ABSTRACT We propose a new, sunken continent beneath the North Atlantic Ocean that we name Icelandia. It may comprise blocks of full-thickness continental lithosphere or extended, magma-inflated continental layers that form hybrid continental-oceanic lithosphere. It underlies the Greenland-Iceland-Faroe Ridge and the Jan Mayen microplate complex, covering an area of ~600,000 km 2 . It is contiguous with the Faroe Plateau and known parts of the submarine continental rifted margin offshore Britain. If these are included in a “Greater Icelandia,” the entire area is ~1,000,000 km 2 in size. The existence of Icelandia needs to be tested. Candidate approaches include magnetotelluric surveying in Iceland; ultralong, full-crust-penetrating reflection profiling along the length of the Greenland-Iceland-Faroe Ridge; dating zircons collected in Iceland; deep drilling; and reappraisal of the geology of Iceland. Some of these methods could be applied to other candidate sunken continents that are common in the oceans.

ABSTRACT The Pliocene–Quaternary igneous record of the Tyrrhenian Sea area features a surprisingly large range of compositions from subalkaline to ultra-alkaline and from ultrabasic to acid. These rocks, emplaced within the basin and along its margins, are characterized by strongly SiO 2 -undersaturated and CaO-rich to strongly SiO 2 -oversaturated and peraluminous compositions, with sodic to ultrapotassic alkaline and tholeiitic to calc-alkaline and high-K calc-alkaline affinities. We focused on the different models proposed to explain the famous Roman Comagmatic Region, part of the Quaternary volcanism that spreads along the eastern side of the Tyrrhenian area, in the stretched part of the Apennines thrust-and-fold belt. We reviewed data and hypotheses proposed in the literature that infer active to fossil subduction up to models that exclude subduction entirely. Many field geology observations sustain the interpretation that the evolution of the Tyrrhenian-Apennine system was related to subduction of the western margin of Adria continental lithosphere after minor recycling of oceanic lithosphere. However, the lateral extent of the subducting slab in the last millions of years, when magmatism flared up, remains debatable. The igneous activity that developed in the last millions of years along the Tyrrhenian margin is here explained as originating from a subduction-modified mantle, regardless of whether the large-scale subduction system is still active.

ABSTRACT Formation of the Central European tektites, known as moldavites, has been associated with a large meteorite impact in southern Germany 14.8 m.y. ago. The geochemical link between moldavites and their source materials, and the processes of their possible chemical differentiation still remain uncertain. Some differences in chemical composition between moldavites and sediments of corresponding age from the surroundings of the Ries crater could be explained by a hypothesis according to which biomass covering the pre-impact area contributed to the source materials. In a comparison of the geochemical compositions of a large representative set of moldavites and suitable Ries sediments, enrichment in elements K, Ca, Mg, and Mn and depletion of Na in moldavites, similar to redistribution of these elements during their transfer from soil to plants, could indicate the unconventional biogenic component in moldavite source materials. Simple mixing calculations of the most suitable Ries sediments and a model biogenic component represented by burned biomass residue are presented. The plausibility of the estimated biomass contribution considering reconstructions of the middle Miocene paleoenvironment in the pre-impact Ries area is discussed. No significant vapor fractionation is required to explain the observed variability of moldavite chemical composition.

ABSTRACT Australasian tektites represent the largest group of tektites on Earth, and their strewn field covers up to one sixth of Earth’s surface. After several decades of fruitless quest for a parent crater for Australasian tektites, mostly in the main part of the strewn field in Indochina, the crater remains undiscovered. We elaborate upon a recently suggested original hypothesis for the impact in the Alashan Desert in Northwest China. Evidence from geochemical and isotopic compositions of potential source materials, gravity data, and geographic, paleoenvironmental, and ballistic considerations support a possible impact site in the Badain Jaran part of the Alashan Desert. In further support of an impact location in China, glassy microspherules recovered from Chinese loess may be the right age to relate to the Australasian tektite event, perhaps as part of the impacting body. The most serious shortcomings of the commonly accepted Indochina impact location include signs of little chemical weathering of source materials of Australasian tektites, unlike highly weathered sedimentary targets in Indochina, and questionable assumptions about transport of distal ejecta.

