Shallow Magma Intrusion Research Articles

Dynamics of carbon dioxide emission at Mammoth Mountain, California

Mammoth Mountain, a dormant volcano in the eastern Sierra Nevada, California, has been passively degassing large quantities of cold magmatic CO 2 since 1990 following a 6-month-long earthquake swarm associated with a shallow magmatic intrusion in 1989. A search for any link between gas discharge and volcanic hazard at this popular recreation area led us to initiate a detailed study of the degassing process in 1997. Our continuous monitoring results elucidate some of the physical controls that influence dynamics in flank CO 2 degassing at this volcano. High coherence between variations in CO 2 efflux and variations in atmospheric pressure and wind speed imply that meteorological parameters account for much, if not all of the variability in CO 2 efflux rates. Our results help explain differences among previously published estimates of CO 2 efflux at Mammoth Mountain and indicate that the long-term (annual) CO 2 degassing rate has in fact remained constant since ∼1997. Discounting the possibility of large meteorologically driven temporal variations in gas efflux at other volcanoes may result in spurious interpretations of transients that do not reflect actual geologic processes.

Fold uplift and synkinematic stratal architectures in a region of active transtensional tectonics and volcanism, eastern Tyrrhenian Sea

The detailed stratigraphic analysis of synkinematic stratal architectures permits the calculation of fold uplift rates and the reconstruction of fold kinematics. This approach was applied to active folds in the Bay of Pozzuoli, eastern Tyrrhenian Sea. This area features late Quaternary volcanism, shallow magmatic intrusions, and transtensional tectonics. Seismostratigraphic analysis of synkinematic strata is constrained by a physical correlation to an already established stratigraphic succession in the neighboring Campi Flegrei and by tephra layers in a gravity core located in Pozzuoli Bay. A stratigraphic resolution in thousands of years was established. Structural and stratigraphic analyses yield quantitative data on the timing of fold inception, kinematics, and the amount and rates of uplift. The folds extend over ∼2 km, and display a curved axial pattern and wedge-shaped packages of synkinematic strata. The rate of fold uplift ranges from 1 to 20 mm/yr. The structures feature limb rotation kinematics and decreasing uplift rates. The uplift rates and geometry of the Pozzuoli Bay folds suggest that they are of tectonic origin and are a product of detachment folding. Their limited longitudinal extent and arcuate axial trends are compatible with an east-trending left transtensional shear zone that was active along the eastern Tyrrhenian Sea during the late Quaternary. A relative sea-level curve was calculated for an anticline culminating in the city of Pozzuoli. This relative sea-level curve highlights the relationship between the deformation timing and the genesis of related abrupt changes in sedimentary facies and erosional surfaces.

The mechanism of phreatic eruptions

We investigate the mechanism for initiating phreatic eruptions following the emplacement of a shallow magmatic intrusion into water‐saturated permeable rock which contains subsidiary low‐permeability crack networks and disconnected cracks. Heat from the intrusion causes the local groundwater to boil and ascend through the main permeable crack network. As the ascending superheated steam heats the overlying rock, the water in the subsidiary networks and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected cracks can lead to rapid horizontal crack propagation, resulting in an increase in crack length by more than an order of magnitude. According to the model, the eruption process starts near a free surface and migrates rapidly along thermoelastic isostresses as a result of multiple breakage of the thin surface layers above the cracks. For certain crack and rock parameters, however, the crack propagation mechanism, instead of leading to a dynamic eruption, may generate a highly cracked zone that may be removed later by fluid transport processes. The proposed mechanism gives rise to precursory phenomena observed in conjunction with many phreatic eruptions. According to the model developed here, phreatic eruptions are most likely to occur only for a rather restricted set of rock parameters. For example, the country rock should not be too strong (σt ≅ 10 MPa) and should be characterized by two‐scale permeability structure involving a main crack network of relatively high permeability (≳10−12 m2) and a subsidiary crack network with much lower permeability (<10−17 m2). Moreover, the model works better if the mean crack aspect ratio is relatively large (β ∼ 10−1) and the crack concentration is not too low (Ω > 10−2). These restrictions may explain indirectly why phreatic eruptions are not ubiquitous in volcanic regions.

