Dated Lava Flows Research Articles

Benchmarking a new 2.5D shallow water model for lava flows

Lava flows associated with effusive volcanic eruptions require accurate modelling in order to forecast potential paths of destruction.This study presents a new depth-averaged model that overcomes the classical shallow water hypothesis by incorporating several enhancements, allowing for a more precise representation of the flow dynamics and behaviour: (i) a parabolic profile which captures the vertical variations in velocity within the flow; (ii) a non-constant vertical profile for temperature, enabling a more realistic representation of thermal gradients within the flowing lava; (iii) a viscoplastic temperature-dependent viscosity model to account for the non-Newtonian behaviour of lava; (iv) a transport equation for temperature accounting for the thermal heat exchanges with the environment and the soil. The first two modifications allow us to describe, under reasonable assumptions, the vertical structure of the flow, and for this reason, we put our model in the class of 2.5D models.To assess the performance of our modified model, comprehensive benchmark tests are conducted using both laboratory experiments and real-world lava flow data related to the 2014–2015 Pico do Fogo, Cape Verde, effusive eruption. The benchmarking analysis demonstrates that this model accurately reproduces, with short execution times, essential flow features such as flow front advancement and cooling processes.

Full‐Vector Paleosecular Variation Curve for the Azores: Enabling Reliable Paleomagnetic Dating for the Past 2 kyr

AbstractFor archeomagnetic dating, high‐quality directional and intensity paleosecular variation curves are needed. The Azores Archipelago in the mid‐Atlantic Ocean provides a wealth of volcanic products erupted during the Holocene, making it an ideal location to (1) gather paleomagnetic data from well dated lava flows and (2) construct a paleosecular variation (PSV) curve that enables paleomagnetic dating of volcanic products with unknown age. Here, we present new full‐vector paleomagnetic data from Pico Island, and combine the new data with existing data from neighboring islands to construct a new full‐vector PSV curve for the Azores Archipelago. An extensive rock‐magnetic study underpins the quality of our paleomagnetic carriers. From Pico Island, we obtained 21 new mean site directions; and 15 paleointensity estimates with the multimethod paleointensity approach from 12 sites, the age was known for 14 and 10 sites, respectively. By bootstrapping the non‐Gaussian uncertainty estimates of the radiocarbon age calibrations and the confidence intervals associated with the direction and paleointensity estimates, we produce the first full‐vector PSV curve with confidence intervals for the Azores covering the past 2 kyr. The PSV curve reveals a period of low inclination between ∼900 and 1560 AD, with minimum values of 32°. The potential of our new full‐vector PSV curve is demonstrated by successfully dating five lava flows from Pico Island.

Dating young (

Dating recent lava flows is a critical issue to assess the volcanic hazards for nearby human populations, but traditional methods such as radiocarbon dating are often not applicable. We propose here a simple statistical method that relates the age of lava flows of the Piton de la Fournaise volcano (PdF) to the size of Agarista salicifolia (Ericaceae), a long-lived pioneer tree that quickly establishes after eruption and able to live > 600 years. We measured the diameter at base of 711 trees on 20 dated lava flows (between 1401 CE and 2007 CE). We used a log-log linear model to assess the relationship between maximum diameter at base of Agarista salicifolia and the age of lava flows, and showed a very strong correlation (R2 = 0.987, p < 10-15). We then used this calibrated model to estimate the age of 11 lava flows between 1447 CE (CI: 1349–1531) and 1823 CE (CI: 1806–1839). These new ages, combined with existing radiocarbon ages and historical records, indicate three clusters of eruptions (1460–1630, 1690–1840, since 1970 CE) affecting both the caldera and the flanks of PdF. We interpret such discontinuous dynamics made of periods of intense and low activity as evidencing pulses of high magma supply since at least the 11th century. Overall, our work shows that dating lava flow with calibration based on the size distribution of long-lived pioneer trees represents an accurate alternative method to redefine the hazard map of lava flow inundation. The existence of long-lived pioneer trees in several volcanic areas provides the opportunity to use the same framework in order to better understand eruption recurrence patterns.

