Open Questions in Geoscience

[Are you a geoscientist yourself? This list is in perpetual progress, so if you have additions, please send them to d.g.c(AT)csic.es with a citation] 

What keeps Earth scientists busy? This list of unsolved problems aims at providing an updated, fully-referenced account of the main current scientific questions, disputes, paradoxes, and challenges in Geoscience, with a special focus on the Solid Earth.

1. The Early Earth and how did the Solar System determine our present world. How special was our planet's initial setting?

Advances such as those achieved in meteorite geochemistry lead to new exciting hypotheses about the early stages of our planet, but as usual, answers are outnumbered by the new knowledge gaps: 

1. How did the Earth and other planets form? Were planets formed in situ? Or are orbital changes relatively frequent? What determined the different deep layering of the solar planets? [McKinnon, 2012, Science on Mercury]  What is the precise composition of their interior and, in particular, what is the 3rd element present in the Earth's core that explains its anomalous density? [BBC article] 
2. Was there ever a collision of the Earth with another planet Theia, giving birth to our satellite? [Canup, 2013, Science] There is compelling evidence, such as the similar primordial composition of the Earth and the Moon [Mastrobuono-Battisti et al., 2015, Nature], and the measures of a shorter duration of the Earth's rotation and lunar month in the past, pointing to a Moon much closer to Earth during the early stages of the Solar System. [Williams, CSPG Spec. Pubs., 1991]
3. What is the long-term heat balance of Earth? How did its internal temperature decay since it formed by accretion of chondrites? How abundant are radiogenic elements in the interior? The Sun was 30% fainter 3 billion yrs back and hence the equilibrium temperature of the Earth should have been around -110 C. So how did the "faint young Sun" coexist with liquid water and life more than 3 billion years ago? 
4. Evidence for glacial deposits at tropical latitudes suggest that the Earth has been through several periods of full, planetary-scale glaciation, the first of them starting about 2.5 billion years ago. How could the faint Sun warm this snowball Earth up (snowball/faint sun paradox)? [Wired; Marty et al., 2013, Science]
5. How inherent to planetary evolution is the development of a magnetic field, a key requisite for life development? [Zimmer, 2005, Science, Elkins-Tanton, 2013, Nature]. When did our magnetic field form? Short after 4 billion years ago as seemingly recorded in the oldest rocks? [Biggin et al., 2016; Phys.org] or 1 billion years ago when models predict that the Earth's solid core was formed? [Sci. News, 2015] And how could the geomagnetic dynamo work in an early Earth much hotter than today?
6. What made plate tectonics a dominant process only on Earth? [outreach paper] Water? [Bouffard, 2013; Regenauer-Lieb et al., 2001, Science]. What makes an exoplanet a good candidate to host plate tectonics? A large size or the presence of water? [Korenaga, 2010, Astroph. J. Lett.; van Heck & Tackley, 2011, EPSL] How did the Earth cool down before plate tectonics?[Moore & Webb, 2013, Nature]. What are the feedbacks between climate, water, weathering, and plate tectonics that lead to habitable conditions? [Foley, 2015, Astroph. J.]
7. Both the crust and the oceans seem to have existed already by 4.3-4.4 Ga, very soon after the formation of the sun, 4.57 Ga (based on computer simulations and on dating of chondrules and calcium-aluminum inclusions in meteorites; Bonanno et al., 2002) and also shortly after the formation of the Earth 4.54 Ga. How could liquid water be hosted in such a hot, primitive Earth? [Wilde et al., 2001, Nature; Mojzsis et al., 2001, Nature]
8. Was the Earth's crust formed during the early stages of its evolution or is it the result of a gradual distillation of the mantle that continues today along with crustal recycling? Is the crust still growing or does its recycling at subduction zones compensate for crust formation at mid-ocean ridges and other volcanic areas?
9. Why is plate tectonics episodic? What drives the formation of supercontinents in the hundred/thousand time scales? What causes their eventual breakup? Why does this occur in pulses? [Voice et al., 2011, The Journal of Geology]. 