Geology of Tok Island, Korea: eruptive and depositional processes of a shoaling to emergent island volcano

Detailed mapping of Tok Island, located in the middle of the East Sea (Sea of Japan), along with lithofacies analysis and K-Ar age determinations reveal that the island is of early to late Pliocene age and comprises eight rock units: Trachyte I, Unit P-I, Unit P-II, Trachyandesite (2.7±0.1 Ma), Unit P-III, Trachyte II (2.7±0.1 Ma), Trachyte III (2.5±0.1 Ma) and dikes in ascending stratigraphic order. Trachyte I is a mixture of coherent trachytic lavas and breccias that are interpreted to be subaqueous lavas and related hyaloclastites. Unit P-I comprises massive and inversely graded basaltic breccias which resulted from subaerial gain flows and subaqueous debris flows. A basalt clast from the unit, derived from below Trachyte I, has an age of 4.6±0.4 Ma. Unit P-II is composed of graded and stratified lapilli tuffs with the characteristics of proximal pyroclastic surge deposits. The Trachyandesite is a massive subaerial lava ponded in a volcano-tectonic depression, probably a summit crater. A pyroclastic sequence containing flattened scoria clasts (Unit P-III) and a small volume subaerial lava (Trachyte II) occur above the Trachyandesite, suggesting resumption of pyroclastic activity and lava effusion. Afterwards, shallow intrusion of magma occurred, producing Trachyte III and trachyte dikes.

Burial history of the Penrith Sandstone (Lower Permian) deduced from the combined study of fluid inclusion and palaeomagnetic data

Abstract The Lower Permian Penrith Sandstone in the Vale of Eden represents the deposits of fossil aeolian dunes, which are well cemented by syntaxial quartz overgrowths in the northern part of the outcrop. The overgrowths possess a complex internal growth fabric as revealed by scanning electron microscopy-cathodoluminescence. Fluid inclusions are common in the overgrowths and occur in three associations. Small, subrounded, mostly liquid-rich, two-phase, primary aqueous inclusions occur in the concentrically banded growth zone. Similar primary inclusions occur concentrated in growth imperfections aligned orthogonally to the growth direction. A third association comprises irregular, elongate inclusions that occur along healed microfractures. Salinities in all three associations vary from 13.5 to 23 wt.% NaCl equivalents, whereas measured homogenization temperatures range from 69 to 280°C. This variation results from necking of the inclusion populations after vapour phase nucleation and, as a consequence, the measured homogenization temperatures are geologically meaningless. However, the smallest inclusions yield consistent T h data of around 70°C, which is equivalent to a T t of around 95°C in the absence of methane. A variety of iron oxyhydroxides carry the magnetic remanance in the Penrith Sandstone. The characteristic magnetization is carried by hematite, which occurs in several textural forms. This remanence is multicomponent and shows directional scatter along a NNW-SSE great circle. Individual components are difficult to isolate because of the similarity of their unblocking temperatures, but are best interpreted as a mixture of Tertiary normal and reverse elements. Above 530°C a weak, but stable, Early Permian magnetization can be isolated from a number of specimens. A model is proposed in which quartz authigenesis and magnetization took place during burial from alkaline-rich fluids derived from the Eden Shales. Authigenic hematites precipitated before and during quartz cementation at temperatures of around 95°C. The fluid inclusion microthermometry and thermal demagnetization data are consistent with the Penrith Sandstone attaining a maximum burial temperature of about 120°C. For necking to have caused entrapment of inclusions containing both liquid and vapour suggests that the Penrith Sandstone underwent burial into the all-liquid field, trapped inclusions and then underwent cooling, equivalent to a temperature decrease of 30–40°C. This temperature decrease may be a consequence of regional uplift or, alternatively, the Penrith Sandstone may have been heated and cooled following a transient flush of hot, migrating fluids, possibly related to the shallow intrusion of magma into the basin in Tertiary times. The origin of the Tertiary remanence in the Penrith Sandstone is compatible with fluid inclusion data and is considered to be a combination of chemical remanent magnetization and partial thermoremanent magnetization which was blocked during the Tertiary cooling.