Glaciovolcanic emplacement of an intermediate hydroclastic breccia-lobe complex during the penultimate glacial period (190–130 ka), Ruapehu volcano, New Zealand

Abstract An intermediate-composition hydroclastic breccia deposit is exposed in the upper reaches of a deep glacial valley at Ruapehu volcano, New Zealand, indicating an ancient accumulation of water existed near the current summit area. Lobate intrusions within the deposit have variably fluidal and brecciated margins, and are inferred to have been intruded while the deposit was wet and unconsolidated. The tectonic setting, elevation of Ruapehu, and glacial evidence suggest that the deposit-forming eruption took place in meltwater produced from an ancient glacier. The breccia-lobe complex is inferred to have been emplaced at &amp;gt; 154 ± 12 ka, during the penultimate glacial period (190–130 ka) when Ruapehu’s glaciers were more extensive than today. This age is based on overlying radiometrically dated lava flows, and by correlation with a well-constrained geochemical stratigraphy for Ruapehu. Field relations indicate that the glacier was at least 150 m thick, and ubiquitous quench textures and jigsaw-fit fracturing suggest that the clastic deposit was formed from non-explosive fragmentation of lava in standing water. Such features are unusual for the high flanks of a volcanic edifice where steep topography typically hinders accumulation of water or thick ice, and hence the formation and retention of hydroclastic material. Although not well-constrained for this time, the vent configuration at Ruapehu is inferred to have contributed to an irregular edifice morphology, allowing thick ice to locally accumulate and meltwater to be trapped.

Extending Global Continuous Geomagnetic Field Reconstructions on Timescales Beyond Human Civilization

AbstractStudy of the late Quaternary geomagnetic field contributes significantly to understanding the origin of millennial‐scale paleomagnetic secular variations, the structure of geomagnetic excursions, and the long‐term shielding by the geomagnetic field. A compilation of paleomagnetic sediment records and archeomagnetic and lava flow data covering the past 100 ka enables reconstruction of the global geomagnetic field on such long‐term scales. We use regularized inversion to build the first global, time‐dependent, geomagnetic field model spanning the past 100 ka, named GGF100k (Global Geomagnetic Field over the past 100 ka). Spatial parametrization of the model is in spherical harmonics and time variations with cubic splines. The model is heavily constrained by more than 100 continuous sediment records covering extended periods of time, which strongly prevail over the limited number of discrete snapshots provided by archeomagnetic and volcanic data. Following an assessment of temporal resolution in each sediment's magnetic record, we have introduced smoothing kernels into the forward modeling when assessing data misfit. This accommodates the smoothing inherent in the remanence acquisition in individual sediment paleomagnetic records, facilitating a closer fit to both high‐ and low‐resolution records in regions where some sediments have variable temporal resolutions. The model has similar spatial resolution but less temporal complexity than current Holocene geomagnetic field models. Using the new reconstruction, we discuss dipole moment variations, the time‐averaged field, and paleomagnetic secular variation activity. The new GGF100k model fills the gap in the geomagnetic power spectrum in the frequency range 100–1,000 Ma−1.

Of mosses and men: Plant succession, soil development and soil carbon accretion in the sub-Arctic volcanic landscape of Hekla, Iceland

