49 research outputs found
Preeruptive flow focussing in dikes feeding historical pillow ridges on the Juan de Fuca and Gorda Ridges
Linear, hummocky pillow mound volcanism dominates at slow and intermediate spreading rate mid-ocean ridges. Volcanic hummocks are thought to be formed by low effusion rates or as a result of flow focussing during effusive fissure style eruptions in which the initial dike intercepts the seafloor and erupts along its entire length. In this study, high-resolution autonomous underwater vehicle (AUV) bathymetry is used to accurately map the extents of four historical fissure eruptions of the Juan de Fuca and Gorda ridges: on the North Gorda, North Cleft, and CoAxial ridge segments. The four mapped eruptions take the form of pillow mounds, which are similar in both lithology and dimension to hummocks on the Mid-Atlantic Ridge. Pillow mounds may be isolated, or coalesce to form composite mounds, aligned as ridges or as clustered groups. In three of the four mapped sites, the eruptions were discontinuous along their lengths, with pillow mounds and composite mounds commonly separated by areas of older seafloor. This style of discontinuous eruption is inconsistent with typical en echelon fissure eruptions and is probably due to a mildly overpressured, fingering dike intersecting the seafloor along parts of its length
Character of magnetic excitations in a quasi-one-dimensional antiferromagnet near the quantum critical points: Impact on magneto-acoustic properties
We report results of magneto-acoustic studies in the quantum spin-chain
magnet NiCl-4SC(NH) (DTN) having a field-induced ordered
antiferromagnetic (AF) phase. In the vicinity of the quantum critical points
(QCPs) the acoustic mode manifests a pronounced softening accompanied
by energy dissipation of the sound wave. The acoustic anomalies are traced up
to , where the thermodynamic properties are determined by fermionic
magnetic excitations, the "hallmark" of one-dimensional (1D) spin chains. On
the other hand, as established in earlier studies, the AF phase in DTN is
governed by bosonic magnetic excitations. Our results suggest the presence of a
crossover from a 1D fermionic to a 3D bosonic character of the magnetic
excitations in DTN in the vicinity of the QCPs.Comment: 5 pages, 4 figures. Accepted for publication by Phys. Rev
Blending HF Radar and Model Velocities in Monterey Bay Through Normal Mode Analysis
Nowcasts of the surface velocity field in Monterey Bay are made for the period August 1-9, 1994, using HF radar observations blended with results from a primitive equation model. A spectral method called normal mode analysis was used. Objective spatial and temporal filtering were performed, and stream function, velocity potential, relative vorticity, and horizontal divergence were calculated over the domain. This type of nowcasting permits global spectral analysis of mode amplitudes, calculation of enstrophy, and additional analyses using tools like empirical orthogonal functions. The nowcasts reported here include open boundary flow information from the numerical model. Nowcasts using no open boundary flow information, however, still provide excellent results for locations within the observation footprint. This method, then, is useful for filtering high-resolution data like HF radar observations, even when open boundary flow information is unavailable. Also, since the nowcast velocity gradient fields were much less noisy than the observations, this may be an effective method for preconditioning high-resolution observation sets for assimilation into a numerical model. Copyright 2000 by the American Geophysical Union
Evolution of Fine‐Scale Segmentation at Intermediate‐Spreading Rate Ridges
Mid‐ocean ridge axes are marked by segmentation of the axes and underlying magmatic systems. Fine‐scale segmentation has mainly been studied along fast‐spreading ridges. Here we offer insight into the third‐ and fourth‐order segmentation of intermediate‐spreading ridges and their temporal evolution. The Alarcón Rise and the Endeavour Segment have similar spreading rates (49 and 52.5 mm/year, respectively) but contrasting morphologies that vary from an axial high with a relatively narrow axial summit trough to an axial valley. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, lava geochemistry, and ages from sediment cores is combined with available seismic reflection profiles to analyze variations in (1) geometry and orientation of the axial summit trough or valley, (2) seafloor depth near the axis, and (3) distribution of hydrothermal vents, (4) lava chemistry, and (5) flow ages between contiguous axes. Along both intermediate‐spreading segments, third‐ and fourth‐order discontinuities and associated segments are similar in dimension to what has been observed along fast‐spreading ridges. The Alarcón Rise and the Endeavour Segment also allow the study of the evolution of fine‐scale segmentation over periods of 300 to 4,000 years. Comparison between old and young axes reveals that the evolution of fine‐scale segmentation depends on the intensity of the magmatic activity. High magmatic periods are associated with rapid evolution of third‐order segments, while low magmatic activity periods, dominated by tectonic deformation and/or hydrothermal activity, are associated with little to no change in segmentation
