Thick Volcanic Deposits Research Articles

InSight Constraints on the Global Character of the Martian Crust

AbstractAnalyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low‐density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three‐layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m−3. These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet.

Geochemistry of Silurian-Devonian volcanic rocks in the Coastal Volcanic belt, Machias-Eastport area, Maine: Evidence for a pre-Acadian arc

The Coastal Volcanic belt in easternmost Maine consists of Silurian–Devonian volcanic rocks related to the convergence of Avalonia with the leading edge of composite Laurentia just before and during early stages of the Acadian orogeny. New major- and trace-element analyses from the Machias-Eastport area indicate that the Early to Late Silurian volcanic rocks in the Dennys, Edmunds, and Leighton Formations formed in a continental arc environment, in contrast to some earlier interpretations. These rocks have a continuous compositional range from basalt, through andesite, to rhyolite. The mafic rocks in these formations are calc-alkaline and show subduction-related geochemical signatures on various discrimination diagrams, whereas the felsic rocks have geochemical characteristics of volcanic arc granites. In contrast, the thick volcanic deposits of the younger Late Silurian Eastport Formation are bimodal and tholeiitic and have transitional to within-plate geochemical signatures. Thus, the transition over time, as the Acadian seaway closed, from subduction and arc settings to continental extension can been seen in this rock sequence, and it constrains the time of Avalonia’s docking. Regionally, volcanic rocks in the Kingston terrane of southern New Brunswick are likely correlatives of the earlier arc-volcanic rocks (Dennys, Edmunds, and Leighton Formations), while Eastport Formation analogues are found in the coastal Maine magmatic province, Cranberry Island series, and probably the Passamaquoddy Bay volcanic series, which similarly appear to have formed after the transition to an extensional environment occurred, although other extensional settings cannot be entirely ruled out. Basalts in the post-Acadian Perry Formation indicate continental extension was also active in the Late Devonian.

Persistence of seed bank under thick volcanic deposits twenty years after eruptions of Mount Usu, Hokkaido Island, Japan

The topsoil that contained the seed bank became buried under thick tephra after the eruptions of Mount Usu during 1977 and 1978. To determine the seed bank potential of the topsoil 20 yr after the eruptions, i.e., in 1998, 408 100-cm(3) samples were excavated under 115-185 cm of volcanic deposits. The topsoil was collected at 10-cm intervals along the horizontal scale and was divided into a 0-5 cm deep upper layer and a 5-10 cm deep lower layer. The seed bank was estimated by both the germination (GM) and flotation (FM) methods. In total, 23 species with an average seed density of 1317 seeds/m(2) were identified by GM, and 30 species with a density of 2986 seeds/m(2) were extracted by FM. The dominant species was Rumex obtusifolius, and perennial herbs, such as Carex oxyandra, Viola grypoceras, and Poa pratensis, were common. For nine species this study provided the first records for field seed longevity >20 yr. The seed density in the upper layer was double that in the lower layer, and the horizontal distribution was heterogeneous even at 10-cm intervals. We concluded that the seed bank has retained the original structure of the seed bank under the tephra and will persist longer with soil water content between 20 and 40%, no light, and low temperature fluctuations (±0.17°C of standard deviation in a day).

Tree regeneration patterns on Mount Usu, northern Japan, since the 1977?78 eruptions

Although the summit of Mount Usu was deforested by the 1977–78 eruptions, vegetative regeneration on the caldera rim was rapid due to the erosion of thick volcanic deposits by snow and rain. To obtain the mechanisms underlying regeneration patterns after the eruptions, we monitored the growth of permanently-marked stems from 1983 to 1990. Regeneration was from resprouting-branches buried in the volcanic deposits on the caldera rim, while on the crater basin, where thick volcanic deposits accumulated, regeneration was from seedlings. The seedling regeneration lagged approximately 3 years behind vegetative regeneration. Stem densities averaged 14,000 ha-1 in the vegetatively-regenerated community on the caldera rim, and 28,000 ha-1 in the seedling regeneration on the crater basin. Populus maximowiczii accounted for ca. 75% of total stems on the caldera rim, while P. maximowiczii accounted for ca. 30% on the crater basin where Salix integra and Betula platyphylla var. japonica were also common. In both stands, immigration and mortality rates were very low. The growth of vegetatively regenerated stems expressed as stem height and diameter was significantly faster than that of stems grown from seedlings. Herbivory damage on the terminal shoots of tall stems was restricted on the caldera rim and was restricted for B. platyphylla var. japonica on the crater basin, perhaps due to fast growth supporting herbivore avoidance or low palatability. Height growth was restricted when neighbors established in close proximity, especially in the seedling-regenerated forest. The results suggest that vegetative regeneration is rapid due to three mechanisms: 1) faster plant growth; 2) herbivore avoidance; and 3) decreased interference by neighboring.

