Ronadh Cox

• Ph.D
• Professor at Williams College

Erosion via lavaka formation is widespread in Madagascar, but controls on why and where lavakas occur are not understood. Geographic information system analysis reveals a spatial correlation between lavaka abundance and the frequency of seismic events: most lavakas occur in or near areas where recorded earthquakes (magnitude 0.5-5.6) are most frequ...

Numerical simulations show that impactors can penetrate Europa's ice, creating conduits to the underlying ocean. Breaching becomes inevitable when transient cavity depth exceeds 90% of ice thickness. Results indicate that an 0.5 km comet would penetrate 5 km ice, and a 5 km comet could breach 40 km ice. Thinner ice would be breached more frequently...

Extreme wave impacts on coastal infrastructure, including bridges, is a growing problem. Highest energy events are difficult to study because direct observations in real time are almost impossible. Also the non-linear dynamics are difficult to model or to reproduce in scaled experiments. Coastal boulder deposits incorporat-ing megagravel (y axis >4...

Lavaka, large gullies in Madagascar, can cause problems for farmers and for infrastructure, but these features also have beneficial aspects that have generally been overlooked. They provide plant refugia, and commonly host species that would otherwise not thrive on the grassy uplands. Farmers sometimes make use of lavaka to plant crops that require...

Gully evolution remains poorly understood, largely because multidecadal analysis is lacking. Large gullies (called lavaka) that pepper Madagascar’s highlands are generally attributed to human impact; but longitudinal data are few, and anthropogenic causation is inferred not verified. We focus on Lac Alaotra, Madagascar’s largest lake and wetland, i...

Cosmogenic nuclide analysis of river sediment provides insight into catchment wide erosion rates and dynamics. Here, we investigate spatial patterns and controls on 10Be-inferred erosion rates in Madagascar, a moderately seismically active microcontinent surrounded by passive margins with locally steep topography and a climate that varies from humi...

Plain Language Summary Europa is an ocean world with an outer ice shell in our Solar System. Outer ice shells on ocean worlds serve as both a barrier and facilitator for internal ocean habitability and the detectability of its potential inhabitants. A prominent feature we observe on the surface of ice shells is highly variable craters from impacts...

Coastal boulder deposits (CBD) are wave-emplaced supratidal accumulations that record extreme inundation on rocky coasts. They are poorly understood but are of growing importance as we seek to better understand the extremes of wave power on coastlines. The Aran Islands, Ireland, host CBD in varying settings ranging from sheer cliff tops to wide sho...

Coastal boulder deposits (CBD) provide what are sometimes the only remaining signatures of wave inundation on rocky coastlines; in recent decades, CBD combined with initiation of motion (IoM) analyses have repeatedly been used as primary evidence to infer the existence of ancient tsunamis. However, IoM storm wave heights inferred by these studies h...

Supratidal coastal boulder deposits (CBD) result from extreme marine inundation on rocky shores. They are important for understanding coastal waves, and have predictive value for future events. But they are poorly studied, and their interpretation remains contentious, with debate on whether they record storms, tsunami, or both. In the case of older...

Storm waves, after breaking or overtopping, generate strong onshore flows that do significant mechanical work, including eroding and transporting large boulders. The waves can be amplified on approach, and the flows themselves may be further intensified by local topographic effects. These processes are currently poorly parameterised, but are of gre...

Graphical abstractCoastal boulder deposits on Inishmore, one of the Aran Islands off Ireland's west coast, showcase many of the classic features. Note the adult human for scale (in the yellow circle). The cliff‐collapse block in the foreground is the largest ever documented to be moved by storm waves. Weighing 620t, it was shifted several metres du...

Coastal boulder deposits (CBD), transported by waves at elevations above sea level and substantial distances inland, are markers for marine incursions. Whether they are tsunami or storm deposits can be difficult to determine, but this is of critical importance because of the role that CBD play in coastal hazard analysis. Equations from seminal work...

