to form the core.

Thus, it is widely accepted that magma oceans are significant events in the earliest stages of planetary evolution and set the initial conditions for their future evolution. Meaning that understanding how a magma ocean evolved on Earth is key to understand why life exists on our planet.

How did magma oceans form?

The late stage of formation of the Earth and other rocky planets was an energetic process, where violent impacts between protoplanets happened. The energy provided by impacts was the largest energy source to heat and melt the Earth. There were however other sources of heat I should mention such as the conversion of gravitational energy of formation into heat, heat losses from the core at the core-mantle boundary, radioactive decay (mainly short-lived isotopes such as 26Al and 60Fe, which are important in the early stages of planetary formation), electromagnetic induction heating and tidal heating (caused by the proximity to another large object). Basically, a large amount of energy was being provided to the juvenile rocky planets. Thus, again, it is very likely they underwent one or multiple large-scale melting events.

It is probable that in the last giant impact to hit the Earth, the impactor was the size of Mars. Such a big impact had enough energy to melt at least a substantial part of the mantle. Oh, and this impact is also believed to have formed the Moon. Pretty cool, right? The Moon-forming impact was most likely in the origin of the last major, deep and global magma ocean on Earth. But how did the magma ocean evolve in such a way that set up the initial conditions for the evolution of a tectonically active planet, able to sustain water at the surface, an atmosphere, and life?

BOOOMMM! And the Earth was molten! And the Moon was formed! Artist’s depiction of a collision between two planetary bodies. Such an impact between Earth and a Mars-sized object likely formed the Moon. Credit: NASA/JPL-Caltech

How did the magma ocean evolve on Earth?  

Fortunately for us, the magma ocean solidified, and Earth’s conditions are very different from back then. According to the conventional view, as a magma ocean loses heat from the surface, its temperature drops, and it starts crystalizing from the bottom-up. When there are enough crystals present, a solid matrix is formed, and because overall this structure flows slower, the heat loss is also much slower, and thus crystallization is prolonged. Estimates for the transition from a liquid- to a solid-dominated regime range from thousands of years to several hundred million years. However, fully crystallizing the mantle takes billions of years. It might sound a bit strange that the uncertainty in the lifetime of a magma ocean exhibits such large differences. This is due to the fact that we don’t know much about such a system (we never observed one…), and the data available is rare and hard to interpret. These are the sort of uncertainties that we have to deal with when studying the evolution of a magma ocean on Earth:

  • Was an atmosphere present? After the Moon-forming impact the surface temperatures were very high (~2000 K), so the heat transfer from the interior to the surface of the magma ocean is expected to be very rapid. But, these values can be buffered, and timescales of crystallization extended if an atmosphere is present. In the beginning an atmosphere composed of vaporized silicates (literally vaporized rock!) covered the Earth. Afterwards, as the magma ocean cools down, it degasses through bubbles that rise up and burst at the surface to form a thick and insulating steam-dominated atmosphere. Such atmosphere can substantially extend the cooling time by millions of years. As we will see next, this has important implications in the mode of crystallization of a magma ocean. Finally, once sufficient cooling is attained, the atmosphere collapses into a water ocean, and our planet starts to look more like the “Blue Planet” we recognize today.

How does the magma ocean crystallize? In the classic view, shown here, it crystallizes from the bottom-up. Credit: Diogo Lourenço

  • Could the surface be solid? The existence of a solid lid at the surface would slow down the cooling of a magma ocean, and therefore increase its solidification time. However, it is unlikely that such a lid developed in bigger planets because of: (1) the possible existence of an insulating atmosphere that keeps the temperature at the surface high, (2) the major possibility of any solidified material to sink, and (3) any small impactor during the cooling of the magma ocean would disrupt the formed lid. The only likely way to form a relatively complete conductive lid on a planetary-scale magma ocean is by flotation of buoyant minerals to the surface. Like in the Moon, remember? However, this mechanism is likely to occur only on small planets.
  • How do crystals in a magma ocean behave? One of the most important and complex questions regarding the crystallisation of a magma ocean is whether the crystals continue to be suspended in the liquid, or whether they settle, being removed from the liquid magma ocean. This is a challenging problem to address due to the lack of information about the conditions in a magma ocean, and the complexity of the physics of settling versus entrainment in a vigorous fluid flow. In general, longer timescales of magma ocean freezing imply relatively slow convection within the magma ocean and may allow for crystal settling. On the other hand, short timescales imply fast turbulent convection, and hence no crystal settling. On Earth, with all the uncertainties, both styles are acceptable to have operated, but most likely a combination of the two types of solidification existed in a magma ocean, where at first crystals remained entrained, and as the magma ocean cools down crystals started to settle.
  • Full-mantle overturns? Another complexity in the evolution of a magma ocean is that full-mantle overturns are predicted. Two types have been proposed: (1) Thermal overturns, where the (partially-molten) mantle resulting from the solidification of a magma ocean is gravitationally unstable. These instabilities can grow to a solid-state overturning of the cumulate mantle. (2) “Compositional” overturns. As the magma ocean crystallizes, a progressive enrichment of iron and incompatible elements in the magma is expected, and ultimately, this leads to an unstable compositional density stratification because magma ocean cumulates will be denser as the crystallisation front proceeds. A single overturn and a pronounced stratification of the compositionally stratified cumulate layers are expected, which could delay mantle solid convection for billions of years. However, it might be the case that stratification is (partially) erased through progressive mixing due to multiple incremental cumulate overturns, instead of a single one.