Synoptic Lagrangian maps: Application to surface transport in Monterey Bay
Here we report on an effort to describe in detail the evolution of surface water particles in Monterey Bay from the time they first enter until the time they leave. The data used for this study are objective mappings from hourly surface currents obtained from high frequency (HF) radar measurements in Monterey Bay for the period 2 June through 4 August 1999. The basic concept is simple: compute the origin and fate of a large number of particles for every hour during the analysis period. However, analyzing and displaying the enormous amount of computed trajectory information required a new data compression technique: synoptic Lagrangian maps produced by representing each trajectory by its origin/fate and its residence time. The results show unexpected complexity and variability not apparent in the Eulerian current archive. For example, the fraction of particles that escaped to the open ocean during this period varied from about 17 to more than 92 percent. Mean particle residence times ranged from 4.5 to 11 days. The distribution of particle residence times and transport pathways varied over time scales from hours to weeks, and space scales from 2 to 40 km. The wide range of variability in particle properties reported here shows that surface transport studies in Monterey Bay require detailed wind and tidal current information over the entire bay, as well as information about the flow along the open ocean boundary
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Origin of volcanic seamounts at the continental margin of California related to changes in plate margins
Volcanic samples collected with the Monterey Bay Aquarium Research Institute's ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB-like to relatively enriched OIB. High-precision ⁴⁰Ar/³⁹Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction-dominated to a transform margin. They differ in composition, age, and origin from other more widespread near-ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling-Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.Keywords: Seamounts, Chemistry, Ar-Ar age datesKeywords: Seamounts, Chemistry, Ar-Ar age date
Critical Properties of Condensation of Field-Induced Triplet Quasiparticles
A review on the field-induced magnetic ordering is given, together with some
results of a quantum Monte Carlo simulation focused on the critical behevior
near the quantum critical point.Comment: Proceedings of SPQS, Sendai, 200
Phenomenological model for the remanent magnetization of dilute quasi-one-dimensional antiferromagnets
We present a phenomenological model for the remanent magnetization at low
temperatures in the quasi-one-dimensional dilute antiferromagnets
CH_{3}NH_{3}Mn_{1-x}Cd_{x} Cl_{3}\cdot 2H_{2}O and
(CH_{3})_{2}NH_{2}Mn_{1-x}Cd_{x}Cl_{3}\cdot 2H_{2}O. The model assumes the
existence of uncompensated magnetic moments induced in the odd-sized segments
generated along the Mn(^{2+}) chains upon dilution. These moments are further
assumed to correlate ferromagnetically after removal of a cooling field. Using
a (mean-field) linear-chain approximation and reasonable set of model
parameters, we are able to reproduce the approximate linear temperature
dependence observed for the remanent magnetization in the real compounds.Comment: 5 pages, 2 figures; final version to appear in Physical Review
Evaluating Connectivity between Marine Protected Areas Using CODAR High-Frequency Radar
To investigate the connectivity between central California marine protected areas (MPAs), back-projections were calculated using the network of high-frequency (HF) radar ocean surface current mapping stations operated along the California coast by the member institutions of the Coastal Ocean Currents Monitoring Program with funding provided by California voters through Propositions 40 & 50 and administered by the State Coastal Conservancy. Trajectories of 1 km resolution grids of water particles were back-projected from ten MPAs each hour, out through 40 days in the past, from each day in 2008, producing a map of where surface waters travel over a 40-day period to reach the MPAs - and visualizations of the length of time the waters travel along these paths. By comparing the travel times of those back-projected track-points that crossed between MPA regions, the connection time between MPAs along the State\u27s central coast was assessed. Repeating these calculations resulted in a connectivity matrix between the MPAs in the region, and may be useful for assessing connectivity for the important invertebrate and fish larvae that are restricted to the surface ocean during a fraction of their lifecycle