Lava flows pose a hazard in volcanic environments and reset ecosystem development. A succession of dated lava flows provides the possibility to estimate the direction and rates of ecosystem development and can be used to predict future development. We examine plant succession, soil development and soil carbon (C) accretion on the historical (post 874 AD) lava flows formed by the Hekla volcano in south Iceland. Vegetation and soil measurements were conducted all around the volcano reflecting the diverse vegetation communities on the lavas, climatic conditions around Hekla mountain and various intensities in deposition of loose material. Multivariate analysis was used to identify groups with similar vegetation composition and patterns in the vegetation. The association of vegetation and soil parameters with lava age, mean annual temperature, mean annual precipitation and soil accumulation rate (SAR) was analysed. Soil carbon concentration increased with increasing lava age becoming comparable to concentrations found on the prehistoric lavas. The combination of a sub-Arctic climate, gradual soil thickening due to input of loose material and the specific properties of volcanic soils allow for continuing accumulation of soil carbon in the soil profile. Four successional stages were identified: initial colonization and cover coalescence (ICC) of Racomitrium lanuginosum and Stereocaulon spp. (lavas &lt;70 years of age); secondary colonization (SC) – R. lanuginosum dominance (170−700 years); vascular plant dominance (VPD) (&gt;600 years); and highland conditions/retrogression (H/R) by tephra deposition (70−860 years). The long time span of the SC stage indicates arrested development by the thick R. lanuginosum moss mat. The progression from SC into VPD was linked to age of the lava flows and soil depth, which was significantly deeper within the VPD stage. Birch was growing on lavas over 600 years old indicating the development towards birch woodland, the climax ecosystem in Iceland.

Paleomagnetism and Aeromagnetic Survey From Tancitaro Volcano (Central Mexico) - Paleo-Secular Variation at Low Latitudes During the Past 1 Ma

The Tancitaro volcano (TV) is part of the Michoacan-Guanajuato monogenetic volcanic field (MGVF) in the central-western sector of the Trans-Mexican Volcanic Belt (TMVB). Results of a paleomagnetic study of radiometrically dated lava flows from Tancitaro volcano were used to investigate the paleosecular variation (PSV) and time averaged field (TAF) at low latitudes. Ar-Ar dates range from ~70 to 960 kyr spanning the Brunhes and Matuyama polarity chrons. All samples yielded well defined normal polarity magnetization. Two flows are correlated to the Jaramillo polarity event, which provide a useful marker for the volcanic activity in the MGVF. For the PSV and TAF analysis, paleodirections were combined with previously reported high standard results. The aeromagnetic survey around the Tancitaro volcano was characterized by a trend of regional anomalies over the volcanic structures. The residual field showed several positive and negative anomalies. The Tancitaro volcano is marked by a broad positive anomaly suggesting the presence of a large underground source. Spectral analysis of this anomaly field gives an average estimate to the top of the source bodies between 2-3 km.

Rock-magnetic and paleomagnetic survey on dated lava flows erupted during the Bruhnes and Matuyama chrons: the Mascota Volcanic Field revisited (Western Mexico)

A rock magnetic and paleomagnetic investigation was performed on some selected, radiometrically dated lava flows from the Mascota Volcanic Field (MVF), western Trans- Mexican Volcanic Belt. A set of rock-magnetic experiments and standard paleomagnetic analysis were carried out on 19 sites spanning the time interval from 2268 to 72 kyr. The paleomagnetic directions are anchored to absolute radiometric ages while no such information was available in previous studies. This makes possible to correctly evaluate the fluctuation of Earth’s magnetic field from Pliocene to Pleistocene and reveal the firm evidence of possible Levantine excursion. Both Ti-poor and Ti-rich titanomagnetites seem to carry the remanent magnetization with Curie temperatures ranging from 350°C to 537°C. Thirteen flows correspond to the Brunhes chron, one of them exhibits transitional directions, while the remaining six sites belong to the Matuyama chron. New and existing dataset for MVF were used to estimate the paleosecular variation parameters. The selected data include 35 Plio-Quaternary lava flows. After excluding the poor quality data, as well as the transitional directions, the mean paleodeclination is 356.1° and oaleoinclination 39.9°, which agree well with the geocentric axial dipole (GAD) and the expected paleodirections for the Plio-Pleistocene, as derived from the reference poles for the stable North America. The corresponding mean paleomagnetic poles are paleolongitude 226.7° and paleolatitude 86.0°. The virtual geomagnetic pole scatter for the MVF is 15.2°, which is consistent with the value expected from model G at latitude of 20° (this model provides an interpretation of the paleosecular variation at different latitudes for the time of interest). The combined paleomagnetic data, supported by positive reversal test, indicate no paleomagnetically detectable vertical-axis rotations in the study area. The evidence of one transitional directions was detected, which may correspond to the Levantine excursion (360-370 kyr) or unnamed event between 400-420 kyr.