How does the magma ocean crystallize? In a more more recent view, shown here, it crystallizes from the middle-out. Iron (Fe) tends to concentrate in the magma. Eventually, when the BMO is almost solid, the last-forming solids are highly iron-enriched, and probably dense enough to become stable against entrainment in the solid mantle. Credit: Diogo Lourenço based on Labrosse et al. (2007)

  • Did the magma ocean really crystallize from the bottom to the top? Alright, here we go for another strange bit. The conventional view presented above is that the magma ocean crystallised from the bottom-up. Makes sense, yes? However, evolution from a completely molten state resulting from the Moon-forming impact might have been different from the conventional view. Recent measurements suggest that it is possible that liquids are denser than solids at lowermost mantle depths, opening the possibility for alternative scenarios of crystallisation of a magma ocean from mid-mantle depths. This would imply that as the magma ocean crystallizes, a shallow and a basal magma ocean (BMO) are formed. The solid layer grows upward and downward, however it grows faster towards the surface because of rapid heat loss to the atmosphere. The BMO’s heat loss is buffered by the mantle, so it crystallizes much slower. Moreover, the basal melt layer is also cooling slower because iron and incompatible trace elements (such as heat-producing elements) would tend to be concentrated in the magma. Eventually, the BMO would almost completely solidify, with the last-forming solids being highly iron-enriched, and probably dense enough to become stable against complete entrainment in the solid mantle. These dense and solid piles could account for some of the observed large-scale features with reduced seismic velocities around the core-mantle boundary we observe today. Understanding whether the Earth had a BMO, and if so, its evolution, is a first-order question that has important implications for planetary thermochemical evolution, for example some reservoirs isolated from the rest of the mantle can be the result from the crystallisation of a BMO. Also, it would have affected the thermal history of the Earth and affected the history of the geodynamo in our planet.

Answering these questions, which are all connected to the chemical and dynamical evolution of a magma ocean, is very important in order to understand the initial conditions for solid-state mantle convection, which on Earth led to plate tectonics and life (but did not in other terrestrial planets). We should keep in mind that magma oceans are common and are still being formed in other galaxies, so maybe planets with magma oceans will one day become targets for direct imaging. Who knows?

I will leave you with some links and further reading below if you really got excited with this text! If you really really got motivated then you can also read my PhD thesis, I’m still recovering from that time, so it will make me happy to know it taught something to other people.

Alright, enough! Hope you enjoyed the read and remember to not swim in magma oceans (you might get arrested)! Bye-bye!

References and further reading:

Abe, Y. (1997). Thermal and chemical evolution of the terrestrial magma ocean. Physics of the Earth and Planetary Interiors, 100:27-39.

Ballmer, M. D., Lourenço, D. L., Hirose, K., Caracas, R., and Nomura, R. (2017). Reconciling magma-ocean crystallization models with the present-day structure of the earth’s mantle. Geochemistry, Geophysics, Geosystems, 18(7):2785–2806.

Elkins-Tanton, L. T. (2012). Magma Oceans in the Inner Solar System. Annual Review of Earth and Planetary Sciences, 40(1):113–139.

Labrosse, S., Hernlund, J. W., and Coltice, N. (2007). A crystallizing dense magma ocean at the base of the Earth’s mantle. Nature, 450(7171):866–869. 