ABSTRACT The original location and tectonic setting of the prominent Paleocene dike swarm in the British Isles are reconstructed for a “tight fit” of the North Atlantic region prior to any Cenozoic opening of the ocean basin between Greenland and Europe. The present-day northwest-southeast–oriented swarm originally trended toward southern Greenland and the locations of magmatic rocks of comparable age along the eastern and western margins of Greenland and approximately the position of the Iceland hotspot at 70–60 Ma in a “fixed hotspot” model. This raises the possibility that the northeast-southwest–oriented extensional stress field in which the dikes and associated central igneous complexes were emplaced may have been generated by impingement on the base of the lithosphere by a rising plume beneath present-day West Greenland. It is speculated, on the basis of seismic tomography and three-dimensional modeling, that the Paleocene igneous activity in the British Isles may have resulted from flow of a hot “finger” of upper mantle outward from the plume, perhaps controlled by preexisting lithospheric structures and the distant location of a second Paleocene volcanic province in central Europe.

ABSTRACT The true partitioning between continental and oceanic lithosphere in oceans is unclear. According to early models, oceanic-type accretion generates pairs of linear magnetic anomalies, which are indicators of oceanic lithosphere and can be used as isochrons formed by seafloor spreading. However, seaward-dipping reflectors at conjugate volcanic passive margins also generate linear magnetic anomalies. The thick wedges of the inner seaward-dipping reflectors are associated with magnetic anomalies that are clearly distinct in shape and amplitude from those recorded in the distal oceanic realm. However, linear magnetic anomalies indistinguishable from those related to oceanic crust exist in the outer seaward-dipping reflector domain of many volcanic passive margins. Located seaward of the inner seaward-dipping reflectors, the crust of outer seaward-dipping reflectors is thus generally considered to be “oceanic.” However, the outer seaward-dipping reflector crust may be interpreted as tectonically exhumed mid-to-lower magma-intruded continental crust covered with syntectonic basalts. Although both oceanic crust and outer seaward-dipping reflector crust are associated with thick lava sections, the linear magnetic anomalies of outer seaward-dipping reflectors represent pre-oceanization magnetic anomalies that develop along extended continental lithosphere. We illustrate the consequence of these uncertainties on the type of lithosphere between Greenland and Europe. Here, depending on latitude, 20%–100% of the lithosphere previously thought to be oceanic might, on the contrary, be continental. Since more than 50% of passive margins worldwide are volcanic, poor mapping of seaward-dipping reflector–bearing crust types, and misinterpretation of linear magnetic anomaly–bearing distal volcanic passive-margin crust, could have led to widespread overestimation of the age of continental breakup and the extent of oceanic lithosphere in oceans.

ABSTRACT When Warren Hamilton passed away in October 2018, he left behind the manuscript for a synthesis paper that was published in Earth-Science Reviews in 2019: “Toward a myth-free geodynamic history of Earth and its neighbors.” Integrating hundreds of detailed studies across four worlds and billions of years, the paper’s outlook is heterodox, presenting alternatives to conventional wisdom in every paragraph for almost 50 pages. During the last years of his life, Hamilton had worked steadily on this paper, which he viewed as the culmination of his long career. This chapter tells the story of how Hamilton wrote his last paper, summarizes a few of the many ideas it contains, and describes how, with help from his colleagues, the paper was posthumously completed and published.

ABSTRACT In this paper, I use Thomas S. Kuhn’s model of scientific change to frame a brief, broad-brushed biographical sketch of the career of Warren B. Hamilton. I argue that Hamilton’s career can usefully be interpreted as encompassing a full “Kuhn cycle,” from a period of crisis in his early work, to one of normal science in midcareer, and back to something resembling crisis in his later research. Hamilton entered the field around mid-twentieth century when earth science can plausibly be described as being in a period of crisis. The then dominant fixist paradigm was facing an increasing number of difficulties, an alternative mobilist paradigm was being developed, and Hamilton played an important role in its development. The formulation of plate tectonics in the 1960s saw the overthrow of the fixist paradigm. This inaugurated a new phase of normal science as scientists worked within the new paradigm, refining it and applying it to different regions and various geological phenomena. Hamilton’s mid-career work fits largely into this category. Later, as the details of the plate-tectonic model became articulated more fully, and several of what Hamilton perceived as weakly supported conjectures became incorporated into the paradigm, problems began again to accumulate, and earth science, in Hamilton’s estimation, entered a new period of crisis. Radically new frameworks were now required, and Hamilton’s later work was dedicated principally to developing and articulating these frameworks and to criticizing mainstream views. Small incremental improvements are constantly being made, but larger and more fundamental upgrades are incorporated only erratically. — Hamilton (2011b)