10. (1)What diagnostic criteria can we use to recognize Wilson cycles? Which was the first supercontinent and the first Wilson Cycle? When was plate tectonics established on early Earth with its greater mantle temperatures?

    (2) What triggers supercontinent dispersal? Are the driving forces located in the Earth’s mantle or provided by subduction zones? How does subduction initiate?

    (3) How frequent are switches in subduction zone polarity? How do new subduction zones form near their predecessor? What controls the localization of deformation along the same plate boundaries over geological time? How can we recognize when inheritance is controlled by crust or mantle?

    (4) Is continental lithosphere constructed from coherent domains that are rheologically anisotropic and how is that anisotropy developed? And how does it determine the geometry of rifted margins? [Wilson et al., 2019].

11. Why does the Earth’s atmosphere contain so much nitrogen, unlike Venus & Mars? [Mikhail & Sverjensky, 2014, Nat.Geosc.]
12. Earth-like planets (in terms of mass and distance to their stars) are now known to be abundant in our galaxy (two out of three stars may have one [e.g., Cassan et al., 2012, Nature]), but how many of them develop widespread durable water chemistry? How much of our water was supplied by comets or asteroids after the Earth's formation? [outreach article]

2. What is going on in the Earth’s Interior?

Our rock-sampling reach is limited to the upper 12 km of the Earth's crust, but the keys to extend our knowledge often lay far deeper than that. Indirect measurements such as seismic wave tomography, together with geodynamic and petrological modeling, become crucial: 

1. What are the chemical composition and mechanical properties of rocks in the Earth’s mantle at the extreme pressure and temperature they undergo? As planets age and cool off, their internal and surface processes coevolve, chemically and mechanically, shaping also the atmospheric composition. Therefore this question has direct implications for our understanding of the environmental evolution of the Earth. [Kerr, 2005, Science]
2. What are the dynamic processes in the Earth interior that accommodate and fuel plate tectonics? As seismometers spread more evenly over the planet's surface, the seismic imaging of the interior will rapidly improve, providing a detailed distribution of seismic wave velocity. Simultaneously, lab-based mineral physics must better constrain what these mechanical wave velocities tell us about the hot, deep rocks of uncertain composition in the mantle. Only then will computer models be able to test the proposed geodynamic models by trying to fit quantitatively those data and other geophysical observations such as gravity variations. [Kerr, 2005, Science]
   

   

   Computer model based on [Glatzmaier &
   Roberts, 1995] of the magnetic field lines.
   The dense clusters of lines are within the
   Earth's core

3. Sedimentary and volcanic rocks have recorded changes of the magnetic field throughout the evolution of the Earth. What causes the sudden reversals of the paleomagnetic field? How does the geomagnetic field link to the iron convection properties at the deep Earth? What controls the multidecadal changes in magnetic field? Is the inner core moving relative to the rest of the planet? [Song & Richards, 1996); Yang & Song, 2023] Or inversely, what can we learn about the mechanical behavior of the materials at those depths from the geomagnetic field? [more context in Buffett, 2012, Nature] Are the magnetic reversals too fast to be related to core dynamics? [pop.sci.1] [pop.sci.2] [Biggin et al., 2012, Nat. Geosc.] Could their frequency be related to the distribution of tectonic plates? [Petrelis et al., 2011, GRL]. What causes superchrons (periods longer than 10 Myr without magnetic reversals)? Something internal to the core, or induced externally by the mantle/subducting slabs? Was the geomagnetic field always dipolar, or was it more asymmetric in the past? [pop.sci.]
4. Are intraplate hotspots really made by deep sources of uprising materials (mantle plumes) coming from the deepest Earth's mantle? Or can they be explained by shallower convection? [e.g., Morgan, 1971, Nature; Yellowstone case: Fouch, 2012, Geology].
5. What is the history of and what controls the excursions of the rotation pole relative to the surface geography, known as true polar wander? [Creveling et al., 2012, Nature]
6. What are the properties of deep rocks? How can we translate the heterogeneity in density, seismic wave velocity, or electromagnetic resistivity presently observed in the mantle and the lithosphere into variations of the mineralogical composition? And how do these measures relate to the dynamics of the Earth and to key mechanical properties such as the viscosity? [Faccenna & Becker, 210, Nature]. 