Volcano monitoring by microgravity and energy budget analysis

Microgravity monitoring of active volcanoes can provide evidence of sub-surface mass and/or density changes that precede eruptions. If, in addition, precursory increases in thermal emissions are observed, an integrated mechanistic model for volcanic activity may be developed with the potential for forecasting eruptions. Crater-lake volcanoes provide an interesting target for such studies since thermal output can be monitored simply through lake water calotimetry. Here we summarize 10 years of gravity and thermal data from Poás volcano, Costa Rica. Mass/energy balance calculations demonstrate that, in the steady-state, the large thermal inertia of the crater lake acts as a buffer to short-term changes in the energy input from the cooling magma feeder pipe. Since February 1986, it is postulated that there has been gradual emplacement of a shallow magma intrusion associated with vesiculation and gas loss to the surface. This follows from unambiguous, gravity increases, constrained by elevation control, that are coincident in time with a period of long-term increased energy input to the crater lake. Progressive reduction of the lake volume by evaporation/seepage culminated in an (April 1989) ash eruption providing a good documented record of combined gravity and thermodynamic precursors to volcanic activity.

High Heat Flow Anomaly around Hatsushima Biological Community in the Western Sagami Bay, Japan.

Heat flow values have been determined at sites that are densely distributed around biological communities located along a topographic inflection line to the southeast of Hatsushima Island in the western Sagami Bay through four research cruises since 1988. Anomalously high heat flow values up to 2, 000 mW/m2 were observed at the largest community, where a high methane content anomaly had been found indicating fluid venting activity. Within 1 km of the inflection line, which is presumed to be a surface expression of an active fault, heat flow values are high and variable (about 400 mW/m2), whereas they are low and uniform (50 mW/m2) in a flat area more than 1 km away from the inflection line.Since this area is located near a convergent plate boundary and also near the volcanic front of the Izu-Bonin arc, either a seepage of the pore fluid at the inflection line or a conductive heating by the shallow magma intrusion below the community is thought to be a possible cause of the high heat flow anomaly. From simple estimations, however, neither of them was found to be adequate to explain the observation. Probably a pore fluid circulation is occurring in the fault system beneath the community that is possibly driven by a shallow magma below the fault and/or the compressional stress field.

Deformations and Inferred Stress Field for Ellipsoidal Sources at Long Valley, California, 1975–1982

Ground deformation and seismicity observed from 1975 to the present support the existence of magma beneath the central part of Long Valley caldera, California. Constraints on the source geometry are obtained from deformation modeling of uplift and trilateration data over the period 1975–1982. The deformation data are modeled with an elastic half‐space incorporating an ellipsoidally shaped magma chamber under uniform pressure, the Hilton Creek fault, and the South Moat fault. The best fit to the deformation data indicates a nearly spherically shaped source beneath the central part of the caldera at a depth of 9.5 ± 1 km. However, significant deviations of some horizontal displacement data suggest that movements along the faults are probably more complicated than the assumed dislocations. The predicted normal stress on the vertical South Moat fault plane varies from weak tension above 4 km depth to strong compression below that depth and the shear stress is right‐lateral in the eastern half and left‐lateral in the western half of the fault. It is unlikely that such a stress field could trigger slip movement on the fault. Tangential stress at the surface of the magma chamber has a maximum tension near its intersection with the South Moat fault. It is possible that the 1983 earthquake swarms in the South Moat were triggered by shallow magma intrusions beneath the South Moat of the caldera. A narrow prolate ellipsoidal source, which produces about the same uplift as the spherical source model, leads to a horizontal tensile stress that is about 1 order of magnitude higher in the region directly above the source. However, the crack opening location for magma injection is at the top of the source in this case and would not be related to the seismicity in 1983.