A high-resolution 40Ar/39Ar lava chronology and edifice construction history for Ruapehu volcano, New Zealand

Ruapehu is an active ~150km3 andesite-dacite composite volcano located at the southern end of the Taupo Volcanic Zone, New Zealand. The growth of the present-day edifice has occurred throughout coeval eruptive and glacial histories since ~200ka. We present high-precision 40Ar/39Ar eruption ages for 46 samples and whole-rock major element geochemical data for 238 samples from lava flows. These new and existing data are interpreted in the context of geomorphologic and geologic mapping, volcano-ice interaction processes and glacier reconstructions to present an improved chronostratigraphic framework and new edifice evolution history for Ruapehu. Sub-glacial to ice-marginal effusive eruption of medium-K basaltic-andesites and andesites constructed the northern portion of the exposed edifice between ~200 and 150ka, and a wide southeast planèze as well as parts of the northern, eastern and western flanks between ~160 and 80ka. None of the dated lava flows have ages in the range of 80–50ka, which may reflect an eruptive hiatus. Alternatively the lack of ages may be the result of erosion and burial of lavas and syn-eruptive glacial conveyance of lava flows to the ring-plain during glacial advance at 70–60ka. From ~50–15ka edifice growth occurred via effusive eruptions onto the glaciated flanks of the volcano, resulting in construction of ice-bounded planèzes and ridges. The distribution of dated ice-marginal lavas indicates a general decrease in glacier thicknesses over this time, except for a short-lived period centred at ~31ka when peak ice cover was reached. The compositional ranges of medium- to high-K andesite and dacite lava flows within 50–35ka eruptive packages define broadly bimodal high- and low-MgO trends. Lavas erupted at ~35–22ka have compositions that fill a transitional geochemical field between older dacites and younger andesites. Large-scale retreat of flank glaciers since ~15ka has resulted in intra-valley lava flow emplacement at elevations below ~1500m on the edifice. Between 15 and 10ka unstable cones were constructed through effusive activity in the presence of remnant summit and upper flank glaciers and the emplacement of eruptive deposits on top of hydrothermally altered and fragmental sub-glacial and ice-marginal deposits. Debuttressing of two northern summit cones and a southern summit cone as ice underwent continued post-glacial retreat pre-conditioned the edifice for two major sector collapses. Ultimate triggering of the collapse events and deposition of debris avalanche deposits on the northern and south-eastern flanks may have been the result of tectonic or volcanic activity. The northern collapse scar is infilled by a new cone comprising <10ka lava flows that form the upper northern and eastern flanks of the modern edifice. Late Holocene-Recent eruptive activity has occurred through Crater Lake, which occupies the site of the collapsed southern cone. Deglaciation did not induce enhanced magma flux rates at Ruapehu as indicated by calculated eruptive volumes and their chronologies. However, volcano-ice interactions have had a fundamental influence on eruptive styles, sector collapse events, and the shape of the edifice by modifying the distribution, morphology and preservation of eruptive deposits for at least 200kyr. Our approach to integrating field and geochronological datasets from Ruapehu is directly relevant to the evolution of glaciated Quaternary composite cones worldwide, particularly those along the circum-Pacific continental volcanic arcs.