Depth distribution of earthqakes in a cold
and a hot oceanic lithosphere (Ozaki &
Hirth, 2016)

7. What are the causes and the huge magma sources for Large Igneous Provinces and massive flood basalts such as the Columbia River Basalts? What brings over a million km3 of magma to the surface in just a few million years? Where are the chambers where such huge volumes accumulate? 
8. What is the depth distribution of the deformation properties (rheology) in the oceanic lithosphere and why are the earthquakes distributed bimodally in old plates and unimodally in hot plates? [Ozaki & Hirth, 2016; and a pop.sci. article]

3. How tectonic-plate motions turn into seismicity and volcanism? Will we predict these hazards?

The successful adoption of the plate tectonics paradigm has lead to a myriad of new questions about its limits and about the lessons for risk mitigation.

Velocity of the earth's surface at the Indian-Asian collision,

from GPS data (arrows relative to Eurasia). Blue star 

indicates the 2008 China earthquake.

Source: CALTECH

1. What is the relative importance of the forces driving plate tectonics: slab pull, slab suction, mantle drag, and ridge push? [e.g., Conrad & Lithgow-Bertelloni, 2004; Negredo et al., GRL, 2004, vs. van Benthem & Govers, JGR, 2010]. What is the force balance and the geochemical cycle in subduction zones? [Emry et al., 2014, JGR] How much water (and how deep) penetrates into the mantle? [Ranero et al., 2003, Nature] How much subcontinental erosion takes place under subduction areas? [Ranero et al., 2000, Nature]
2. What determines the formation of cratons like Africa and how stable are they? What triggers extension at the East African Rift on a continent that is largely surrounded by spreading centers and, therefore, expected to be mainly in compression? What is the role of shallow mantle edge-driven convection? What is the formation mechanism of intracratonic sedimentary basins, such as the Taoudeni Basin (West African Craton), the Congo Basin, or the Duero Basin in N Iberia? [van Hinsbergen, 2011, The Formation and Evolution of Africa, Geol-Soc. Londonsp. pub 357, 378pp.]
3. What triggers plate subduction: Computer models consistently require inherited weaknesses to initiate lithospheric subduction, but what caused them in the real world?[Regenauer-Lieb et al., 2001, Science]. What happens after the collision of two continents? Does continental collision diminish the rate of plate subduction, as suggested by the slab-pull paradigm? [Alvarez, EPSL, 2010] How frequent are the processes of mantle delamination and slab break-off? What determines their occurrence? [Magni et al., GRL, 2013; Durezt & Gerya, Tectonoph., 2013]
4. Why are orogens curved when seen from space? [Weil & Sussman, 2004, GSASP 383]
5. How does the long-term deformation derived from paleomagnetism and structural geology link quantitatively to the present-day motions derived from GPS and from neotectonic patterns of crustal deformation? [Calais et al., EPSL, 2003] How do the last two relate to each other? [Wang et al., 2012, Nature] Can we learn from regional structure of the crust/lithosphere from that link (or viceversa)? 
6. Are plate interiors moving in steady-state linear motion? How rigid are these and why/when did they deform? [Davis et al., (2005) doi:10.1038/nature04781, and Wernicke & Davis, (2010) doi:10.1785/gssrl.81.5.694]. 
7. How is the relative motion between continents accommodated in diffuse plate boundaries? (eg., the Iberian/African plate boundary). What determines the (a)seismicity of a plate contact? 
8. How/when does deformation propagate from the plate boundaries into plate interiors? [e.g., Cloetingh et al., 2005, QSR] 
9. What is the rheological stratification of the continental lithosphere: like a jelly sandwich? Or rather like a creme brulée? [Burov & Watts, 2006]. Is the lower crust ductile? Is strength concentrated at the uppermost mantle? Or just the other way around? [e.g., McKenzie et al., 2000, JGR; Jackson, 2002, GSA Today; Handy & Brun, 2004; and a nice recent post]. 

