长白山地区火山碎屑粒度特征研究

长白山地区全新世火山活动活跃,发育了良好的火山空降、火山碎屑流、火山涌流和火山泥石流堆积物。这些堆积物交错堆积,野外区分较为困难。在火山碎屑地层剖面调查基础上,系统采集了各种类型的火山碎屑堆积物样品。在实验室通过粒度参数和概率累积曲线分析,对堆积物成因类型进行了判别,讨论了火山空降堆积物和火山碎屑流堆积物随着与火口距离变化的规律。首次对研究区内粒度范围为62.5～0.02μm的细火山灰进行了粒度分析,对火山碎屑流和火山碎屑涌流中细火山灰端元分布特征和地质意义进行了分析和讨论     

Plinian eruptions and their products

Plinian eruptions are amongst the most powerful of explosive volcanic events, and the extensive pumice deposits which they produce have an exceptionally wide dispersal because of the great eruptive plume height. Historical data on 12 plinian eruptions, and available quantitative data on the deposits of these and 37 other plinian eruptions are collated in this review to characterise further the plinian eruptive style and its products and to establish the known limits of their variation. The deposit volumes have been recomputed according to a standard procedure to provide a better basis for comparison, and they vary over 4 orders of magnitude to reach a maximum of about 100 km³. Almost all volcanic magma compositions apart from the most mafic are represented among the juvenile products; rhyolitic and dacitic deposits account for 80% of the total volume and basaltic ones less than 1%. Compositional zoning is very common. Plinian eruptions are of open vent type and produce deposits which tend to be homogeneous in grain size and constitution through their thickness. Considerable departures from homogeneity often however exist. Finer grained beds which often interrupt the continuity can be produced by a number of different mechanisms, the features of which are summarised. In a significant proportion of plinian deposits the finer beds are the deposits of intraplinian pyroclastic flows, or are related to such flows; pyroclastic flows such as may be attributable to column collapse thus do not form exclusively at the end of the plinian phase. The most recent work indicates that major phreatoplinian eruptions dominated by the copious inflow of water into the vent can produce deposits quite as widely dispersed and as voluminous as the biggest plinian eruptions, and it appears that the characteristics of the grain size populations of the two types tend to converge in the most powerful eruptions.     

Characterization of pyroclastic fall and flow deposits from the 1815 eruption of Tambora volcano,Indonesia using ground-penetrating radar

Ground-penetrating radar (GPR) is used to image and characterize fall and pyroclastic flow deposits from the 1815 eruption of Tambora volcano in Indonesia. Analysis of GPR common-mid-point (CMP) data indicate that the velocity of radar in the sub-surface is 0.1 m/ns, and this is used to establish a preliminary traveltime to-depth conversion for common-offset reflection profiles. Common-offset radar profiles were collected along the edge of an erosional gully that exposed approximately 1–2 m of volcanic stratigraphy. Additional trenching at select locations in the gully exposed the contact between the pre-1815 eruption surface and overlying pyroclastic deposit from the 1815 eruption. The deepest continuous, prominent reflection is shown to correspond to the interface between pre-eruption clay-rich soil and pyroclastics that reach a maximum thickness of 4 m along our profiles. This soil surface is distinctly terraced and is interpreted as the ground surface augmented for agriculture and buildings by people from the kingdom of Tambora. The correlation of volcanic stratigraphy and radar data at this location indicates that reflections are produced by the soil-pyroclastic deposit interface and the interface between pyroclastic flows (including pyroclastic surge) and the pumice-rich fall deposits. In the thickest deposits an additional reflection marks the interface between two pyroclastic flow units.     

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km² reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (>150 km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105 km³ of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350 km³ of deposits. Thus, the total volume of this tephra amounts over 380–490 km³. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.     