Repeat photogrammetry is increasingly the go-too tool for long-term geomorphic monitoring, but quantifying the differences between structure-from-motion (SfM) models is a developing field. Volumetric differencing software (such as the open-source package CloudCompare) provides an efficient mechanism for quantifying change in landscapes. In this cas...

It is a long-standing maxim that coarser sediment sizes are associated with steeper beach-face slopes; but because most work has focused on sandy beaches, few data are available for the pebble, cobble, or boulder size ranges. Little is known, therefore, about how beach morphology and grain size relate at the coarser size grades. We compiled data fr...

Coastal boulder deposits (CBD) are supratidal clastic deposits that include isolated boulders, small clusters, and extensive ridge systems built of stacked, imbricated clasts. Individual boulders can weigh many 10s to 100s of tonnes. They are emplaced by waves along steep rocky coasts. Some are on cliffs as high as 50m above high water, and others...

Coastal boulder deposits (CBD) are archives of extreme wave events. They are emplaced well above high tide, and may include megagravel clasts weighing tens or even hundreds of tonnes. But do they represent storms or tsunami? Many are interpreted as tsunami deposits based simply on clast size and inferences about transport, despite the fact that the...

Recent research has shown that interaction with bathymetry can lead to wave height amplification of up to a factor of 12. These extreme waves are important for coastal erosion and very large boulder movement but long-term field records in such high-energy conditions are challenging to acquire, and very scarce. Over three winters (2016-2019) we depl...

MEGAGRAVEL (https://www.youtube.com/watch?v=u9PuDY7QyBo) is a short documentary film introducing coastal boulder deposits (CBD) to a general audience. CBD accumulate above high tide on certain rocky coasts. Some are on tops of tall cliffs, others are found up to a quarter of a kilometer inland; and they can include clasts weighing 10s or 100s of to...

Erdmann and co-authors (2018) present interesting observations of coastal boulder deposits along Ireland's western seaboard, Unfortunately, some of their interpretations are undermined by inaccurate information; and two of their main conclusions—that boulders greater >50 t were not moved during the recent strong storms, and that boulders 20–30 t we...

Boulders are fractured and excavated by large waves along steep coastlines, contributing to coastal erosion. Intense fluid pressures enter cracks in attached bedrock, inducing bending stresses that may result in hydraulic fracture of a block. We consider the dynamic response of an attached rock beam to loading during wave impact. We verify the comp...

Boulder quarrying by large waves along steep, high-energy coastlines contributes to erosion both by causing inland migration of cliff faces and by vertical lowering of coastal platform surfaces. It also leads to the formation of coastal boulder deposits (CBD) above and inland of the high water mark. We describe mechanisms by which hydraulic fractur...

Surface sediment data is scarce for beaches that are made of material close to the cobble size range, partly because of the difficulty of direct sampling for sediment size analysis. An object-detection tool for semiautomatic analysis of clast geometry, originally developed for riverbeds, was applied to the coarse-clastic beach of Nantian, Taiwan. C...

Before-and-after photos of supratidal coastal boulder deposits (CBD) in the west of Ireland show that storms in the winter of 2013–2014 transported boulders at elevations up to 29 m above high water, and at inland distances up to 222 m. Among the clasts transported are eighteen weighing more than 50 t, six of which exceed 100 t. The largest boulder...

The pressure load at a vertical barrier caused by extreme wave run-up is analysed numerically, using the conformal mapping method to solve the two-dimensional free surface Euler equations in a pseudo-spectral model. Previously this problem has been examined in the case of a flat-bottomed geometry. Here, the model is extended to consider a varying b...

Coastal boulder deposits (CBD) are emplaced above high water (AHW) during extreme wave events, and occur worldwide on high-energy rocky coasts. In many cases it is unclear whether storms or tsunami were responsible for boulder dislodgement and transport. CBD are sometimes interpreted as tsunami-related based mainly on the large size of included bou...