10. 

    Not only erosion and sediment transport are affected by the
    tectonic patterns of deformation, but also the inverse
    seems to be true. 

    Does the climate-controlled erosion and surface transport of sediment modify the patterns of tectonic deformation? Does vigorous erosion cause localized deformation in the core of mountain belts and prevent the propagation of tectonic shortening into the undeformed forelands? Does the deposition of sediment on the flank of mountains stop the frontal advance of the orogen? Is there any field evidence for these effects predicted from computer models? [Philip Allen's blog] [Willett, 1999, JGR; Whipple, 2009; Garcia-Castellanos, EPSL, 2007]
11. Can earthquakes be predicted? [Heki, 2011, GRL; Freed, 2012, Nat.Geosc.]. How far away can they be mechanically triggered? [Tibi et al., 2003, Nature]. Little is known about how faults form and when do they reactivate [ex.6], and even worse, there seems to be no clear pathway to solve this problem in the near future. Unexpected breakthroughs needed. 
12. How frequent are volcanic flank failures, what are their causes [Hürlimann & Martí, 2000, GRL; Masson et al., 2002, Earth Sci. Rev.], how to assess their risk, and how large are the potential tsunamis they generate? [Ramalho et al., Science, 2015] 



Cape Verde volcanic flank collapse, potentially

responsible for large boulders moved by a

tsunami more than 200 m above

sea level [Ramalho et al., Science, 2015]

13. How can the prediction of volcanic eruptions be improved? What determines the rates of magma accumulation in the chamber and what mechanisms make magmas eruptible? [ex.7][ex.7b]
14. The elevation of the continents does not match everywhere the predictions from the classical principle of isostasy for the Earth's outer rigid layer (the lithosphere). This deviation is known as dynamic topography, by opposition to isostatic topography. But what are the mechanisms responsible? Can we learn about the mantle dynamics by estimating dynamic topography? [ref.1] Can the hidden loads needed to explain the accumulation of sediment next to orogens (foreland basins) be linked to these dynamic forces? [Busby & Azor, 2012, Wiley]
15. How do land-forming processes react to climate change at a variety of scales, ranging from the Milankovitch cycles to the late Cenozoic cooling of the Earth? Is there a feedback from erosion into climate at these time scales, through the Carbon cycle and the weathering of silicates, for example? What is the role of the surface uplift and erosion of Tibet on the drawdown of atmospheric CO2 over the Cenozoic? [Garzione, 2008, Geology]

4. The gap between the Earth's landscape history and the present environmental changes

The shape of the planet's solid surface, its topography, is the key feature that connects many of the disciplines within Earth science, probably because it is the feature that most affects our daily life. It is today common wisdom that landscape forms from a complex interplay between plate tectonics and climate, through mechanical, chemical, and biological processes acting at the surface of the Solid Earth. Topographic data is becoming now available at resolutions finer than a few meters, and the sedimentary record is also being archived at unprecedented rates. But:

Drainage patterns in Yarlung Tsangpo River, China (NASA)