Estimation of tephra volumes from sparse and incompletely observed deposit thicknesses

We present a Bayesian statistical approach to estimate volumes for a series of eruptions from an assemblage of sparse proximal and distal tephra (volcanic ash) deposits. Most volume estimates are of widespread tephra deposits from large events using isopach maps constructed from observations at exposed locations. Instead, we incorporate raw thickness measurements, focussing on tephra thickness data from cores extracted from lake sediments and through swamp deposits. This facilitates investigation into the dispersal pattern and volume of tephra from much smaller eruption events. Given the general scarcity of data and the physical phenomena governing tephra thickness attenuation, a hybrid Bayesian-empirical tephra attenuation model is required. Point thickness observations are modeled as a function of the distance and angular direction of each location. The dispersal of tephra from larger well-estimated eruptions are used as leverage for understanding the smaller unknown events, and uncertainty in thickness measurements can be properly accounted for. The model estimates the wind and site-specific effects on the tephra deposits in addition to volumes. Our technique is exemplified on a series of tephra deposits from Mt Taranaki (New Zealand). The resulting estimates provide a comprehensive record suitable for supporting hazard models. Posterior mean volume estimates range from 0.02 to 0.26 km ³. Preliminary examination of the results suggests a size-predictable relationship.     

Characterization of volcanic deposits with ground-penetrating radar

 Field-based studies of surficial volcanic deposits are commonly complicated by a combination of poor exposure and rapid lateral variations controlled by unknown paleotopography. The potential of ground-penetrating radar (GPR) as an aid to volcanological studies is shown using data collected from traverses over four well-exposed, Recent volcanic deposits in western Canada. The deposits comprise a pumice airfall deposit (3–4 m thick), a basalt lava flow (3–6 m thick), a pyroclastic flow deposit (15 m thick), and an internally stratified pumice talus deposit (60 m thick). Results show that GPR is effective in delineating major stratigraphic contacts and hence can be used to map unexposed deposits. Different volcanic deposits also exhibit different radar stratigraphic character, suggesting that deposit type may be determined from radar images. In addition, large blocks within the pyroclastic deposits are detected as distinctive point diffractor patterns in the profiles, showing that the technique has potential for providing important grain-size information in coarse poorly sorted deposits. Laboratory measurements of dielectric constant (K') are reported for samples of the main rock types and are compared with values of K' for the bulk deposit as inferred from the field data. The laboratory values differ significantly from the "field" values of K'; these results suggest that the effectiveness of GPR at any site can be substantially improved by initial calibration of well-exposed locations. Received: 10 May 1996 / Accepted: 27 December 1996     

Paleomagnetic evidence for low-temperature emplacement of the phreatomagmatic Peperino Albano ignimbrite (Colli Albani volcano, Central Italy)

The Peperino Albano (approximately 19–36 ka old) is a phreatomagmatic pyroclastic flow deposit, cropping out along the slopes of the associated Albano maar (Colli Albani volcano, Italy). The deposit exhibits lateral and vertical transitions from valley pond to veneer facies, as well as intracrater facies. We present the results of a paleomagnetic study of thermal remanent magnetization (TRM) of the lithic clasts of the Peperino Albano ignimbrite that provide quantitative estimates of the range of emplacement temperatures across the different facies of the ignimbrite. Emplacement temperatures estimated for the Peperino Albano ignimbrite range between 240° and 350°C, with the temperatures defined in the intracrater facies being generally lower than in the valley pond and veneer facies. This is possibly due to the large size of the sampled clasts in the intracrater facies which, when coupled with low temperature at the vent, were not completely heated throughout their volume during emplacement. The emplacement temperatures derived from the paleomagnetic results are in good agreement with the presence of un-burnt plants at the base of the ignimbrite, indicating that the temperature of the pyroclastic flow was lower than the temperature of ignition of wood. Paleomagnetic results from the Peperino Albano confirm the reliability of the paleomagnetic approach in defining the thermal history of pyroclastic flow deposits.     