Survival characteristics of buried seeds 10 years after the eruption of the Usu volcano in northen Japan

In the crater basin of the Usu volcano, where the vegetation has almost completely been destroyed by 1977–1978 eruptions, a study was conducted in 1987 to determine the survival characteristics of buried seed populations. A total of 17 618 seeds were extracted from six 25-L former topsoil blocks by a flotation test. Data showed that even after 10 years burial by 65–140 cm volcanic deposists, 16.6% of the seeds, representing 25 species, were viable. There were few predators in the former topsoil below the thick volcanic deposits. Seed volume of most species was less than 2.0 mm3 and smaller seeds had a greater rate of survival than larger seeds. Seed survival rates of Poa annua, Rumex obtusifolius, Alopecurus aequalis var. amurensis, and Viola grypoceras were positively correlated with the thickness of volcanic deposits, suggesting that the most important factor for seed survival is stable soil temperature with little diurnal fluctuations. Absence of light may be an important variable in seed survival, but this factor did not sufficiently explain survival under the thick volcanic deposits. Key words: buried seed, burial depth, seed size, seed survival, soil temperature, volcano Usu.

Species turnover and diversity during early stages of vegetation recovery on the volcano Usu, northern Japan

AbstractTo study the rate of revegetation during succession on the volcano Usu, northern Japan, vegetation structure and species composition were monitored from 1984 to 1988 in permanent quadrats near the summit of the volcano, which had been almost completely deforested by 1 to 3 m thick volcanic deposits in 1977–78. Analyses of vegetation structure included species richness, species diversity (IT), evenness (J'); year‐to‐year changes were quantified using Community Coefficient (CC), and Percentage Similarity (PS). While species turnover rates fluctuated and vegetation cover gradually increased, species richness, diversity and evenness did not fluctuate much. The diversity parameters showed slightly different levels for three habitat types distinguished. CC values comparing subsequent years with the starting year 1984 suggest only minor changes in qualitative species composition; PS values decreased more rapidly, indicating larger changes in species cover. A few well‐rooted perennial plants were predominant, therefore, PS decline resulted from dominance‐controlled community structure. PS‐value detected not only habitat difference, but also the rate of the volcanic succession, suggesting that PS is most effective to evaluate successional pace.

Contribution of buried seeds to revegetation after eruptions of the volcano Usu, Northern Japan

Soon after the 1977–78 eruptions of the volcano Usu, there were created many gullies in which former topsoil, i.e., well-developed soil accumulated before the eruptions, was eroded in the crater basin, whereas the outside of the gully was covered with thick volcanic deposits. The short-dispersal-seed plants were the most abundant in the inside of gully where the former topsoil was exposed, however, they have not immigrated from external environments. The germination tests of the seeds extracted from the former topsoil demonstrated that viable seeds were buried at 1683.3/m2 for 9 years after the eruptions. At least, 12 herbaceous species, e.g.,Rumex obtusifolius, Geum macrophyllum var.sachalinense andPoa annua, were derived from the buried seeds in the former topsoil. Their contribution to revegetation was estimated to be 40.0% in the inside of gully where the former topsoil was exposed in 1983, suggesting that buried seeds are one of the most important seed source for revegetation even in the case of a volcano which produced pumice and ash by eruption.

ANALYSIS OF REVEGETATION DYNAMICS ON THE VOLCANO USU, NORTHERN JAPAN, DEFORESTED BY 1977–1978 ERUPTIONS

Vegetation dynamics were monitored from 1983 to 1987 near the summit of the volcano Usu which had almost completely been deforested by 1–3 m thick volcanic deposits in 1977–1978. A permanent quadrat survey for five years confirmed the high mortality of seedlings. Since seedling mortality appeared mostly due to the frequent erosion of the volcanic deposits, ephemeral annuals and well‐rooted perennials or woody plants were successful here. The high mortality of seedlings resulted in intensive fluctuation of plant density in all habitats. However, owing to the expansion of perennials, once they became established, the plant cover increased from year to year. In particular, Polygonum sachalinense and Petasites japonicus var. giganteus, seeds of which were frequently provided from their community vegetatively recovered on the wall of the caldera rim soon after the eruptions, were remarkably contributive to the revegetation. The excavation of those perennial plants demonstrated that they were derived not only from the seeds but also vegetatively from the old stumps buried in the former topsoil by extending their rhizomes up to the present ground surface.

Buried seed populations on the volcano Mt. Usu, northern Japan, ten years after the 1977-78 eruptions

In 1987, a study on buried seed populations was conducted in the crater basin of Mt. Usu, a volcano located in northern Japan, where the vegetation had beeen almost completely destroyed by eruptions in 1977 and 1978. The former vegetation had consisted of grassland and broad-leaved forest. In the areas formerly occupied by this grassland and forest, 2128.0 and 1985.3 seeds per square meter, respectively, were extracted from 12 blocks of the former topsoil using a floattion method. This revealed that many seeds were still viable even after ten years of burial under thick volcanic deposits. The seeds were distributed more in sandy soil than in rocky soil of the former topsoil. Twenty-five species, most of which favored grassland, were detected in both the former grassland and forest. From comparison of α-and β-diversities between the grassland and forest, the structure of the buried seed population was considered likely to have been more diverse in the forest than in the grassland. The determinants of composition of the buried seed populations were discussed with reference to the former vegetation and soil characteristics.