1,721,097 research outputs found
On the triggers, resulting flow types and frequencies of subaqueous sediment density flows in different settings
No full textTurbidity currents, and other types of underwater sediment density flow, are arguably the most important flow process for moving sediment across our planet. Direct monitoring provides the most reliable information on the varied ways in which these flows are triggered, and thus forms the basis for this contribution. Recent advances in flow monitoring make this contribution timely, although monitoring is biased towards more frequent flow types. Submarine deltas fed by bedload dominated rivers can be very active with tens of events each year. Larger events are generated by delta-lip failures, whilst smaller events can be associated with motion of up-slope migrating bedforms. River-fed submarine canyons are flushed every few years by powerful long run-out flows. Flows in river-fed delta and canyon systems tend to occur during months of elevated river discharge. However, many flows do not coincide with flood peaks, or occur where rivers do not reach hyperpycnal concentrations, and are most likely triggered by failure of rapidly deposited sediment. Plunging of hyperpycnal river floodwater commonly triggers dilute and slow moving flows in lakes and reservoirs, and has been shown to produce mm-thick fine-grained deposits. It is proposed here that such thin and fine deposits are typical of flows triggered by hyperpycnal river floods, rather than thicker sand layers with traction structure or displaying inverse-to-normal grading. Oceanographic canyons are detached from rivers mouths and fed by oceanographic processes (wave and tide resuspension, longshore drift, etc.). Most events in these canyons are associated with large wave heights. Up-slope migrating crescentic bedforms are seen, similar to those observed in river-fed deltas. Oceanographic processes tend to infill canyons, which are flushed episodically by much more powerful flows, inferred to result from slope failure. This filling and flushing model is less applicable to river-fed canyons in which flushing events are much more frequent. Oceanographic canyons may result from rapid sea level rise that detaches river mouths from canyon heads, and they can remain active during sea level highstands. Deep-water basin plains are often dominated by infrequent but very large flows triggered by failure of the continental slope. Recurrence intervals of these flows appear almost random, and only weakly (if at all) correlated with sea level change. Turbidites can potentially provide a valuable long-term record of major earthquakes, but widespread slope failure is the only reliable criteria for inferring seismic triggering. However, not all major earthquakes trigger widespread slope failure, so that the record is incomplete in some locations
Self-Organization of River Networks to Threshold States
No full textNumerical models in which grid-cells self-organize, so that the shear stress in each cell equals a predetermined threshold value, are remarkably successful in producing the fundamental structure of river networks. Such models are a reasonable approximation of networks characterized by near-constant dimensionless shear stress (т* α bed shear stress/bed grain size), with each cell's threshold value being proportional to median-bed grain size. Previous work has shown that this is the case for channels cut into alluvium, and that the characteristic narrow range of т* is different for gravel- and sand-bed channels. Channels in the Italian Apennines that are slowly incising into weak bedrock, but which are covered by a veneer of gravel for most of the time, are shown to be characterized by the same т* as alluvial gravel-bed channels. Thus threshold models capture the fundamental behavior of many river networks, even in areas of long-term bedrock incision
Anatomy of a submarine pyroclastic flow and associated turbidity current: July 2003 dome collapse, Soufrière Hills volcano, Montserrat, West Indies
No full textThe 12 to 13 July 2003 andesite lava dome collapse at the Soufrière Hills volcano, Montserrat, provides the first opportunity to document comprehensively both the sub-aerial and submarine sequence of events for an eruption. Numerous pyroclastic flows entered the ocean during the collapse, depositing approximately 90% of the total material into the submarine environment. During peak collapse conditions, as the main flow penetrated the air–ocean interface, phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling. The bulk of the flow was submerged and rapidly mixed with sea water forming a water-saturated mass flow. Efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth. The coarsest components (?60% of the total volume) were deposited proximally from a dense granular flow, while the finer components (?40%) were efficiently elutriated into the overlying part of the flow, which evolved into a far-reaching turbidity current.<br/
How are subaqueous sediment density flows triggered, what is their internal structure and how does it evolve? Direct observations from monitoring of active flows