During the winter of 2013-2014 the west coast of Ireland was exposed to 6 storms over a period of 8 weeks with wind speeds equating to hurricane categories 3 and 4. During this period, the largest significant wave height recorded at the Marine Institute M6 wave buoy, approximately 300km from the site, was 13.6m (on 26th January 2014). However, this...

The sedimentology of coastal boulder deposits (CBD) is little known. These deposits, and especially the largest clasts within them—which commonly weigh many 10s of tonnes—are of increasing interest because they attest to extreme energy wave events acting well above the high-tide mark (in some cases > 40 m above high water and > 100 m inland). Recen...

Coastal boulder deposits (CBD) accumulate above the high-water mark along steep coastlines with open-ocean exposure. They include megagravel with masses in the many 10s of tonnes, and therefore preserve a record of high-energy events; but because they are activated only by very large waves they can be essentially dormant for long periods of time, a...

Eleuthera, on the northeastern side of the Bahamas archipelago, has a steep ocean-facing coastline fully exposed to Atlantic storms. Cliffs of Pleistocene limestone rising to about 20 m a.s.l. are topped with modern boulder deposits that include (in order of increasing clast concentration) isolated clasts, scattered-boulder fields, and boulder ridg...

Io, the solar system’s most volcanically active body, also has distinctive mountains, most of which are not volcanoes. The majority of Io’s lavas emanate from low-relief paterae and form extensive flows rather than edifices, whereas the ~140 Ionian mountains are high-standing massifs, rising several km above the plains. The mountains appear to be t...

Hydrocode modeling shows that impacts penetrate to water if transient depth exceeds 0.8x ice thickness. For likely ice thicknesses, impact breaching at Europa is feasible on geologically short timescales, which may permit surface-ocean exchange.

The characteristic gullies of central Madagascar–lavakas–vary greatly in abundance over short distances, but existing understanding does not explain why some hillsides should have high concentrations of lavakas when nearby slopes have fewer. We present a GIS analysis of lavaka abundance in relation to bedrock geology and topography, covering two ar...

Ireland’s Aran Islands are an excellent place to test whether coastal boulder deposits—including individual rocks weighing several tens of tonnes near sea level and clasts weighing several tonnes transported at tens of meters above sea level—require a tsunami for emplacement or whether storm waves can do this work. Elongate deposits of cobbles, bou...

Simulations using iSALE indicate that multiring basin Tyre could represent full crustal melt-through following impact of a 2-km impactor into 20-km crust; and further indicate that more energetic impacts could punch holes directly through the crust.

Large chaos areas have more complex boundary shapes than small ones. Chaos areas are strongly concentrated at low latitudes. Both observations match predictions for impact-penetration features.

Zircons in transport in the modern Amazon River range from coarse silt to medium sand. Older grains are smaller on average: Mesozoic and Cenozoic grains have average equivalent spherical diameter (ESD) 122 ± 42 μm (lower fine sand), whereas grains >2000 Ma have average ESD 67 ± 14 μm (upper coarse silt). As a full Wentworth size class separates the...

In situ-produced and meteoric 10Be are both powerful tools for tracing the production and transport of hillslope sediment. In situ-produced 10Be is used to infer sediment production rates as well as investigate sediment sources and transport. Meteoric 10Be may also be useful for inferring sediment production and transport rates in some landscapes,...

The central highlands of Madagascar are characterized by rolling hills thickly mantled with saprolite and cut in many areas by dramatic gullies known as lavakas. This landscape generates sediment to rivers via diffusive downslope movement of colluvium and event-driven advection of material from active lavakas; these two sediment sources have very d...

Ice thickness estimates and impactor dynamics indicate that some impacts must breach Europa’s ice crust; and outcomes of impact experiments using ice-over-water targets range from simple craters to chaos-like destroyed zones, depending on impact energy and ice competence. Firstorder impacts—into thick ice or at low impact energy—produce craters. Se...