Topographic constraints on magma accumulation below the actively uplifting Uturuncu and Lazufre volcanic centers in the Central Andes

Geodetic surveys of Volcan Uturuncu and the Lazufre volcanic complex in the Central Andes of South America reveal sustained surface uplift from magmatic intrusion at depth. However, the decadal timescales of geodetic surveys are short relative to the timescales of magma chamber growth. Thus, from ­geodesy alone, it is difficult to infer the deformation and hence magma accumulation history of these volcanoes. Here we combine data from InSAR, long-wavelength topography, GPS and high-resolution topographic surveys of lake shorelines and rivers, and lava flow morphology to constrain the spatial and temporal evolution of magmatism at Uturuncu and Lazufre. Near Uturuncu, dated lake shorelines show no evidence of tilting since ca. 16 ka, and we find no evidence of deformation in the long-wavelength topography. A lack of net surface displacement suggests that uplift related to a rising diapir must be less than a century old, or, more likely, magmatic inflation at Uturuncu is transient over millennial timescales and is therefore not recorded in the topog­raphy. At Lazufre, we also find no evidence for sustained uplift recorded in Late Pleistocene lake shorelines. However, the orientations of multiple dated lava flows suggest that the long-wavelength dome at the center of Lazufre’s uplift has persisted since at least 400 ka. Additionally, we find that the radial distribution of volcanic vents at Lazufre, coupled with the presence of an apical graben, is consistent with experimental and theoretical predictions of magmatic doming. The dome’s longevity indicates significant magma storage at depth, and therefore Lazufre is likely a highly evolved pre-caldera magmatic system. These two case studies demonstrate that combining geomorphic and geophysical data sets to extend the geodetic record back in time can help determine the style and magnitude of magma transport in volcanic systems.

Full vector archaeomagnetic records from Anatolia between 2400 and 1350 BCE: Implications for geomagnetic field models and the dating of fires in antiquity

Anatolia, as one of the busiest crossroads of ancient civilizations, provides an ideal platform for archaeomagnetic studies. Previous results from the Middle East have suggested the occurrence of a strong peak in geomagnetic intensity at ∼1000 BCE associated with dramatic field strength variations that could require a radical rethinking of geodynamo theory. The behavior of the field in the centuries preceding this peak remains poorly constrained, however. Here we present the results of full-vector archaeomagnetic experiments performed on 18 sets of samples from three archaeological sites belonging to Assyrian Trade Colony and Hittite periods. Associated rock magnetic analyses showed that the major magnetic carrier is magnetite chemically stable up to 700 °C and the magnetic mineral assemblage is composed mostly of non-interacting PSD grains.The directional results are compared with existing data and with the most recent global geomagnetic field models pfm9k.1b and SHA.DIF.14k. The directions are in remarkably good agreement with SHA.DIF.14k which is based on archaeomagnetic and lava flow data. Together with our earlier results from Anatolia, we triple the existing database of directions for the 700 year long period 2200–1500 BCE, over a large region from Greece to Azerbaijan, and from Moldavia/Ukraine to Egypt.Three archaeointensity methods: thermal IZZI-Thellier, microwave Thellier and the multi-specimen protocol (MSP) produced virtual axial dipole moment estimates (9.0–10.9×1022 Am2) that are somewhat higher than contemporaneous (regional and global) data and model predictions suggesting that the field was already substantially stronger than today more than 800 years prior to the reported peak. In addition to constraining geomagnetic variability, our data also allow us to assign relative dates to inferred fire events in the Assyrian Trade Colony Period sites. This allows us to conclude that the fire events at the largest site, Kültepe, were not all contemporaneous with one another and with the abandonment of the site as has been previously hypothesized.

Secular variation of the Earth’s magnetic field and application to paleomagnetic dating of historical lava flows in Chile

The recent geomagnetic secular variation is mainly characterized by the large growth of the South Atlantic Magnetic Anomaly during the last three centuries, first documented by the geomagnetic model gufm1 (Jackson et al., 2000). We report new paleomagnetic results (directions and paleointensities) from several sites in two well dated lava flows in Chile, the 1835 AD eruption of the Osorno volcano and the 1751 AD eruption of the Llaima volcano. In addition, paleointensities were obtained on 14 samples from bricks of shelters built along the main road across the Andes from Santiago (Chile) to Mendoza (Argentina) in 1770±5 AD. The results confirm the high reliability of the global geomagnetic model gufm1 for the last three centuries with a large amplitude of the secular variation in inclination (∼20°) and intensity (∼25μT). Results from three 14C dated volcanic units in the time interval 1400–1750 AD indicate that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of existing global geomagnetic models. At this stage, precise paleomagnetic or archeomagnetic dating in South America using global models should be restricted to the last 3 centuries.To illustrate the potential of paleomagnetic dating in region and time interval with very large geomagnetic secular variation, we report paleomagnetic data from several sites in historical lava flows (1700–1900 AD) from the Antuco, Llaima and Villarrica volcanoes that permit to refine the ages of the major historical effusive volcanic events.