Labrosse, S., Hernlund, J. W., and Hirose, K. (2015). Fractional Melting and Freezing in the Deep Mantle and Implications for the Formation of a Basal Magma Ocean. In The Early Earth Accretion and Differentiation, edited by Badro, J. and Walter, M., Hoboken, NJ. 

Lourenço, D. L. (2017). The influence of melting on the thermo-chemical evolution of rocky planets’ interiors. PhD thesis, ETH Zurich. [link]

Rubie, D. C., Nimmo, F., and Melosh, H. J. (2007). Formation of Earth’s Core, In Treatise on Geophysics, edited by Gerald Schubert, Elsevier, Amsterdam, pages 51-90.

Solomatov, V. S. (2007). Magma Oceans and Primordial Mantle Differentiation, In Treatise on Geophysics, edited by Gerald Schubert, Elsevier, Amsterdam, pages 91-119.

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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.


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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Wichura, H. et al., 2015. P. Natl. Acad. Sci. USA 112 (13), 3910-3915.

The art of the 15-minute talk

The art of the 15-minute talk

We’ve all attended conferences with those dreaded 15-minute talks and we have no problem picking out which talks were amazing and which talks were abysmal. However, when it comes to our own talks, it’s hard to judge them, find out how they can be improved or break away from long-established habits (such as our layout or talking pace). This week, Matthew Herman, postdoc at the Tectonophysics research group at Utrecht University in the Netherlands, guides you towards your best 15-minute talk yet!

At some point in your career as an Earth Scientist, you will hopefully have a chance to give a 15-minute talk at a meeting, a colloquium series, or simply in your lab group. This provides a great opportunity to advertise your hard work to your colleagues in an amount of time that is well within a human attention span. Ultimately, your goal in this talk is to effectively communicate your discovery to your audience. In the process, you get to explain the importance of your field, pose a crucial research question in that field, demonstrate cutting-edge analyses and applications, and, finally, provide an answer to that initial research question, sometimes for the very first time.

Despite all the latent potential for a 15-minute talk to captivate and teach the audience, many of these presentations end up being uninformative. I do not intend this as a judgment regarding the significance or quality of the science. I have seen incomprehensible talks from people whose research is crucial to our understanding of the Earth system. Alternatively, I have seen talks presenting incremental scientific advancements that were truly enlightening. But from all the diverse presentations I have seen, there are common elements that either dramatically improved or reduced my understanding of the subject matter. My aim here is to provide what I think are some of these key characteristics that make up a really excellent talk, so that next time you have the opportunity to present, you will inspire your audience.

I think there are two general things to keep in mind for your 15-minute talk: (a) you have limited time with your audience, and (b) the expertise of your audience can vary a lot. This means that you should design a presentation that fits your extensive understanding into a brief window and tailor the details for the particular audience that will be attending. If this makes it seem like it will take time and effort to construct an effective talk, that is because it is true! Even if you have a well-received publication, simply transferring figures, analyses, and interpretations from the paper into your talk is almost guaranteed to lead to an ineffective presentation – it will probably be too long, too technical, and too difficult to see from the back of the room. If you really want your audience to concentrate on your work for the full 15 minutes, take the time (potentially up to a few weeks) to craft a great talk. And one more thing: you really should practice your talk ahead of time. Actually, I cannot emphasize this point enough: PRACTICE.

Note: If you are short on time right now, I have included a checklist at the end to summarize the main points.

How long?

Imagine: you are in the audience and the end of the talk is not in sight. You shift in your seat uncomfortably as you glance at your watch. The speaker does not appear to notice the amount of time since they started, but you definitely do: 14:30… 15:00… 15:30. Finally, two full minutes after the end of the scheduled time slot, the speaker asks if there are any questions, but of course there is no time for that. Many otherwise good talks have been ruined for me by the presenters going into overtime. All I can now remember about them is by how much they exceeded the final bell. As a speaker, you have 15 minutes – choose a topic and present it in the allotted time frame. In fact, target your talk for 12-13 minutes so your audience can ask questions at the end.

This, and that, and these…

The detailed structure of the talk is flexible, but should probably contain the following items: background/motivation (Why should we, the audience, care?); a research question or hypothesis (What is being tested?); observations, models, and analysis (How is the research question being answered?); and interpretations and conclusions (GIVE US THE ANSWERS!).

i. Background
Try to avoid dwelling on the background for too long. I know many of us (myself included) enjoy pedantically explaining the rich history of our field leading up to the present day. But you do not have the time in a 15-minute talk. As you are constructing your presentation, you should budget no more than 2-3 minutes at the start to establish the context for your research problem. At that point, your audience should be oriented and ready to be amazed by your results.