Paleomagnetic determination of emplacement temperatures of pyroclastic deposits: an under-utilized tool

Paleomagnetic data from lithic clasts collected from Mt. St. Helens, USA, Volcán Láscar, Chile, Volcán de Colima, Mexico and Vesuvius, Italy have been used to determine the emplacement temperature of pyroclastic deposits at these localities and to highlight the usefulness of the paleomagnetic method for determining emplacement temperatures. At Mt. St. Helens, the temperature of the deposits (T _dep ) at three sites from the June 12, 1980 eruption was found to be ≥532°C, ≥509°C, and 510–570°C, respectively. One site emplaced on July 22, 1980 was emplaced at ≥577°C. These new paleomagnetic temperatures are in good agreement with previously published direct temperature measurements and paleomagnetic estimates. Lithic clasts from pyroclastic deposits from the 1993 eruption of Láscar were fully remagnetized above the respective Curie temperatures, which yielded a minimum T _dep of 397°C. Samples were also collected from deposits thought to be pyroclastics from the 1913, 2004 and 2005 eruptions of Colima. At Colima, the sampled clasts were emplaced cold. This is consistent with the sampled clasts being from lahar deposits, which are common in the area, and illustrates the usefulness of the paleomagnetic method for distinguishing different types of deposit. T _dep of the lower section of the lithic rich pyroclastic flow (LRPF) from the 472 A.D. deposits of Vesuvius was ~280–340°C. This is in agreement with other, recently published paleomagnetic measurements. In contrast, the upper section of the LRPF was emplaced at higher temperatures, with T _dep ~520°C. This temperature difference is inferred to be the result of different sources of lithic clasts between the upper and lower sections, with the upper section containing a greater proportion of vent-derived material that was initially hot. Our studies of four historical pyroclastic deposits demonstrates the usefulness of paleomagnetism for emplacement temperature estimation.     

Flow directions inferred from imbrication in the Handa pyroclastic flow deposit in Japan

Flow direction patterns have been determined by imbrication measurements of pumice and lithic fragments of the Handa pyroclastic flow deposit, in order to estimate the source vent location and to analyze the flow behavior. The pyroclastic flow deposit studied is dacitic in composition, 2 km² in volume, and >32,300 Y.B.P. in age. Flow directions from 52 outcrops indicate a source vent located within the area of recent lava domes of Kuju Volcano. The distribution of the pyroclastic flow deposit and the flow direction patterns determined by imbrication suggest that the pyroclastic flow accurately followed the topographic relief at the time of eruption. The presence of imbrication indicates the change of flow-regime from turbulent condition to laminar condition according to the distance from the source vent. Imbrication is visible within the lower-half reaches of the pyroclastic flow distribution, where the pyroclastic flow had developed the laminar flow characteristics of a dense gravity current.     

Flow behavior of large-scale pyroclastic flows — Evidence obtained from petrofabric analysis

The grain orientations within the matrix of two large-scale welded, two small-scale nonwelded and two nonwelded low-aspect ratio pyroclastic flow deposits are measured to analyze flow behavior. Preferred grain alignments are especially apparent in the middle part of layer 2 of each deposit. Preferred grain alignments do not vary laterally within a 10 m interval. The grain alignments obtained are radial from the source caldera, especially in proximal to medial and plateau-forming facies of pyroclastic flow deposits. Grain alignments are controlled by valley-channel directions for the valley-ponded facies of pyroclastic flow deposits, especially at medial to distal locations. Such local topographic factors strongly affect the data for high-aspect ratio and smallscale deposits, and weakly affect the data for widespread low-aspect ratio pyroclastic flow deposits. This work suggests that grain alignment analysis should be used with care when attempting to determine the location of an unknown source.     

Depositional ramps: asymmetrical distribution structure in the Ata pyroclastic flow deposit,Japan

The asymmetrical distribution of the welded Ata large-scale pyroclastic flow deposit in Southern Kyushu, Japan was identified. This distribution pattern was defined as depositional ramps. Depositional ramps can be identified in valleys wider than 1 km and become smaller-scale with increasing distance from the source. Upslope directions of depositional ramps are generally radially away from the source caldera, suggesting that the structure was formed by the flow of pyroclastic material radially away from the source. The original depositional surface was reconstructed based on field mapping and density measurements of the pyroclastic flow deposit. Depositional ramps having a dip angle of more than 9° were reconstructed on the vent-facing slopes of the topography underlying the valley-filling deposits in the area within 10 km of the caldera rim. Such a dip angle is much larger than previously described dip angles. The size and gradient of the depositional ramps decreases with increasing distance from the source. Depositional ramps are recognized commonly in densely welded pyroclastic flow deposits. A high emplacement temperature is required to form the depositional ramps. This suggests that the pyroclastic flow was transported as a dense, fluidized layer to minimize heat loss.     

