Caldera Deposits Research Articles

Three-dimensional joint inversion of surface wave dispersion and gravity data using a petrophysical approach: an application to Los Humeros Geothermal Field

SUMMARY We present a method to jointly invert surface wave dispersion data and gravity measurements for 3-D shear wave velocity and density models. We implemented a petrophysical approach to combine the kernels of both methodologies in a single process. The synthetic experiments show that jointly inverted models recover shear wave velocity and density better than separate inversions. In particular, density models benefit from the good vertical resolution of surface wave dispersion data, while shear velocity models benefit from the good lateral resolution of gravity data. We also proposed two methods to stabilize the solution when using high-grade polynomials. We applied the methodology to the Los Humeros Geothermal area to demonstrate its applicability in a complex geological scenario. Compared with separate inversion, the joint inversion contributes to enhancing key aspects of the geothermal system by (i) delimitating better the geometry of the caldera deposits in the first 0–2.8 km deep by increasing the vertical resolution in density, (ii) delimitating better the lateral borders of low-Vs bodies at different depths interpreted as a part of a complex magmatic chamber system and (iii) estimating the local shear wave velocity–density relationship that conforms to other known relationships for sedimentary and igneous rocks but with some differences that bring us additional information.

The Campi Flegrei Deep Drilling Project (CFDDP): New insight on caldera structure, evolution and hazard implications for the Naples area (Southern Italy)

AbstractThe 501 m deep hole of the Campi Flegrei Deep Drilling Project, located west of the Naples metropolitan area and inside the Campi Flegrei caldera, gives new insight to reconstruct the volcano‐tectonic evolution of this highly populated volcano. It is one of the highest risk volcanic areas in the world, but its tectonic structure, eruptive history, and size of the largest eruptions are intensely debated in the literature. New stratigraphic and 40Ar/39Ar geochronological dating allow us to determine, for the first time, the age of intracaldera deposits belonging to the two highest magnitude caldera‐forming eruptions (i.e., Campanian Ignimbrite, CI, 39 ka, and Neapolitan Yellow Tuff, NYT, 14.9 ka) and to estimate the amount of collapse. Tuffs from 439 m of depth yield the first 40Ar/39Ar age of ca. 39 ka within the caldera, consistent with the CI. Volcanic rocks from the NYT were, moreover, detected between 250 and 160 m. Our findings highlight: (i) a reduction of the area affected by caldera collapse, which appears to not include the city of Naples; (ii) a small volume of the infilling caldera deposits, particularly for the CI, and (iii) the need for reassessment of the collapse amounts and mechanisms related to larger eruptions. Our results also imply a revaluation of volcanic risk for the eastern caldera area, including the city of Naples. The results of this study point out that large calderas are characterized by complex collapse mechanisms and dynamics, whose understanding needs more robust constraints, which can be obtained from scientific drilling.

Eruption products of the 1883 eruption of Krakatau and their final settlement

<p>Firstly the volume of pyroclastic ejecta during the 1883 eruption of Krakatau is re-examined. To revise the volume of flow deposits, the author basically follows Verbeek’s observation while to estimate the fall deposits, as the last resort, the author assumes that volume ratios fall / flow are common to similar caldera eruptions, and the ratios determined by the caldera- forming eruptions of Novarupta and Pinatubo are applied to the Krakatau eruption. Verbeek’s estimation of the total volume of ejecta, 12 km<sup>3</sup> is revised to 19 km<sup>3</sup>. This is significantly different from the volume of disrupted volcano edifice, 8 km<sup>3</sup>. Such a result does not support the predecessors’ hypothesis that calderas are formed by collapses of volcano edifices into magma reservoirs in replacement of the total ejecta. Through the discussion on the volume estimation of volcanic ejecta on and around Krakatau, the author recognizes that such estimation should be originally very difficult to attain enough accuracy. Much importance of “caldera deposits” to post-eruption settlements of the ejecta is emphasized. In relation to caldera formation, mechanical stability of a cavity in the crust is discussed. Lastly, upon the basis of subsurface structure, especially caldera deposits, a structural image of Krakatau caldera is presented.</p>

Krakatau caldera deposits: revisited and verification by geophysical means

<p> </p><p>One of the differences between volcanic craters and calderas is that the latter bottoms are flatways filled with caldera deposit with lower density in comparison to country rocks. The 1883 Krakatau eruption affords us important knowledge on caldera formation even if it was not observed with modern sophisticated instruments. First, volcanic activities of the Krakatau Islands before and after the 1883 eruption are reexamined: previous suppositions involving a caldera-forming eruption of the proto-Krakatau prior to 1883 proved to be unsupported by the bathymetric topographies and gravity anomalies on and around the Krakatau Islands. Then, Anak Krakatau is interpreted as a parasitic cone of the main Krakatau volcano. As supplementary knowledge to discussion of caldera deposits, the results of drillings at several calderas in Japan and Mexico are introduced. Mass deficiency of the caldera deposit at Krakatau caldera is estimated by the gravity anomaly observed there and converted to probable volume with suitable density. For quantitative examination of the subsurface structure beneath the Krakatau complex, spatial distributions of seismic S-wave attenuation and Vp/Vs ratios have been already studied by temporary seismological observations and their results have been published. The high ratios of Vp/Vs observed approximately at Krakatau caldera may be attributable to the caldera deposit that is low density and contain much water. As additional remarks, a zone having both the characters, S-wave attenuation and zones of relatively high Vp/Vs ratio, may be a probable magma reservoir centering at a depth of about 10 km.</p><p><span style="font-size: 12.000000pt; font-family: 'TimesNewRomanPSMT';"><br /></span></p>