ABSTRACT A robust, geology-based Proterozoic continental assembly places the northern and eastern margins of the Siberian craton against the southwestern margins of Laurentia in a tight, spoon-in-spoon conjugate fit. The proposed assembly began to break apart in late Neoproterozoic and early Paleozoic time. Siberia then drifted clockwise along the Laurussian margin on coast-parallel transforms until suturing with Europe in late Permian time. The proposed drift path is permitted by a geocentric axial dipole (GAD) magnetic field from Silurian to Permian time. However, the Proterozoic reconstruction itself is not permitted by GAD. Rather, site-mean paleomagnetic data plotted on the reconstruction suggest a multipolar Proterozoic dynamo dominated by a quadrupole. The field may have resembled that of present-day Neptune, where, in the absence of a large solid inner core, a quadrupolar magnetic field may be generated within a thin spherical shell near the core-mantle boundary. The quadrupole may have dominated Earth’s geomagnetic field until early Paleozoic time, when the field became erratic and transitioned to a dipole, which overwhelmed the weaker quadrupole. The dipole then established a strong magnetosphere, effectively shielding Earth from ultraviolet-B (UV-B) radiation and making the planet habitable for Cambrian fauna.

ABSTRACT Seismic tomography methods that use waves originating outside the volume being studied are subject to bias caused by unknown structure outside this volume. The bias is of the same mathematical order and similar magnitude as the local-structure effects being studied; failure to account for it can significantly corrupt derived structural models. This bias can be eliminated by adding to the inverse problem three unknown parameters specifying the direction and time for each incident wave, a procedure analogous to solving for event locations in local-earthquake and whole-mantle tomography. The forward problem is particularly simple: The first-order change in the arrival time at an observation point resulting from a perturbation to the incident-wave direction and time equals the change in the time of the perturbed incident wave at the point where the unperturbed ray entered the study volume. This consequence of Fermat’s principle apparently has not previously been recognized. Published teleseismic tomography models probably contain significant artifacts and need to be recomputed using the more complete theory.

ABSTRACT Field data reveal how the discharge ( Q ), channel area ( A ), and average water velocity ( V avg ) of natural streams functionally depend on the effective stage ( h ) above channel bottom. A graphical technique allows the level that corresponds to a dry channel, denoted “ h 0 ,” to be determined, permitting the dependent variables Q , A , and V avg to all be expressed as simple functions of h , equal to h L – h 0 , where h L is the local stage that is typically reported relative to an arbitrary, site-specific datum. Once h 0 is known, plots of log Q , log A , and log V avg versus log h can be constructed using available data. These plots define strong, nearly linear trends for which the slopes (1) quantify the power relationships among these variables; (2) show that V avg varies nearly linearly with h , unlike behaviors assumed in the Chezy and Manning equations; (3) distinguish the individual contributions of A and V avg to discharge, which is their product; (4) provide quantitative means with which to compare different sites; and (5) offer new insights into the character and dynamics of natural streams.

ABSTRACT Lateral accelerations require lateral forces. We propose that force imbalances in the unique Earth-Moon-Sun system cause large-scale, cooperative tectonic motions. The solar gravitational pull on the Moon, being 2.2× terrestrial pull, causes lunar drift, orbital elongation, and an ~1000 km radial monthly excursion of the Earth-Moon barycenter inside Earth’s mantle. Earth’s spin superimposes an approximately longitudinal 24 h circuit of the barycenter. Because the oscillating barycenter lies 3500–5500 km from the geocenter, Earth’s tangential orbital acceleration and solar pull are imbalanced. Near-surface motions are enabled by a weak low-velocity zone underlying the cold, brittle lithosphere: The thermal states of both layers result from leakage of Earth’s internal radiogenic heat to space. Concomitantly, stress induced by spin cracks the lithosphere in a classic X-pattern, creating mid-ocean ridges and plate segments. The inertial response of our high-spin planet with its low-velocity zone is ~10 cm yr –1 westward drift of the entire lithosphere, which largely dictates plate motions. The thermal profile causes sinking plates to thin and disappear by depths of ~200–660 km, depending on angle and speed. Cyclical stresses are effective agents of failure, thereby adding asymmetry to plate motions. A comparison of rocky planets shows that the presence and longevity of volcanism and tectonism depend on the particular combination of moon size, moon orbital orientation, proximity to the Sun, and rates of body spin and cooling. Earth is the only rocky planet with all the factors needed for plate tectonics.