Palaeomagnetic estimates of emplacement temperatures of pyroclastic deposits on Santorini,Greece

Thermal remanent magnetism provides a method of quantitatively determining the emplacement temperature of individual lithic clasts in a volcaniclastic rock. The technique is reviewed and applied to two types of Quaternary pyroclastic deposit on Santorini. Emplacement-temperature estimates for lithic clasts from two co-ignimbrite lithic breccias (Cape Riva and Middle Pumice eruptions) range from 250°C to 580°C, showing unambiguously that the breccias were emplaced hot. Good precision on temperature estimates (about ±20°C) were obtained from the Cape Riva breccias. Lithics in a Plinian airfall deposit from the Middle Pumice eruption give less precise results because the primary magnetisation has been partly overprinted by chemical (and/or viscous) remanence, and some clasts may have rotated during compaction of the deposit. Temperatures from proximal airfall are consistent with welding of the deposit within 1.5 km from vent. Temperature estimates for lithic clasts further from vent scatter, but a falloff of temperature away from vent can be recognised if an average emplacement temperature for the whole deposit is identified at each location. The study highlights some difficulties in interpreting quantitative temperature estimates for prehistoric pyroclastic deposits.     

The AD 1362 Öræfajökull eruption,S.E. Iceland: Physical volcanology and volatile release

The explosive rhyolitic eruption of Öræfajökull volcano, Iceland, in AD 1362 is described and interpreted based on the sequence of pyroclastic fall and flow deposits at 10 proximal locations around the south side of the volcano. Öræfajökull is an ice-clad stratovolcano in south central Iceland which has an ice-filled caldera (4–5 km diameter) of uncertain origin. The main phase of the eruption took place over a few days in June and proceeded in three main phases that produced widely dispersed fallout deposits and a pyroclastic flow deposit. An initial phase of phreatomagmatic eruptive activity produced a volumetrically minor, coarse ash fall deposit (unit A) with a bi-lobate dispersal. This was followed by a second phreatomagmatic, possibly phreatoplinian, phase that deposited more fine ash beds (unit B), dispersed to the SSE. Phases A and B were followed by an intense, climactic Plinian phase that lasted ∼ 8–12 h and produced unit C, a coarse-lapilli, pumice-clast-dominated fall deposit in the proximal region. At the end of Plinian activity, pyroclastic flows formed a poorly-sorted deposit, unit D, presently of very limited thickness and exposed distribution. Much of Eastern Iceland is covered with a very fine distal ash layer, dispersed to the NE. This was probably deposited from an umbrella cloud and is the distal representation of the Plinian fallout. A total bulk fall deposit volume of ∼ 2.3 km³ is calculated (∼ 1.2 km³ DRE). Pyroclastic flow deposit volumes have been crudely estimated to be < 0.1 km³. Maximum clast size data interpreted by 1-D models suggests an eruption column ∼ 30 km high and mass discharge rates of ∼ 10⁸ kg s^− 1. Ash fall may have taken place from heights around 15 km, above the local tropopause (∼ 10 km), with coarser clasts dispersed below that under a different wind regime. Analyses of glass inclusions and matrix glasses suggest that the syn-eruptive SO₂ release was only ∼ 1 Mt. This result is supported by published Greenland ice-core acidity peak data that also suggest very minor sulphate deposition and thus SO₂ release. The small sulphur release reflects the low sulphur solubility in the 1362 rhyolitic melt. The low tropopause over Iceland and the 30-km-high eruption column certainly led to stratospheric injection of gas and ash but little sulphate aerosol was generated. Moreover, pre-eruptive and degassed halogen concentrations (Cl, F) indicate that these volatiles were not efficiently released during the eruption. Besides the local pyroclastic flow (and related lahar) hazard, the impact of the Öræfajökull 1362 eruption was perhaps restricted to widespread ash fall across Eastern Iceland and parts of northern Europe.     