1. Can we use these data to derive past tectonic and climatic conditions? Will we ever know enough about the erosion and transport processes? Is the stochasticity of meteorological and tectonic events overprinting the landscape? And how much has life contributed to shape the Earth's surface? 
2. Can classical geomorphological concepts such as 'peneplanation' or 'retrogressive erosion' be understood quantitatively? [Lavé, 2015, Nature] Old mountain ranges such as the Appalachian or the Urals seem to retain relief for > 10^8 years, while fluvial valleys under the Antarctica are preserved under moving ice of kilometric thickness since the Neogene [Rose et al., 2013, EPSL]. What controls the time-scale of topographic decay? [Egholm, 2013, Nature]
3. What causes the formation of plateaus in mountain regions? [Sinclair, 2016, Nature]. Are these flat areas inherited from older flat regions, then uplifted and incised? Or are they formed at high elevation by drainage captures (river piracy)?[Whipple, 2017, Geology]. 
4. What are the erosion and transport laws governing the evolution of the Earth’s Surface?[Willenbring et al., Geology, 2013; Wainwright et al., 2015] What are the more relevant mechanisms for fluvial erosion: pluckin? abrasion?[Beer et al., 2017, JGR-ES] Rivers transport sediment particles that are at the same time the tools for erosion but also the shield protecting the bedrock. How important is this double role of sediment for the evolution of landscapes? [Sklar & Dietrich, Geology, 2001 (tools and cover effect); Cowie et al., Geology, 2008 (a field example)]. What causes low-relief areas amidst mountain ranges? [Sinclair, 2017].
5. Can we predict sediment production and transport for hazard and scientific purposes? [NAS SP report, 2010; Geology, 2013]
6. What mechanisms govern landsliding? Can the risk for landslide be better assessed? [Wieczorek, 1996, "Landslide triggering mechanisms." Landslides: Investigation and mitigation, 247]
7. What do the preserved 4D patterns of sediment flow tell us from the past of the Earth? Is it possible to quantitatively link past climatic and tectonic records to the present landforms? Is it possible to separate the signals of both processes? [e.g., Armitage et al., 2011, Nature Geosc]. 
8. Can we differentiate changes in the tectonic and climate regimes as recorded in sediment stratigraphy? Some think both signals are indeed distinguishable [Armitage et al., 2011, Nat.Geo.]. Others (Jerolmack & Paola, 2010, GRL], argue that the dynamics intrinsic to the sediment transport system can be 'noisy' enough to drown out any signal of an external forcing. 



Tectonic aneurysms purportedly form by
the acceleration of tectonic rock
exhumation driven by enhanced erosion.

9. Does surface erosion draw hot rock towards the Earth’s surface? Do tectonic folds grow preferentially where rivers cut down through them, causing 'river anticlines' (upwarping of the crust with a deep transverse incision)? [Simpson, 2004, Geology]. When/how do these anticlines develop into extreme cases known as tectonic aneurysms? [Zeitler et al., 2001, GSA today].
10. How do the patterns of river networks form? [eg. Devauchelle et al., 2012, PNAS; Perron et al., 2012, Nature]. And what information about the past do these patterns contain? Can we quantitatively reconstruct past ecology or climate from old river patterns? [e.g., Hartley et al., 2010, J. Sedim. Res.]
11. Do we need a new geological epoch called Anthropocene? When do the Homo Sapiens start to have a significant impact on the Earth System? 8000 BP?[Ruddiman, 2003, Climatic Change]; 2000 BP? [Scalenghe, 2011, The Holocene]; 1850 AD? [Crutzen & Steffen, 2003]

5. How do the Oceans interact with the Solid Earth?

While playing fundamental roles in Earth dynamics, the oceans have remained largely unknown until WWII, when seafloor exploration became common. Today, the mapping of the seafloor relief, magnetic anomalies, gravity, etc, and the drilling of the oceanic sediment have become routine, and deep direct exploration of the bottom of the oceans is revealing an entire new world to oceanographers and marine geologists.

1. What are the processes shaping the submarine relief and performing submarine sediment transport? What mechanisms control the formation of the turbiditic currents that deliver sediment from the continental platforms to the deepest parts of the ocean? How episodic are these? What determines the formation of submarine mud volcanos? 
2. How resilient is the ocean to chemical perturbations? 
3. How do the geographical changes imposed by tectonics modify the ocean circulation and then then global climate? For example during the closure of the Panama Isthmus: [Haug et al., 1998, Nature]
   

   

   Artistic view of the refill of the

   Mediterranean after the Messinian salinity

   crisis. Author: Pibernat & G-C.

   License: CC-BY-SA. Downloadable here.

