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The initial cooling of pahoehoe flow lobes 总被引：1，自引：0，他引：1
In this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows. The accurate modeling of this
initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as
well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation.
This model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not
an empirical fit to field data. We find that the only significant processes are (a) heat loss by thermal radiation, (b) heat
loss by atmospheric convection, (c) heat transport within the flow by conduction with temperature and porosity-dependent thermal
properties, and (d) the release of latent heat during crystallization. The numerical model is better able to reproduce field
measurements made in Hawai'i between 1989 and 1993 than other published thermal models. By adjusting one parameter at a time,
the effect of each of the input parameters on the cooling rate was determined. We show that: (a) the surfaces of porous flows
cool more quickly than the surfaces of dense flows, (b) the surface cooling is very sensitive to the efficiency of atmospheric
convective cooling, and (c) changes in the glass forming tendency of the lava may have observable petrographic and thermal
signatures. These model results provide a quantitative explanation for the recently observed relationship between the surface
cooling rate of pahoehoe lobes and the porosity of those lobes (Jones 1992, 1993). The predicted sensitivity of cooling to
atmospheric convection suggests a simple field experiment for verification, and the model provides a tool to begin studies
of the dynamic crystallization of real lavas. Future versions of the model can also be made applicable to extraterrestrial,
submarine, silicic, and pyroclastic flows.
Received: 26 November 1994 / Accepted: 1 December 1995 相似文献
2.
David A. Clague Jonathan T. Hagstrum Duane E. Champion Melvin H. Beeson 《Bulletin of Volcanology》1999,61(6):363-381
The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their
eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic
directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the
vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within
several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km3, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been
nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit.
The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions
for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000
BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive
events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also
reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each
of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO
near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated
termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above
the 'Ailā'au flows, cannot be older than this age.
Received: 10 October 1998 / Accepted: 12 May 1999 相似文献
3.
Multiple tracers of groundwater input (salinity, Si, 223Ra, 224Ra, and 226Ra) were used together to determine the magnitude, character (meteoric versus seawater), and nutrient contribution associated with submarine groundwater discharge across the leeward shores of the Hawai'ian Islands Maui, Moloka'i, and Hawai'i. Tracer abundances were elevated in the unconfined coastal aquifer and the nearshore zone, decreasing to low levels offshore, indicative of groundwater discharge (near-fresh, brackish, or saline) at all locations. At several sites, we detected evidence of fresh and saline SGD occurring simultaneously. Conservative estimates of SGD fluxes ranged widely, from 0.02–0.65 m3 m− 2 d− 1at the various sites. Groundwater nutrient fluxes of 0.04–40 mmol N m− 2 d− 1 and 0.01–1.6 mmol P m− 2 d− 1 represent a major source of new nutrients to coastal ecosystems along these coasts. Nutrient additions were typically greatest at locations with a substantial meteoric component in groundwater, but the recirculation of seawater through the aquifer may provide a means of transferring terrestrially-derived nutrients to the coastal zone at several sites. 相似文献
4.
High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones
on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea),
Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted
during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed
of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana
Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep
(∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights <300 m. The flat tops commonly have
either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep
sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m.
The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most
of the rejuvenated-stage flat-topped cones formed on slopes <10° and formed adjacent semicircular steps down the flank of
Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive
eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as
continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues
to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava
flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process
repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There
is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that
facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining
pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably
ca. 0.1 km3/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion
rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will
not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify
between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water
will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield
stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine
flat-topped cones but formed under different cooling conditions.
Received: 30 September 1999 / Accepted: 9 March 2000 相似文献
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