Rates of pedogenic processes in volcanic landscapes of late Pleistocene to Holocene age in Central Mexico

The Transmexican Volcanic Belt and its many volcanic fields of different ages offer good opportunities to study soil development on volcanic tephra of intermediate to basaltic composition. We studied a soil chronosequence within the Sierra Chichinautzin volcanic field (SCVF), south of the basin of Mexico, and aimed to establish the rates of pedogenic processes. This field has been active for around 50,000 years, and produced 221 cinder cones with their respective lava flows. We selected 11 sites located on 14C dated lava flows of ages between 1800 and 30,500 BP, at 3100–3200 m above sea level, covered with pine–fir forest, with an ustic soil moisture regime and an isomesic soil temperature regime. We also included a younger site (1000 BP) and three older sites (>100,000 years), two at 3100 masl and one at 2600 masl, from nearby volcanic fields to widen the time frame of the chronosequence. Soil profile samples were analysed for total organic carbon as well as for mineral neoformations related to clay contents, selective chemical extractions, and X-ray diffraction analyses.Within the SCVF the total soil thickness, carbon accumulation and Al, Si and Fe extracted with acid ammonium oxalate increased linearly with age on surfaces up to 10,000 years old at rates of 19 cm ky−1, 4.1 kg C m−2 ky−1, 4.6 kg Al, 2.7 kg Si and 2 kg Fe m−2 ky−1, respectively. Crystalline clay and iron oxide formation reach a maximum at the oldest site located at 2600 masl of 1650 kg clay m−2 and 70 kg Fed m−2. Their increase is linear up to ages of 10,000 years at rates of 22 kg clay m−2 ky−1, and 2.8 kg Fed m−2 ky−1. Thereafter the rates are much slower. Allophane and allophane-like minerals dominate in the clay fraction at all sites of the chronosequence, and small amounts of halloysite can be identified in soils older than 6200 years, while kaolinite was only identified at the three oldest sites (>100,000 years).The linear increases of all indicators of pedogenesis in the first 10,000 years are presumably driven by recurrent tephra deposition during this time frame. Thereafter, erosion and colluviation processes seem to disturb pedogenesis, changing its rates and redistributing its products in the landscape.

Paleointensity determination from São Miguel (Azores Archipelago) over the last 3 ka

Paleointensity data from the Atlantic Ocean are rare. We present new paleointensity data from São Miguel (Azores Islands, Portugal) based on 20 paleomagnetic sites from 13 lava flows emplaced over the last 3000years. Ten lava flows are radiocarbon dated, whereas three flows were paleomagnetically dated and one site was dated using stratigraphic relations. All the samples, previously investigated to recover paleodirections, were subjected to IZZI experiments. Importantly, the new data are internally consistent, agree with Moroccan and European datasets, and offer new constraints for global geomagnetic field models. Some of the ages of the paleomagnetically dated lava flows have been revised based on the intensity data presented here. The inferred Virtual Axial Dipole Moments (VADMs) range from 68.2 to 163.5ZAm2. A peak in field strength with an estimated age of around 600 BC is well supported by two sites from the same flow (Furna), and is comparable to the high intensity values found in Portugal for the same age and the earlier field peak at about 1000 BC in the Levant. A gradient in VADM values with latitude from northwestern Africa and across Europe between 100 and 1000 AD is confirmed as well as its absence from between 0 to 100 AD.