4. How did the largest salt deposits in the world form? In the last decade, hydrocarbon reservoirs have been found underneath such salt deposits of kilometric thickness. 
5. In particular, what was the time evolution of the salt giant  accumulated in the Mediterranean during the so called Messinian Salinity Crisis? Was the Mediterranean truly desiccated? What were the effects on climate and biology, and what can we learn from these? [e.g., Hsu, 1983; Clauzon et al., Geology, 1996; Krijgsman et al, 1999, Nature; Garcia-Castellanos & Villaseñor, Nature, 2011]. Were the normal marine conditions truly reestablished by the largest flood documented on Earth, 5.3 million years ago? [Garcia-Castellanos et al., 2009, Nature]

6. What makes our planet inhabitable? How did Climate, Life, and the Solid Earth interact in the past?

Source: R.E..Rhode, Wikipedia

The geological record shows that climate is relatively stable over tectonic time-scales whereas it undergoes abrupt changes in periods ranging from decades to hundreds of thousand years. Past periods when the planet underwent extreme climate conditions may help to understand the mechanisms behind that behavior and its significance for the evolution of the Solid Earth.

1. What caused the largest carbon isotope changes on Earth? [Grotzinger et al., 2011, Nat. Geosc.] How does Earth’s climate system respond to high levels of atmospheric CO2? 
2. And when did life appear on Earth anyway? Are the oldest claimed fossils actually biogenic? Can biomorphic shapes produced by abiotic chemistry trick us about our earliest fossil record? [e.g., Zhang et al., 2018, Nat. Comm.]
3. Was there ever a snow-ball Earth during the earliest stages of Life on Earth? How do ice sheets and sea level respond to a warming climate? 
4. Were there also rivers on Mars, fueling surface transport processes similar to ours? [Hans, 2012] Were large outburst floods comparable to those on Earth? 
5. Nearly all animal phyla appear abruptly in the fossil record during the "Cambrian radiation" of life, coinciding with the first big marine transgression of the Phanerozoic [Dalziel, 2014, Geology]. What was the cause?
6. What were the causes for the mass extinctions at the K-T boundary, the Permian-Triassic or the Late Triassic? Massive volcanism? Meteorites? Microbes? [recent papers: Rampino & Kaiho, 2012, Geology, Lindström et al., 2012, Geology, Chen & Benton, 2012, Nat. Geosc., Keller et al., 2004, PNAS; Rothman et al., 2014, PNAS]. How fast did they occur? How did the geological processes shape the subsequent recovery of biodiversity? And how can these episodes help in understanding the more recent Quaternary extinctions?
7. Why has climate warmed up over the Cenozoic? [Sternai et al., 2019, JGR]. What triggered the extreme climatic variability during the Quaternary and the roughly coeval acceleration in continental erosion and sediment delivery to the margins? [Peizhen, Molnar et al., 2001, Nature; Herman et al., 2013, Nature; Schildgen et al., 2019, Nature]. Was it related to changes in volcanism rates? [Sternai et al., 2019, JGR]. Was it related to the tectonic closure of the Central American Seaway? 
8. How do climate changes translate quantitatively into polar ice volume and sea level changes? What other mechanisms control sea level changes beyond ice accumulation? And most remarkably: what are the causes for third-order quasi-cyclic sea level changes (~0.5 to 3 million years in duration) [Cloetingh & Haq, 2015, Science]. What controls regional patterns of precipitation, such as those associated with monsoons or El Niño, and what are their significance in the long-term?
9. Do we really know the current ice cycle balance in the poles? Estimations are based on models of ice deformation models that seem very imperfect [Bons et al., 2018, GRL].
10. What caused the world-wide Quaternary extinction(s)? Human expansion and megafaunal hunting? Climate Change? [Cohen et al., 2015, Geology]. How sensitive are ecosystems and biodiversity to environmental change? Was the large-fauna extinction ~13,000 yr ago a result of the Younger Dryas climatic event? Was the Younger Dryas caused by an extraterrestrial impact? [ex.11, ex.12] Or may it be linked to the outburst of Lake Agassiz? 
11. How relevant are subsurface microorganisms to earth dynamics by controlling soil formation and the methane cycle? What are the origin, composition, and global significance of deep subseafloor communities? What are the limits of underground life and how much biomass does the underground life amount to? How diverse is it? How much did this life contribute to the geochemical evolution of the Earth? [post]
12. The atmosphere is shaped by the presence of life, a powerful chemical force. The Earth’s evolution seems to affect the evolution of life [see the Cambrian explosion of animal life, for instance; plus this recent paper on that] and inversely, life controls climate [another recent one]. When did this feedback start?[Nutman, et al., 2016, Nature] Is it possible to quantify these links to make reliable predictions that allow filling the data gaps or assessing the chances for extraterrestrial life?
13. How much of the present climate change is anthropogenic? How will growing emissions from a growing global population with a growing consumption impact on climate? 

