What keeps Earth scientists busy? The following open questions aim at providing an updated, fully-referenced account of the main current scientific questions, disputes, and challenges in Geoscience, with a focus on the Solid Earth.
1. The Early Earth and the Solar System
Advances such as those occurred in the geochemistry of meteorites lead to new exciting hypotheses about the early stages of our planet, but as usual, answers are outnumbered by the new knowledge gaps:
- 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? [ref.]
- 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]
- 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? Did a "faint young sun" ever warm a "snowball Earth"? [Wired; Marty et al., 2013, Science]
- 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.]
- 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?
- Why does the Earth’s atmosphere contain so much nitrogen, unlike Venus & Mars? [Mikhail & Sverjensky, 2014, Nat.Geosc.]
- 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? 1 or 4 billion years ago? [Biggin et al., 2015; phys.org] And how could the geomagnetic dynamo work in an early Earth much hotter than today?
- 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 supplied by comets or asteroids? When and how did it reach the Earth? [outreach article]
2. 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:
- 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]
- 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
- 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? 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.]
- 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].
- 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]
- 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].
- 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?
- 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]
|Depth distribution of earthqakes in a cold |
and a hot oceanic lithosphere (Ozaki &
3. Tectonic-plate motion, deformation, and volcanismThe 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.
- 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]
- 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.]
- What triggers plate subduction. Computer models consistently require inherited weaknesses to initiate lithospheric subduction. What generated 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]
- Why are orogens curved when seen from space? [Weil & Sussman, 2004, GSASP 383]
- 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)?
- 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].
- 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?
- How/when does deformation propagate from the plate boundaries into plate interiors? [e.g., Cloetingh et al., 2005, QSR]
- 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].
Not only erosion and sediment transport are affected by the
tectonic patterns of deformation, but also the inverse
seems to be true.
- 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.
- 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]
- 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]
- 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]
- 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]
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]
4. Earth's landscape history and present environmentThe 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 tectonics and climate, through a list of 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)|
- Can we use these data to derive past tectonic and climatic conditions? Will we ever know enough about the erosion and transport processes? Was also the stochasticity of meteorological and tectonic events relevant in the resulting landscape? And how much has life contributed to shape the Earth's surface?
- 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. What controls the time-scale of topographic decay? [Egholm, 2013, Nature]
- What are the erosion and transport laws governing the evolution of the Earth’s Surface?[Willenbring et al., Geology, 2013; Wainwright et al., 2015] 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].
- Can we predict sediment production and transport for hazard and scientific purposes? [NAS SP report, 2010; Geology, 2013]
- What mechanisms govern landsliding? Can the risk for landslide be better assessed? [Wieczorek, 1996, "Landslide triggering mechanisms." Landslides: Investigation and mitigation, 247]
- 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].
- 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.
- 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].
- 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.]
- 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]
Smaller-scale patterns at the limit
between river channels and hillslopes.
Credit: Perron Group, MIT
|Tectonic aneurysms purportedly form by|
the acceleration of tectonic rock
exhumation driven by enhanced erosion.
5. OceansWhile 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.
- 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?
- How resilient is the ocean to chemical perturbations?
- 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 theMediterranean after the Messinian salinitycrisis. Authors: Pibernat & Garcia-Castellanos.License: CC-BY-SA. Downloadable here.
- 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.
- 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. Climate, Life, and Earth
|Source: R.E..Rhode, Wikipedia|
- 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?
- Was there ever a snow-ball Earth during the earliest stages of Life on Earth?
- Were there also rivers on Mars, fueling surface transport processes similar to ours? [Hans, 2012] Were large outburst floods comparable to those on Earth?
- 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?
- 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?
- 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] Was this related to the tectonic closure of the Central American Seaway?
- 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?
- 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?
- 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]
- 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?
- 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
- 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.
- 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.
- 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.
- 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?