ABSTRACT It is well established that plate-tectonic processes operate on a global scale and that spatially separate but temporally coincident events may be linked. However, identifying such links in the geological record and understanding the mechanisms involved remain speculative. This is particularly acute during major geodynamic events, such as the dispersal of supercontinents, where multiple axes of breakup may be present as well as coincidental collisional events. To explore this aspect of plate tectonics, we present a detailed analysis of the temporal variation in the mean half rate of seafloor spreading in the Indian and Atlantic Oceans, as well as plate-kinematic attributes extracted from global plate-tectonic models during the dispersal of Gondwana since ca. 200 Ma. Our analysis shows that during the ~20 m.y. prior to collision between India and Asia at ca. 55 Ma, there was an increase in the mean rate of seafloor spreading in the Indian Ocean. This manifests as India rapidly accelerating toward Asia. This event was then followed by a prompt deceleration in the mean rate of Indian Ocean seafloor spreading after India collided with Asia at ca. 55 Ma. Since inception, the mean rate of seafloor spreading in the Indian Ocean has been generally greater than that in the Atlantic Ocean, and the period of fastest mean half spreading rate in the Indian Ocean was coincident with a slowdown in mean half seafloor spreading rate in the competing Atlantic Ocean. We hypothesize that faster and hotter seafloor spreading in the Indian Ocean resulted in larger ridge-push forces, which were transmitted through the African plate, leading to a slowdown in Atlantic Ocean spreading. Following collision between India and Asia, and a slowdown of Indian Ocean spreading, Atlantic spreading rates consequently increased again. We conclude that the processes in the Indian and Atlantic Oceans have likely remained coupled throughout their existence, that their individual evolution has influenced each other, and that, more generally, spreading in one basin inevitably influences proximal regions. While we do not believe that ridge push is the main cause of plate motions, we consider it to have played a role in the coupling of the kinematic evolution of these oceans. The implication of this observation is that interaction and competition between nascent ocean basins and ridges during supercontinent dispersal exert a significant control on resultant continental configuration.

ABSTRACT We present an interdisciplinary study between philosophy and science that uses a historical case to show some aspects of scientific research. The case in question is that of Alfred F. Rittmann (1893–1980), known as one of the central figures of twentieth-century volcanology. After outlining Rittmann’s scientific background and hypotheses, we briefly examine the set of his theories using Thomas Kuhn’s model of the development of science. We highlight the methodology of multiple working hypotheses and how they contributed to defining his geoscientific paradigm, namely, magmatological tectonics. Rittmann worked on his paradigm throughout his life, even making little-known criticisms on plate tectonics. We present some of them, contextualizing them in twentieth-century as well as current research. His use of multiple working hypotheses, along with his drive to search for synthetic visions between different models, could be a stimulating and pluralistic approach to unsolved geoscientific questions.

ABSTRACT Classic models proposed that continental rifting begins at hotspots—domal uplifts with associated magmatism—from which three rift arms extend. Rift arms from different hotspots link up to form new plate boundaries, along which the continent breaks up, generating a new ocean basin and leaving failed arms, termed aulacogens, within the continent. In subsequent studies, hotspots became increasingly viewed as manifestations of deeper upwellings or plumes, which were the primary cause of continental rifting. We revisited this conceptual model and found that it remains useful, though some aspects require updates based on subsequent results. First, the rift arms are often parts of boundaries of transient microplates accommodating motion between the major plates. The microplates form as continents break up, and they are ultimately incorporated into one of the major plates, leaving identifiable fossil features on land and/or offshore. Second, much of the magmatism associated with rifting is preserved either at depth, in underplated layers, or offshore. Third, many structures formed during rifting survive at the resulting passive continental margins, so study of one can yield insight into the other. Fourth, hotspots play at most a secondary role in continental breakup, because most of the associated volcanism reflects plate divergence, so three-arm junction points may not reflect localized upwelling of a deep mantle plume.

ABSTRACT Volcanic activity is ultimately controlled by processes that take place many kilometers beneath the surface of a planet. The deeper processes are unlikely to reach the surface without some degree of modification at shallower levels. Nevertheless, traces of those deeper processes may still be found when examining the final products at the surface. In this work, it is shown that it is possible to gain insights concerning the integrated contribution of deep structures through the study of the spatial distribution of volcanic vents at the surface. The method here described relies on the systematic use of increasing smoothing factors in Gaussian kernel estimations. The sequences of probability density functions thus generated are equivalent to images obtained with an increasing wavelength, which therefore have the power to penetrate deeper below the surface. Although the resolution of this method is much smaller than the resolution provided by seismic or other geophysical surveys, it has the advantages of ease of implementation, extremely low cost, and remote application. Thus, the reported method has great value as a first-order exploration tool to investigate the deep structure of a planet, and it can make important contributions to our understanding of the volcano-tectonic relationship, not only on Earth, but also across the various bodies of the solar system where volcanic activity has been documented.