7. Broader open questions

1. Many of the questions above are related to the extreme diversity of spatial scales of Earth processes. Direct observation (by sampling or remotely) is mostly limited to a thin layer around the solid surface of the Earth, and physical experimentation is limited to the pressures of the uppermost layers of the planet. Many processes including plate tectonics are known to be driven by the nature of the materials that make up the planet interiors, down to the smallest atomic scales, as thought for instance for the trigger of earthquakes. Answers may arrive via new devices and analytical tools working at the high pressures and temperatures of Earth’s interior.
2. Time scales also pose a problem to know the mechanical and chemical properties of Earth's materials. Partly because we deal with time scales in very different orders of magnitude while we are limited to make observations from the present. But also because scaling the rates of lab experiments (e.g., mineral physics) or analogue models (e.g., sandbox experiments) with the corresponding geological scenarios is not always convincing. 
3. Implementing Episodicity in Gradualism: For historical reasons, geology has generally underestimated the role of episodicity in nature. However, there is a growing view that exceptional events and stochasticity have a relevant role in many of Earth's subsystems. An example for this is the preeminence of large flooding events during erosion, sedimentation, and the evolution of landscape, and the importance of upscaling flood stochasticity into sediment transport models [eg., Sadler, 1981, J. Geol.; Jerolmack & Sadler, 2007, JGR; Finnegan et al., 2014; Baynes et al., 2015, PNAS; Lague, 2010, JGR]. Climate variability at all time-scales has been already mentioned above. Even plate tectonics episodicity may be multi-scaled (during the Archean at least, [ref]).  4D hyperscale data sets in geomorphology are increasingly showing the limits of smooth-process approaches. Future understanding of the Earth will benefit from incorporating the full frequency spectrum (the episodicity) in our models of natural processes, rather than systematically approaching these as gradual phenomena. 
4. Computer models tell whether the complexity of nature can be explained by the interplay between simple processes, but: how can we further model the Earth as a complex system of complex systems? And when can we expect ‘compact’ explanations? 

8. Fundamentals for Earth exploration in an era of new resource needs

[work in progress]

General background, sources and resources:

Note: the specific references given above for each open question are sometimes just examples and may not be the best representative. Furthermore, the list is surely biased towards Solid Earth and Geodynamics, my own fields. The following sources and general references can give you an alternative perspective on the subject.

• 2020, GSA The Future for Geoscience in the Context of Emerging Climate Disruption.
• BESR (2008). Origin and Evolution of Earth: Research Questions for a Changing Planet The National Academies Press DOI: 10.1002/gj.1188
• Science 125th anniv. compilation.
• NAS report Origin and evolution of Earth [pdf] [html]
• NAS report on surface processes.
• Earth Magazine 2012 article.
• IODP 2013-2023 programme.
• Origin and Evolution of Earth. National Research Council. 2008. Origin and Evolution of Earth: Research Questions for a Changing Planet. Washington, DC: The National Academies Press. https://doi.org/10.17226/12161.
• Grand Challenges in the field of Earth science. Frontiers volume, 2015, ISBN: 978-2-88919-711-8.
• Matt Hall has a 2018 webinar and a blog reviewing the history of open questions in geoscience. He is interested in and collecting unsolved problems in applied geophysics.
• Wikipedia has an incomplete list of unsolved problems in Earth Science waiting for your contributions!