Methods for mapping volcanic events overlaid across time

This article describes a number of issues encountered when developing maps of past natural phenomena in the field of volcanology. In order to enable experts to exploit geographical data related to this topic, maps showing the temporal chronology of such events are required. Developing useful maps are made more complex by the coexistence of numerous phenomena in the geographic same space over time, which entails the management of spatial overlays, and includes the difficulty of integrating temporal information in static maps. In this article, we present our approach to the overlaying and temporal ordering of natural phenomena presented as information in maps, using the example of lava flow data. These data are derived from a case study of volcanic hazard affecting La Réunion Island in the Indian Ocean. We explore different methods for mapping evolution over time, for instance ‘map collections’ or ‘small multiple maps’, the use of both color or hue and saturation to represent the dates of events, and the use of ‘semi-transparency’ to preserve the representation of past events overlaid one on the other. Legibility and effectiveness of the map were a prime concern in this exploratory analysis.

A geomagnetic field model for the Holocene based on archaeomagnetic and lava flow data

We propose a new geomagnetic field model for the Holocene period based on archaeomagnetic and lava flow data, avoiding the use of lake sediment data. The source of data comes from the GEOMAGIA50v2 database which has been updated with the new archaeomagnetic and volcanic studies published during the last 3 yr. The model, called SHA.DIF.14k, allows us to analyse the behaviour of the geomagnetic field for the last 14 000 yr: from 12 000 BC to 1900 AD. For the model construction we use the spherical harmonic analysis in space and the penalized cubic B-splines in time. Both spatial and temporal regularization norms are used to constrain the inversion problem and applied at the core–mantle boundary (CMB) to assure the convergence of the model. For the last 3 ka, the model predictions agree with those given by the global model ARCH3k.1 and the European model SCHA.DIF.3k. For older epochs, the new model presents a clear improvement in field resolution with respect to other current models of the geomagnetic field for the Holocene. For the last 9 ka, the time evolution of the dipolar moment obtained from the dipole field shows a clear minimum between 5500 BC and 3000 BC, and the well-known continuous decreasing trend of the geomagnetic field strength for the last millennium and a half. A general view of the time-average evolution of the geomagnetic field flux lobes at the CMB for the northern hemisphere suggests a marked lobe of positive magnetic flux when the dipole moment was maximum. This lobe vanishes when the dipolar field is decreasing. The north polar wander paths of both north magnetic dip and geomagnetic poles were obtained showing an average rate of motion of 5.1 km/yr and 3.7 km/yr respectively. The model shows that the geomagnetic field can be averaged as axial dipolar in ∼2000 yr within an error of 5°, the typical uncertainty of the palaeomagnetic studies. Finally, and following the recent definition of archaeomagnetic jerks, we found 8 critical events in the time evolution of the geomagnetic field for the last 8 ka characterized by a maximum in the hemispheric asymmetry of the proposed model. The model is available in the Earth Ref Digital Archive at http://earthref.org/ERDA/1897/.

Paleomagnetic study of juvenile basaltic–andesite clasts from Andean pyroclastic density current deposits

Additional paleomagnetic data are necessary to improve geomagnetic models of secular variation during the Holocene, especially from the southern hemisphere. In most of the Andean volcanoes from Ecuador to the Chilean central volcanic zone, very well dated lava flows are rare. In contrast, andesitic to basaltic pyroclastic density current (PDC) deposits commonly contain charcoal, facilitating their age determination with 14C. In this study we present the magnetic properties and the paleomagnetic results obtained from three PDC deposits of basaltic to andesitic composition. One is the 2006 PDC deposit from the Ecuadorian Tungurahua volcano and the two others are well-dated PDC deposits from Chilean volcanoes Osorno and Villarica. Although most paleomagnetic studies in pyroclastic rocks deal with the estimation of emplacement temperatures from bulk deposits or accessory and accidental (non-juvenile) clasts, we show that juvenile clasts embedded in PDC deposits provide well-grouped paleomagnetic directions indicating post-emplacement in situ cooling below Curie points. Moreover, the rapid cooling of the juvenile clasts yields an abundance of single domain titanomagnetite grains providing low unblocking temperatures and a reliable material for paleointensity determination.