Full text linkSubaqueous sediment density flows are one of the volumetrically most important processes for moving sediment across our planet, and form the largest sediment accumulations on Earth (submarine fans). They are also arguably the most sparely monitored major sediment transport processes on our planet. Significant advances have been made in documenting their timing and triggers, especially within submarine canyons and delta-fronts, and freshwater lakes and reservoirs, but the sediment concentration of flows that run out beyond the continental slope has never been measured directly. This limited amount of monitoring data contrasts sharply with other major types of sediment flow, such as river systems, and ensure that understanding submarine sediment density flows remains a major challenge for Earth science. The available monitoring data define a series of flow types whose character and deposits differ significantly. Large (> 100 km3) failures on the continental slope can generate fast-moving (up to 19 m/s) flows that reach the deep ocean, and deposit thick layers of sand across submarine fans. Even small volume (0.008 km3) canyon head failures can sometimes generate channelised flows that travel at > 5 m/s for several hundred kilometres. A single event off SE Taiwan shows that river floods can generate powerful flows that reach the deep ocean, in this case triggered by failure of recently deposited sediment in the canyon head. Direct monitoring evidence of powerful oceanic flows produced by plunging hyperpycnal flood water is lacking, although this process has produced shorter and weaker oceanic flows. Numerous flows can occur each year on river-fed delta fronts, where they can generate up-slope migrating crescentic bedforms. These flows tend to occur during the flood season, but are not necessarily associated with individual flood discharge peaks, suggesting that they are often triggered by delta-front slope failures. Powerful flows occur several times each year in canyons fed by sand from the shelf, associated with strong wave action. These flows can also generate up-slope migrating crescentic bedforms that most likely originate due to retrogressive breaching associated with a dense near-bed layer of sediment. Expanded dilute flows that are supercritical and fully turbulent are also triggered by wave action in canyons. Sediment density flows in lakes and reservoirs generated by plunging river flood water have been monitored in much greater detail. They are typically very dilute (< 0.01 vol.% sediment) and travel at < 50 cm/s, and are prone to generating interflows within the density stratified freshwater. A key objective for future work is to develop measurement techniques for seeing through overlying dilute clouds of sediment, to determine whether dense near-bed layers are present. There is also a need to combine monitoring of flows with detailed analyses of flow deposits, in order to understand how flows are recorded in the rock record. Finally, a source-to-sink approach is needed because the character of submarine flows can change significantly along their flow path
Special Issue Introduction: Sediment gravity flows – Recent insights into their dynamic and stratified/composite nature
No full textDeposits of flows transitional between turbidity current and debris flow
No full textThe relationship between submarine sediment gravity flows and the character of their deposits is poorly understood. Annular flume experiments were used to investigate the depositional dynamics and deposits of waning sediment-laden flows. Decelerating fast (>3 m/s) flows with fixed sand content (10 vol%) and variable mud content (0–17 vol%) resulted in only four deposit types. Clean sand with a mud cap that resembled a turbidity current deposit (turbidite) formed if the flow was turbulent when deposition began, or if the muddy fluid had insufficient strength to suspend the sand. The clean sand could contain structures if mud content was low (<6%) and the deceleration period was >300 s. Ungraded muddy sand with a mud cap that resembled a debris-flow deposit (debrite) formed if the flow became laminar before sand could deposit. Clean sand overlain by ungraded muddy sand and a mud cap formed either from a transitional flow or by late-stage settling of sand from a muddy suspension. These deposits resemble enigmatic submarine flow deposits called linked debrite-turbidites. The experiments provide a basis for inferring flow type from deposit character for submarine sediment-laden flows. <br/
Timing and frequency of large submarine landslides: implications for understanding triggers and future geohazard