Dramatic boulder ridges are widespread on cliff-top platforms of Ireland's Aran Islands, emplaced by storm waves at elevations up to 50 m. In places the ridges have overridden 19th-century field walls, and recent movement of large blocks is attested to by modern debris--including nylon ropes, aerosol cans, and plastic bottles--pinned under megaclas...

Chaos terrain is ubiquitous on Europa's surface, but not randomly distributed. The global distribution of chaos areas shows a significant concentration between 30° N and S latitude, decreasing dramatically at higher latitudes. The low-latitude clustering is not an artifact of recognizability, as there is a greater proportion of images with high sol...

Because it is generally assumed that zircon age populations are not significantly fractionated during transport, detrital zircon provenance studies usually characterize locations or units with a single sample. But zircons vary in size by an order of magnitude, ranging from 10s of mum to a few hundred mum in length (i.e. coarse silt to medium sand)....

Madagascar's central highlands are deeply weathered, with 1-2 m of laterite overlying 10s of metres of saprolite. These unstable materials sit at altitudes on the order of 1000m in recently uplifted, steep terrain characterised by convex hills with slopes averaging 25 degrees and local ridge-valley elevation changes of 100- 500 m. In many areas the...

Some chaos areas on Europa may be sites of full-crustal penetration by impacts. Experimental evidence indicates that impact penetration can produce the features associated with chaos areas.

Detrital zircon U-Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the...

Quartzite, conglomerate, and shale of the Mazatzal Group record the filling of a Proterozoic intra-arc basin in central Arizona. UPb ages of zircons from rhyolite ash-flow tuff indicate that deposition began at 1701 ± 2 Ma. Basal deposits of the newly defined Pine Creek Conglomerate formed in an alluvialfan setting, synchronous with the final phase...

The occurrence of quartz-pebble conglomerates (QPC) in the rock record increases backward through time from the Tertiary through the Precambrian. The positive correlation between QPC abundance and age is valid both for numbers of reported QPC and for QPC as a percentage of all conglomerate, and at both the era and the period level. QPC are usually...

Recent decline in the diversity and percent cover of hard corals in reef environments worldwide has been extensively documented (Done, 1992; Gladfelter, 1982; Kuta and Richardson, 1996; Precht and Aronson, 1997; Rogers, 1990). To understand this decline, long-term records of reef environments (e.g., Aronson and Precht, 1997; Hughes, 1994; Precht an...

Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and alga...

This paper explores the use of a Monte Carlo adaptation of runs analysis to analyze turbidite sequences for the presence of asymmetric bed-thickness cycles. Waldron`s test can be used to identify sequences that are dominated by either upward-thickening or upward-thinning cycles, but not both. This adaptation of runs analysis provides a robust techn...

Most studies of sandstone provenance involve modal analysis of framework grains using techniques that exclude the fine-grained breakdown products of labile mineral grains and rock fragments, usually termed secondary matrix or pseudomatrix. However, the data presented here demonstrate that, when the proportion of pseudomatrix in a sandstone exceeds...

This paper reports systematic changes in mudrock composition through time on a single continental crustal block. The changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of a...

On average, compositions of Colorado Province sandstones become more mature in younger sequences. The proportion of framework quartz increases through time at the expense of less robust constituents. The chemical data show that SiO 2 increases through time and that all other major oxides decrease progressively. Normative sandstone compositions indi...

Both sediment recycling and first-cycle input influence the composition of clastic material in sedimentary systems. This paper examines conceptually the roles played by these processes in governing the composition of clastic sediment on a regional scale by outlining the expected effects on sediment composition of protracted sediment recycling and o...

Europa's enigmatic chaos terrain probably represents breakage of the brittle surface ice and exposure of fluid material. Proposed explanations have focussed on endogenic mechanisms and energy sources, but all are disputed. Crust-penetrating bolide impacts are an alternative mechanism by which chaos terrain could be produced, and primary evidence fo...