Late Pleistocene and Holocene activity of the Atacazo–Ninahuilca Volcanic Complex (Ecuador)

The Atacazo–Ninahuilca Volcanic Complex (ANVC) is located in the Western Cordillera of Ecuador, 10 km southwest of Quito. At least six periods of Pleistocene to Holocene activity (N1 to N6) have been preserved in the geologic record as tephra fallouts and pyroclastic flow deposits. New field data, including petrographic and whole-rock geochemical analyses of over forty soil and tephra sections, 100 pumice and lithic samples, and 10 new ¹⁴C ages allow us to constrain: (1) the tephra fall isopachs and detailed characteristics of the last two events (N5–N6) including volume estimates of the tephra and pyroclastic flow deposits and the corresponding volcanic explosivity index (VEI); (2) the petrographical and geochemical correlations between domes, tephras, and pyroclastic flow deposits; and, (3) the timing of the last 4 eruptive events and a period of quiescence that endured a few thousand years (1000–4000).     

Interrelations among pyroclastic surge,pyroclastic flow,and lahars in Smith Creek valley during first minutes of 18 May 1980 eruption of Mount St. Helens,USA

A devastating pyroclastic surge and resultant lahars at Mount St. Helens on 18 May 1980 produced several catastrophic flowages into tributaries on the northeast volcano flank. The tributaries channeled the flows to Smith Creek valley, which lies within the area devastated by the surge but was unaffected by the great debris avalanche on the north flank. Stratigraphy shows that the pyroclastic surge preceded the lahars; there is no notable “wet” character to the surge deposits. Therefore the lahars must have originated as snowmelt, not as ejected water-saturated debris that segregated from the pyroclastic surge as has been inferred for other flanks of the volcano. In stratigraphic order the Smith Creek valley-floor materials comprise (1) a complex valley-bottom facies of the pyroclastic surge and a related pyroclastic flow, (2) an unusual hummocky diamict caused by complex mixing of lahars with the dry pyroclastic debris, and (3) deposits of secondary pyroclastic flows. These units are capped by silt containing accretionary lapilli, which began falling from a rapidly expanding mushroom-shaped cloud 20 minutes after the eruption's onset. The Smith Creek valley-bottom pyroclastic facies consists of (a) a weakly graded basal bed of fines-poor granular sand, the deposit of a low-concentration lithic pyroclastic surge, and (b) a bed of very poorly sorted pebble to cobble gravel inversely graded near its base, the deposit of a high-concentration lithic pyroclastic flow. The surge apparently segregated while crossing the steep headwater tributaries of Smith Creek; large fragments that settled from the turbulent surge formed a dense pyroclastic flow along the valley floor that lagged behind the front of the overland surge. The unusual hummocky diamict as thick as 15 m contains large lithic clasts supported by a tough, brown muddy sand matrix like that of lahar deposits upvalley. This unit contains irregular friable lenses and pods meters in diameter, blocks incorporated from the underlying dry and hot pyroclastic material that had been deposited only moments earlier. The hummocky unit is the deposit of a high-viscosity debris flow which formed when lahars mingled with the pyroclastic materials on Smith Creek valley floor. Overlying the debris flow are voluminous pyroclastic deposits of pebbly sand cut by fines-poor gas-escape pipes and containing charred wood. The deposits are thickest in topographic lows along margins of the hummocky diamict. Emplaced several minutes after the hot surge had passed, this is the deposit of numerous secondary pyroclastic flows derived from surge material deposited unstably on steep valley sides.     

Thermoremanence in red sandstone clasts and emplacement temperature of a quaternary pyroclastic deposit (Catalan Volcanic Zone,ne Spain)

Summary The application of the progressive thermal demagnetization procedure of volcanic rock debris has been frequently used to determine the emplacement temperatures of pyroclastic deposits and thus to characterize the nature of these volcanic deposits. This debris consists of a mixture of juvenile fragments derived from the explosive fragmentation of erupting magma and an assortment of lithic clasts derived mainly from the walls of a volcanic conduit, as well as from the ground. The temperature at which the clasts were deposited can be estimated by analyzing its remanent magnetization. To do this, oriented samples of clasts are subjected to progressive thermal demagnetization and the directions of the resulting remanent vectors provide the necessary information. Clasts of basalt, andesite, limestone, pumice and homebricks have previously been used to estimate the emplacement temperatures of pyroclastic deposits. According to our data, clasts of red sandstones also seem to be good carriers of thermoremanent magnetization. We have carried out a paleomagnetic study on a Quaternary, lithic-rich, massive, pyroclastic deposit from the Puig d'Adri volcano (Catalan Volcanic Zone), which contains a large number of red sandstone clasts. It is concluded that the studied deposit cannot be considered as a lahar or as a pyroclastic surge deposit, considering both the emplacement temperature and the morphological features.Presented at 3rd Biennial Meeting on New Trends in Geomagnetism, Smolenice Castle, West Slovakia, June 22–29, 1992     