In search of long‐term hemispheric asymmetry in the geomagnetic field: Results from high northern latitudes

Investigations of the behavior of the geomagnetic field on geological timescales rely on globally distributed data sets from dated lava flows. We present the first suitable data from the Arctic region, comprising 37 paleomagnetic directions from Jan Mayen (71°N, 0.2–461 ka) and Spitsbergen (79°N, 1–9.2 Ma) and five paleointensity results. Dispersion of the Arctic virtual geomagnetic poles over the last 2 Ma (27.3 ± 4.0°) is significantly lower than that from published Antarctic data sets (32.1 ± 5.0°). Arctic average virtual axial dipole moment (76.8 ± 24.3 ZAm2) is high in comparison to Antarctica over the same time interval (34.8 ± 8.2 ZAm2), although the data are still too sparse in the Arctic to be definitive. These data support a long‐lived hemispheric asymmetry of the magnetic field, contrasting higher, more stable fields in the north with lower average strength and more variable field directions in the south. Such features require significant non‐axial‐dipole contributions over 105−106 years.

The Nd of the Innocence

Geochemistry There are few remaining geologic clues about the structure and geodynamics operating on Earth from its initial differentiation around 4.5 billion years ago until modern plate tectonics began. Debaille et al. analyzed the geochemistry of an ancient layered lava flow from Ontario, Canada, that serves as a window into a partially melted mantle plume from around 2.7 billion years ago. Excess 142Nd contents, which are daughter products of now extinct radiogenic 146Sm, imply that the mantle contained heterogeneities generated by relatively sluggish mixing—especially when considering that the temperatures and convection rates at that time would have been much higher than they are in the modern mantle. Using numerical models, the authors show that the preservation of this signature could occur in a stagnant regime of plate tectonics independently of high convection rates, meaning that early subduction events occurred only in brief and sporadic episodes. Combined with other geochemical data, the ancient lava flow data are consistent with the onset of modern (i.e. continuous) plate tectonics around 2.7 to 3.0 billion years ago. Earth Planet Sci. Lett. 10.1016/j.epsl.2013.04.016 (2013).

Rock-magnetic and paleomagnetic results from the Tepic-Zacoalco rift region (western Mexico)

A rock-magnetic and paleomagnetic investigation was carried out on eleven Pleistocene and Pliocene 40 Ar/ 39 Ar dated lava flows from the Tepic-Zacoalco rift region in the western sector of the Trans-Mexican Volcanic Belt (TMVB) with the aim of obtaining new paleomagnetic data from the study region and information about the Earth’s magnetic field recorded in these rocks. Rock-magnetic experiments including measurement of thermomagnetic curves, hysteresis parameters and isothermal remanence acquisition curves were carried out to find out the carriers of remanent magnetisation and to determine their domain structure. Although some samples were characterised by the presence of a single ferromagnetic phase (magnetite), in most cases more phases were observed. Analysis of hysteresis parameters showed a mixture of single domain and multidomain particles, the fraction of the latter varying between 40% and 80%. Paleomagnetic results were obtained in all sites, although in 7 sites characteristic remanence directions and remagnetisation circles had to be combined in order to calculate site means. The six Pliocene sites not showing intermediate polarity yielded a paleomagnetic pole (latitude  = 81.1, longitude  = 94.3) which roughly agrees with the expected one. Paleomagnetic directions do not indicate significant vertical-axis block rotations in the western TMVB area. Reversed polarities observed can be correlated to the Gilbert chron, normal polarities to the Gauss chron or the Brunhes chron and intermediate polarities to the Cochiti-Gilbert or the Gilbert-Gauss transition. The reversed or intermediate polarity magnetisation recorded in one of the sites (542 ± 24 ka) corresponds either to the West Eifel 4 or the West Eifel 5 excursion, while the reversed