Full text linkLarge submarine landslides can have serious socioeconomic consequences as they have the potential to cause tsunamis and damage seabed infrastructure. It is important to understand the frequency of these landslides, and how that frequency is related to climate-driven factors such as sea level or sedimentation rate, in order to assess their occurrence in the future. Recent studies have proposed that more landslides occur during periods of sea level rise and lowstand, or during periods of rapid sedimentation. In this contribution we test these hypotheses by analysing the most comprehensive global data set of ages for large (>1 km3) late Quaternary submarine landslides that has been compiled to date. We include the uncertainties in each landslide age that arise from both the dating technique, and the typically larger uncertainties that result from the position of the samples used for dating. Contrary to the hypothesis that continental slope stability is linked to sea level change, the data set does not show statistically significant patterns, trends or clusters in landslide abundance. If such a link between sea level and landslide frequency exists it is too weak to be detected using the available global data base. It is possible that controlling factors vary between different geographical areas, and their role is therefore hidden in a global data set, or that the uncertainties within the dates is too great to see an underlying correlation. Our analysis also shows that there is no evidence for an immediate influence of rapid sedimentation on slope stability as failures tend to occur several thousand years after periods of increased sedimentation rates. The results imply that there is not a strong global correlation of landslide frequency with sea level changes or increases in local sedimentation rate, based on the currently available ages for large submarine landslides
Multi-stage volcanic island flank collapses with coeval explosive caldera-forming eruptions
No full textVolcanic flank collapses and explosive eruptions are among the largest and most destructive processes on Earth. Events at Mount St. Helens in May 1980 demonstrated how a relatively small (<5 km3) flank collapse on a terrestrial volcano could immediately precede a devastating eruption. The lateral collapse of volcanic island flanks, such as in the Canary Islands, can be far larger (>300 km3), but can also occur in complex multiple stages. Here, we show that multistage retrogressive landslides on Tenerife triggered explosive caldera-forming eruptions, including the Diego Hernandez, Guajara and Ucanca caldera eruptions. Geochemical analyses were performed on volcanic glasses recovered from marine sedimentary deposits, called turbidites, associated with each individual stage of each multistage landslide. These analyses indicate only the lattermost stages of subaerial flank failure contain materials originating from respective coeval explosive eruption, suggesting that initial more voluminous submarine stages of multi-stage flank collapse induce these aforementioned explosive eruption. Furthermore, there are extended time lags identified between the individual stages of multi-stage collapse, and thus an extended time lag between the initial submarine stages of failure and the onset of subsequent explosive eruption. This time lag succeeding landslide-generated static decompression has implications for the response of magmatic systems to un-roofing and poses a significant implication for ocean island volcanism and civil emergency planning
Deposit structure and processes of sand deposition from a decelerating sediment suspension
No full textTurbidity currents are notoriously difficult to monitor directly, therefore interpretation of their deposits forms the basis for much of our understanding of these flows. The deceleration rate of a flow is a potentially important yet poorly understood control on depositional processes. A series of experiments were conducted in an annular flume, in which fast (up to 3.5 m/s) and highly turbulent flows of sand (up to 250 µm) and water were decelerated at different rates and processes of deposition and deposit character analyzed. Previously poorly documented depositional processes were observed in the experiments. This is because the flows were initially unusually fast and of prolonged duration, with sustained periods of sediment fallout as the flow slowed down. The conditions in these flows are thus likely to be closer to those at the base of a waning turbidity current than is achieved in other relatively slow experimental flows. The collapse of high-concentration, moving, thin (< 5 mm) near-bed layers (laminar sheared layers) were an important mechanism by which the bed aggraded beneath these unsteady flows. At bed aggradation rates in excess of 0.44 mm/s the sequential collapse of laminar sheared layers produced a structureless, poorly graded and poorly sorted deposit (Bouma Ta). When bed aggradation rates fell below 0.44 mm/s the collapsing laminar sheared layers were reworked by turbulence to form planar laminae (Bouma Tb). These laminae are formed in a very different manner than the planar laminae attributed to bedwaves in previous open-channel flow experiments. Collapse of laminar sheared layers is therefore an alternative process for generating the Bouma Tb division. Inverse grading developed at the base of the deposits of slowly decelerated flows. This inverse grading was probably a result of grain sorting in a high-concentration layer that persisted at the base of the flow for many minutes prior to the onset of deposition. <br/
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