Ranking welding intensity in pyroclastic deposits

Welding of pyroclastic deposits involves flattening of glassy pyroclasts under a compactional load at temperatures above the glass transition temperature. Progressive welding is recorded by changes in the petrographic (e.g., fabric) and physical (e.g., density) properties of the deposits. Mapping the intensity of welding can be integral to studies of pyroclastic deposits, but making systematic comparisons between deposits can be problematical. Here we develop a scheme for ranking welding intensity in pyroclastic deposits on the basis of petrographic textural observations (e.g., oblateness of pumice lapilli and micro-fabric orientation) and measurements of physical properties, including density, porosity, point load strength and uniaxial compressive strength. Our dataset comprises measurements on 100 samples collected from a single cooling unit of the Bandelier Tuff and parallel measurements on 8 samples of more densely welded deposits. The proposed classification comprises six ranks of welding intensity ranging from unconsolidated (Rank I) to obsidian-like vitrophyre (Rank VI) and should allow for reproducible mapping of subtle variations in welding intensity between different deposits. The application of the ranking scheme is demonstrated by using published physical property data on welded pyroclastic deposits to map the total accumulated strain and to reconstruct their pre-welding thicknesses.Editorial Responsibility: T. Druitt     

Rounding of pumice clasts during transport: field measurements and laboratory studies

Volcanic clasts in many pyroclastic density current deposits are notably more round than their counterparts in corresponding fall deposits. This increase in roundness and sphericity reflects different degrees of comminution, abrasion and breakup during transport. We performed experimental measurements to determine an empirical relationship between particle shape and mass loss caused by particle–particle interactions. We consider, as examples, pumice from four volcanoes: Medicine Lake, California; Lassen, California; Taupo, New Zealand; Mount St Helens, Washington. We find that average sample roundness reaches a maximum value once particles lose between 15% and 60% of their mass. The most texturally homogeneous clasts (Taupo) become the most round. Crystal-rich pumice abrades more slowly than crystal-free pumice of similar density. Abrasion rates also decrease with time as particles become less angular. We compare our experimental measurements with the shapes of clasts in one of the May 18, 1980 pyroclastic density current units at Mount St Helens, deposited 4–8 km from the vent. The measured roundness of these clasts is close to the experimentally determined maximum value. For a much smaller deposit from the 1915 Lassen eruption, clast roundness is closer to the value for pumice in fall deposits and suggests that only a few volume percent of material was removed from large clasts. In neither field deposit do we see a significant change in roundness with increasing distance from the vent. We suggest that this trend is recorded because much of the rounding and ash production occur in proximal regions where the density currents are the most energetic. As a result, all clasts that are deposited have experienced similar amounts of comminution in the proximal region, and similar amounts of abrasion as they settle through the dense, near-bed region prior to final deposition.     

Deposition temperature of some PDC deposits from the 1982 eruption of El Chichón volcano (Chiapas,Mexico) inferred from rock-magnetic data

Thermal remanent magnetization (TRM) analyses were carried out on lithic fragments from two different typologies of pyroclastic density current (PDC) deposits of the 1982 eruption of El Chichón volcano, in order to estimate their equilibrium temperature (T_dep) after deposition. The estimated T_dep range is 360–400 °C, which overlaps the direct measurements of temperature carried out four days after the eruption on the PDC deposits. This overlap demonstrates the reliability of the TRM method to estimate the T_dep of pyroclastic deposits and to approximate their depositional temperature. These results also constraint the time needed for reaching thermal equilibrium within four days for the studied PDC deposits, in agreement with predictions of theoretical models.