ABSTRACT Relationships between the weather and earthquakes have been suspected for over 2400 yr. However, scientific evidence to support such relationships has grown only since the 1980s. Because faults in Earth’s crust are generally regarded as critically stressed, small changes in stress and pore-fluid pressure brought about by rainfall, snow, and atmospheric pressure and temperature variations have all been proposed to modulate seismicity at local and regional scales. Elastic static stress changes as low as 0.07 kPa and pore-fluid pressure changes as low as 0.5 kPa have been proposed to naturally trigger earthquakes. In the UK, the spatial distributions of onshore earthquakes and rainfall are highly nonuniform and may be related; the wetter and most naturally seismically active areas occur on the west side of the country. We found significant spatial and temporal relationships between rainfall amount and the number of earthquakes for 1980–2012, suggesting larger volumes of rainfall promote earthquake nucleation. Such relationships occur when human-induced seismicity is included or excluded, indicating that meteorological conditions can also modulate seismicity induced by subsurface anthropogenic activities such as coal mining. No significant relationships were observed for monthly time lags, suggesting that the triggering effect of rainfall in the UK is near-instantaneous or occurs within 1 mo. With global climate changing rapidly and extreme weather events occurring more frequently, it is possible that some global regions may also experience changes in the spatial and temporal occurrence of earthquakes in response to changes in meteorologically induced stress perturbations.

ABSTRACT The geodynamic origin of melting anomalies found at the surface, often referred to as “hotspots,” is classically attributed to a mantle plume process. The distribution of hotspots along mid-ocean-ridge spreading systems around the globe, however, questions the universal validity of this concept. Here, the preferential association of hotspots with slow- to intermediate-spreading centers and not fast-spreading centers, an observation contrary to the expected effect of ridge suction forces on upwelling mantle plumes, is explained by a new mechanism for producing melting anomalies at shallow (<2.3 GPa) depths. By combining the effects of both chemical and thermal density changes during partial melting of the mantle (using appropriate latent heat and depth-dependent thermal expansivity parameters), we find that mantle residues experience an overall instantaneous increase in density when melting occurs at <2.3 GPa. This controversial finding is due to thermal contraction of material during melting, which outweighs the chemical buoyancy due to melting at shallow pressures (where thermal expansivities are highest). These dense mantle residues are likely to locally sink beneath spreading centers if ridge suction forces are modest, thus driving an increase in the flow of fertile mantle through the melting window and increasing magmatic production. This leads us to question our understanding of sub–spreading center dynamics, where we now suggest a portion of locally inverted mantle flow results in hotspots. Such inverted flow presents an alternative mechanism to upwelling hot mantle plumes for the generation of excess melt at near-ridge hotspots, i.e., dense downwelling of mantle residue locally increasing the flow of fertile mantle through the melting window. Near-ridge hotspots, therefore, may not require the elevated temperatures commonly invoked to account for excess melting. The proposed mechanism also satisfies counterintuitive observations of ridge-bound hotspots at slow- to intermediate-spreading centers, yet not at fast-spreading centers, where large dynamic ridge suction forces likely overwhelm density-driven downwelling. The lack of observations of such downwelling in numerical modeling studies to date reflects the generally high chemical depletion buoyancy and/or low thermal expansivity parameter values employed in simulations, which we find to be unrepresentative for melting at <2.3 GPa. We therefore invite future studies to review the values used for parameters affecting density changes during melting (e.g., depletion buoyancy, latent heat of melting, specific heat capacity, thermal expansivity), which quite literally have the potential to turn our understanding of mantle dynamics upside down.

ABSTRACT We analyzed 37 large oceanic intraplate earthquakes (M >6). The largest (M >7) are mainly concentrated under the Indian Ocean. Moderate events (6 < M < 7) are sparsely distributed under the Indian Ocean and other oceans where lithospheric ages are between 90 Ma and 20 Ma. Oceanic intraplate events related to mantle plumes or hotspots are rare, though low-velocity anomalies beneath hotspots are a common feature. Tomographic cross sections for Indian Ocean areas with large intraplate earthquakes indicate strong heterogeneity in the mantle. These earthquakes are explained by shallow stress variations caused by a combination of tectonic forces including slab-pull, ridge-push, drag by mantle flow, plume-push, and buoyancy forces as a consequence of low-velocity anomalies in the mantle. Oceanic intraplate seismicity in the Indian Ocean is related to the large-scale, low-velocity anomaly structure around the Ninety East Ridge.