Igneous processes and closed system evolution of the Tharsis region of Mars

A quantitative petrologic model for the formation of the Tharsis rise of Mars has been developed that is consistent with global gravity and topography data. Partial melting of the mantle underlying the Tharsis plateau results in the depletion of basaltic chemical components in the source region. Iron is partitioned preferentially into the basaltic liquid, so that the density of the source region is lowered relative to surrounding unmodified mantle. Basalt is erupted to the surface and/or intruded at shallow levels in the crust, where it solidifies to low‐pressure mineral assemblages or perhaps glass. These low‐pressure materials are less dense than their mantle equivalents; hence the net volume of the Tharsis crust‐mantle system is increased, resulting in the prominent topographic rise with no net gain or loss of mass. The distribution of densities with depth is quantitatively consistent with the isostatic models of Sleep and Phillips [1979, 1985]. Typical values of parameters that are constrained by the gravity and topography data are 150‐km crustal thickness, 250‐km elastic mantle thickness, 26% partial melting in the mantle source region, and 32% addition of intrusive material to the crust. This closed‐system petrologic process is strictly constructional in nature, but shallow intrusion of magma into the crust can cause uplift of overlying strata, as advocated by R. J. Phillips and N. H. Sleep [submitted manuscript, 1987].

Resistivity monitoring of an active volcano using the controlled‐source electromagnetic technique: Kilauea, Hawaii

A controlled‐source electromagnetic monitoring system located near the summit of Kilauea has been in operation since mid‐1979. Prominent changes in the controlled‐source magnetic fields have been observed both preceding and following two seismic swarms that are interpreted to be related to shallow magma intrusions in August 1980 and January 1981. The vertical magnetic field (Hz) sensor nearest the sites of intrusion showed the largest field changes. All comparisons to date have been made at 1 Hz, which corresponds to a depth of penetration of several kilometers. Large and rapid vertical‐magnetic field changes were measured following the intrusion into the upper east rift of Kilauea on August 27, 1980. Similar fluctuations were measured both before and during the intrusion into the southern caldera on January 20, 1981. Located earthquakes and data from a recording tiltmeter within 2 km of the Hz sensor, which showed anomalous field changes over the January 20 event, indicate that the Hz sensor was located nearly over the zone of intrusion. The Hz field changes recorded over both the August 1980 and January 1981 intrusive events appear too large to have been caused by conductivity changes within a horizontally layered section. More probably, they were caused by relatively shallow intrusion of high‐conductivity magma into open high‐angle fractures or by establishing continuity of short sections of preexisting magma storage zones, thereby producing long linear conductors from short, normally discontinuous conductive segments.

Rheological Properties of Magmas

Recent studies of igneous rocks have taken a refreshing new direction, mainly as the result of a greater awareness of the important role played by physical properties of magmas in determining the eruptive behavior and compositional variations of volcanic rocks. This rcview summarizes the present state of knowledge, along with some of the recent rheological studies having a direct bearing on interpretations of volcanic phenomena and processes of crystallization and differentiation of shallow magmatic intrusions. The rheological properties of magmas reflect the inherent structures of molten silicates and will not be thoroughly understood until more is known about the basic nature of silicate liquids. Recent progress in this field has been ably reviewed by Hess (1980) and needs no elaboration here aside from mention of the obvious need for better laboratory measurements in order to place greater constraints on interpretations of silicate melts. For petro­ genetic and volcanological processes, the most conspicuous need is for improved data on rheological properties, chiefly viscosity and yield strength, and for more precise equations for predicting these properties under a wide range of natural conditions. Viscosity, 1'/, is usually defined as the ratio of shear stress to strain rate and is expressed in units of poise (g cm 1 S 1) or Pascal seconds ( 10 poise = 1.0 Pa s). In more general terms, it is the coefficient for transfer of