Aeromagnetic anomaly images of Vulcano and Southern Lipari Islands (Aeolian Archipelago, Italy)

Newly acquired high-resolution, low-altitude aeromagnetic data over Vulcano Island and Southwestern Lipari in the Southern Tyrrhenian Sea resolve the major volcanic features in the area associated with the past and present activity. The magnetic character changes in amplitude and frequency from south-east to north-west. The Primordial Vulcano, the Lentia Complex, the Piano Caldera units, the Fossa Caldera deposits, and the currently active La Fossa cone and Vulcanello represent the main volcanic phases on Vulcano Island. They show a distinct magnetic anomaly pattern, whereas prior to this survey, no magnetic signatures of these features were found.

The 1969-1985 Pozzuoli event and active volcanisms.

Pozzuoli is located at the center of the Campi Flegrei caldera, near Naples and is famous for its anomalous subsidence and upheaval documented since the Roman period. Its secular and gradual subsidence can be interpreted as self-loading compaction of the caldera fills while abrupt upheavals are geologically suspected to be caused by magmagenic movements or steam forces. In order to interpret the origin and the process of the Pozzuoli upheavals, they are compared with active volcanisms represented by the 1977-1982 eruption of Usu volcano in Hokkaido. Usu volcano outburst in 1977 in major pumice eruptions and repeated magmatic and phreatomagmatic eruptions, and manifested remarkable ground deformations accompanying earthquake swarms. In 1969, the ground of Pozzuoli began to upheave with increases in seismicity but finally failed to cause any eruptive phenomena at the surface; nevertheless there are common characteristics of their motives and processes between the two events. The motive of the Usu deformation is clearly due to magma movements while that of the Pozzuoli upheaval has not been completely settled. A quantitative relationship between seismicity and deformation gives a clue for discussing the motive of the Pozzuoli deformations. The discharge rates of seismic energy and the deformation rates are compared between the two events and a certain similarity is found. This suggests that the origin of the Pozzuoli event may be partly magmatic as well as the Usu eruption, but its behavior largely depends on the property of the caldera deposits. When their deformation volumes are taken into consideration, their characteristics become quantitatively conspicuous. The ground at Pozzuoli is much more easily deformed by the upward motive force than Usu volcano. This is due to the rheological property of the caldera deposits of Campi Flegrei, and agrees to the theory that interprets the secular subsidence observed in historical times, as self-loading compaction. It is interesting that there is a point of contact between anomalous movements of the ground along the seashore in Italy and remarkable magmatic movements at the active volcano in Japan.

Origin of the calderas and evolution of Roccamonfina volcano (Roman Region, Italy)

The Roccamonfina volcano consists of a volcanic–fluvial succession dated 549 ka BP (Paleoauruncus) overlain by a 546 ka BP stratacone (Main Cone) formed by leucite-bearing lava and tephra. The main cone was dissected by four nested calderas. A ring fracture concentric with, and outside of, the central caldera is also present. Detailed mapping of the volcano, inspection of Landsat satellite images, and radiometric dating establish that the Gli Stagli and northern calderas subsided between, respectively, 546–474 and 403–374 ka BP, and that the summit part collapsed between >546 and 446 ka BP to form the central caldera. A chronologically intermediate state, dated in the range 376–323 ka BP, was identified between the 549–374 ka BP leucite-bearing High-K Series (HKS) Stage I, and the 317–96 ka BP leucite-free, Low-K Series (LKS) Stage II. This stage is made up of concurrent, HKS-LKS rocks (Stage ‘ICS’). The central, Gli Stagli, and northern calderas are older than BLT and WTT, as supported by the strong discrepancies between calculated volumes of these pyroclastic rocks and the volume of the caldera. Also, caldera collapses due to sector sliding are improbable as tested by the absence of rock-slide avalanche deposits. The collapses occurred incrementally through repetitive draining of the magma chamber(s) resulting from a succession of moderate-sized volcanic events. Gli Stagli caldera was infilled by a lake early after collapse, so that sediments alternated with leucite-tephrite lava flows and tephra within the lake. The sequence was overlain by a thick succession of young pyroclastic rocks. Downfaulting (±downsagging) cut the flat-lying Gli Stagli-filling sequence, resulting in the old Valle caldera. Successive pyroclastic and debris flows, derived by updoming of the central caldera deposits, infilled the last structure, and downfaulted part of the Valle fill producing the young Valle caldera.

Subsurface structure of Toya caldera, Japan as revealed by detailed magnetic survey.

Shipborne and ground magnetic surveys were conducted using a proton precession magnetometer on and around Toya caldera, Hokkaido, Japan. Modeling of the noticeable magnetic anomaly indicates the presence of highly magnetized rocks filling a basement depression. Considering the low gravity anomaly on the caldera, the magnetized infill is identified as low density caldera deposits. The proposed diameter of the depression is only about half that of the present caldera. The evidence suggests that the original caldera wall has been eroded away. Analysis of the magnetic anomalies reveals that the andesitic central cones of Toya caldera are magnetized in a normally polarized direction and with magnetizations of 3-4 A/m. Exceptionally, a magnetization of 9 A/m is es-timated for two lava domes. The origin of the strong magnetization is possibly attributed to the rich content in Fe3O4, on the basis of the chemical analysis of rock samples. Two topographic highs of the lake floor are identified as sunken lava domes because of the considerably high magnetic anomalies.