Example of an introduction/background slide

ii. Research Question
Do not assume that your research question or hypothesis is obvious to everyone. People come to talks for a lot of different reasons; sometimes they are experts in the field, but other times they saw a keyword in your title or abstract, or maybe there were no other interesting sessions. In any case, it is likely that a good percentage of your audience does not know what specifically you are testing if you do not tell them. After setting up the background, verbally or on the screen state your research question or hypothesis.

iii. Observations, Models, and Analysis
This will be the bulk of your presentation. Tailoring your 15-minute talk for your specific audience means you will want to use just the right amount of technical terminology. You should assume some foundational level of knowledge because there is no way to define every term and present the complete theory for your research. But for the most part, I think you should try to minimize technical jargon (particularly uncommon acronyms) in talks. If and when you need to use a term repeatedly, then take 15-30 of your precious seconds to concisely explain the concept, ideally without patronizing or condescending. [Did I mention this was a difficult balance?] Incidentally, explaining a concept has the added benefit of forcing you to understand the concept sufficiently that you can distill its definition into a compact form for your listeners.

The precise minimum level of knowledge you assume for your audience depends on the setting. In the large lecture hall of an international meeting like the EGU General Assembly, the audience may be weighted towards less experience in your field, whereas a special meeting focused on your subject area will likely have a higher percentage of experts.

A related point is that you should avoid all but the most straightforward equations. The reality is that any audience member who does not already understand the equation is not going to understand it from your talk. There is not enough time, and the medium is not amenable to higher level math. Simple equations with a couple variables are okay, but anything with multiple terms, powers, derivatives, etc. are a waste of time.

iv. Interpretations and Conclusions
Honestly, most people are pretty good at this part. This is the most fun and exciting aspect of the talk, plus it means the end is near. A couple minor pieces of advice: (a) make sure you have drawn a clear path from the background through the analyses and into the interpretations, with the common thread being answering your research question; and (b) I think it is best to limit the number of conclusions to 3-4 (consider this in the preliminary design stage of the talk as well!).

Example of a results slide

Good looks matter

I try to follow the advice of the great Jim Henson when it comes to designing the look for my talks: “Simple is good.” I will not harp on making figures, because many other people have discussed how to design good ones. In a nutshell: make them big, use good color schemes and large fonts, and keep them uncluttered. Resist the urge to copy figures straight from papers to your talk. You will probably need to simplify a figure from the published version in order to make it optimal for your talk. Sometimes you just need to design and produce a totally new figure. In fact, making figures is where I spend at least 65% of my time when I am preparing a talk.

In terms of slide layout, use the whole slide. Borders, icons, and backgrounds can be pretty flourishes, but they take up valuable real estate. Every centimeter you use for a border is a centimeter you can no longer use for a making a figure nice and big. And remember there will be people, some with poor eyesight, in the back of the room. As on figures, limit the amount of text. When you do need labels or bullet points, use a classic, simple font (I will scream if I see Comic Sans one more time…) in a large size – I typically use no smaller than 24-point font Helvetica.

Closing remarks

Many of my suggestions are more like guidelines than hard rules. I enjoy seeing creative and innovative presentations. As long as you give yourself enough time to craft an excellent presentation, then take time to practice it in front of friends, it will turn out well. Hopefully we will all see a large collection of great talks in the next few meetings. See you there!

Remember: the goal of the talk is for your audience to understand your science!

• Take time (up to several weeks) to construct your presentation
• Practice before the date of your talk, if possible in front of a test audience

• Target talk length for 12-13 minutes (do not go over 14!)
• Limit background or introductory information to 2-3 minutes
• Explicitly state research question
• Link background, analysis, and interpretations to research question
• Limit conclusions to ≤ 4

Scientific Content
• Choose technical jargon at level appropriate for audience
• Define critical terminology in 30 seconds or less
• Limit acronyms
• Avoid complex equations
• Avoid tables

Visual Content
• Fill space on slide, especially with figures
• Make thin frames to not waste precious room
• Choose large font sizes (≥ 24 pt) in a standard font
• Adjust figures from published version
• Check figure color contrasts (avoid blue/black, yellow/white)
• Use perceptually linear color palettes (no rainbow!)
• Avoid cartoons, animations, and sounds

General Life Advice
• Use common sense (e.g., do not include pictures from the bar in your talk)