Thermal areas on Kilauea and Mauna Loa Volcanoes, Hawaii

Active thermal areas are concentrated in three areas on Mauna Loa and three areas on Kilauea. High-temperature fumaroles (115–362° C) on Mauna Loa are restricted to the summit caldera, whereas high-temperature fumaroles on Kilauea are found in the upper East Rift Zone (Mauna Ulu summit fumaroles, 562° C), middle East Rift Zone (1977 eruptive fissure fumaroles), and in the summit caldera. Solfataric activity that has continued for several decades occurs along border faults of Kilauea caldera and at Sulphur Cone on the southwest rift zone of Mauna Loa. Solfataras that are only a few years old occur along recently active eruptive fissures in the summit caldera and along the rift zones of Kilauea. Steam vents and hot-air cracks also occur at the edges of cooling lava ponds, on the summits of lava shields, along faults and graben fractures, and in diffuse patches that may reflect shallow magmatic intrusions.

Distribution and volcano-structural control of zeolites and other amygdale minerals in the island of Martinique, F.W.I.

Zeolites and other amygdale minerals have a widespread occurrence in Martinique. The central and east part of the island is composed of pre-Pliocene submarine formations (hyaloclastitic tuffs, pillow lavas, swarms of dikes, and domes). Here, zeolites occur in about 10 well-defined areas where specific associations constitute discrete zones of progressive metamorphism. Four zones are established which are believed to reflect temperatures ranging from 100 to 200°C. Within the outer zone I, several of the following minerals occur together: heulandite or clinoptilolite, mordenite, chabazite, mesolite; analcime and thomsonite characterize a narrow zone II; stilbite is widespread in zone III, and laumontite characterizes inner zone IV. The Lamentin thermal area is mainly com posed of Recent monogenetic volcanoes (age < 0.7 m.y.) and alluvium overlying pre-Pliocene volcanic rocks. Hot springs (39–51°C) occur in the central part of the area, but the less restricted occurrence of zeolites, calcite and silica, and scattered crystallization of barite, tourmaline, arsenopyrite and bowlingite, emphasize the larger development of recent hydrothermal activity in the area. The distribution of zeolites and other amygdale minerals is not related to regional metamorphism brought about by burial, but rather to areas of well-defined heat flow where suitable fluids were available. Roots of deeply eroded volcanoes, shallow magmatic intrusions, and zones of intensive faulting could have provided the necessary heat required; such structures are referred to in this communications as “heat flow domes”.

Magmatic eruption of White Island volcano, New Zealand, December 1976–April 1977

White Island volcano erupted fresh andesite during March 1977, in the first emission of new lava bombs and blocks recorded in historic time. Preliminary analytical data indicate that the new lava differs petrographically and chemically from all other analysed White Island lavas. The eruption commenced with ejection of accessory ash derived from crater floor debris, overlying a shallow magma intrusion which has been causing inflation of the crater floor since 1973.

1974 activity of chokai volcano, Japan

Shallow intrusion of magma caused phreatic explosions and mud flows at the snow-covered summit of Chokai volcano, northeast Honshu, Japan, after 153 years of dormancy. Total heat emission by the eruption is estimated at more than 3.0 × 1021 erg. Equivalent amount of magma is about 2.2 × 108 ton. Focal mechanisms of the associated volcanic earthquakes, which had been variable during the period of eruption. became stable after the cessation of the surface activity with pressure axis in a NW direction which is also the strike of the epicenter distribution. This temporal change of focal mechanisms may be interpreted as the result of propagation of increased pore pressure in the direction of the maximum pressure in the post eruptive period. The magmatic pressure which certainly predominated during the eruption period and caused carthquakes with variable mechanisms, decreased through surface activity.