Volcanic Processes: Mechanisms in Material Transport

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For the genetic integration of crater growth and, consequently, the interpretation of crater diameter values of terrestrial, dominantly phreatomagmatic volcanoes, there are fundamentally two end-member models. The first model is the incremental growth model Figure 7A. Growth initiates when the magma first interacts with external water, possibly groundwater along the margin of the dyke intrusions, triggering molten-fuel-coolant interactions MFCI [ - , , ]. These initial interactions excavate a crater on the surface, while the explosion loci along the dyke gradually deepen the conduit beneath the volcano towards the water source, resulting in a widening crater diameter [ ].

This excavation mechanism initiates some gravitational instability of the conduit walls, triggering slumping and wall rock wasting, contributing to the growing crater [ 81 , , , , - , ]. This classical model suggests that. Crater growth envisaged by the incremental growth A and largest explosion models B. In the top figure, the crater grows by each explosion and the subsequent mass wasting processes e. This results in the gradual growth of the crater and the underlying diatreme if any. On the other hand, in the bottom figure the crater can reach its final size in the middle of the eruption history by the largest near-surface explosion and the further explosions make no contribution to its size, geometry and morphology.

However, it is more likely that the pyroclastic succession created at the rim of the crater preserves only a certain stage of the evolution of whole volcanic edifice. For instance, the possibility of juvenile and lithic fragments being erupted and deposited within the ejecta ring from a deep explosion i. Rather than being dominated by the deep-seated eruptions, explosions can occur at variable depths within the diatreme [ ].

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The second model is where the crater geomorphology is dominated by largest explosion event during the eruption sequence. This model of crater growth Figure 7B for phreatomagmatic volcanoes is proposed on the basis of analogues from phreatic eruption, such as Uso craters, Hokkaido, Japan in [ ], and experiments on chemical and nuclear explosions [ ]. In this model, the crater diameter D is a function of the total amount of ejected tephra V ejecta [ ]:. This relationship between crater size and ejected volume was based on historical examples of phreatomagmatic eruptions [ ].

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These historical eruptions are, however, associated with usually polygenetic volcanoes, such as Taupo or Krakatau, and not with classical monogenetic volcanoes, except the Ukinrek maars, near Peulik volcano, Alaska. In this model, the largest phreatomagmatic explosion governs the final morphology of the crater, so the crater size correlates with the peak energy of the maar-forming eruption directly [ ].

Scaled experiments showed that there is a correlation between the energy and the crater depth and diameter [ ] if the explosions take place on the surface [ ]. Most of the explosions modelled in Goto et al. In these multiple explosions, the crater did not grow by subsequent smaller explosions, possibly because the blast pressure was lower than the rock strength when it reached the previously formed crater rim [ ]. As noted by Goto et al. Morphologically, these deep-seated eruptions have a minor effect on crater morphology and diameter, and their deposits rarely appear within the ejecta ring around the crater.

Theoretically, both emplacement models are possible because both mechanisms can contribute significantly to the morphology of the resulting landform. The incremental growth model is based on statigraphy, eye-witnessed historical eruptions and experiments [ 81 , , - , - ], while the largest explosion model is based on analogues of chemical or nuclear explosion experiments, phreatic eruptions and impact cratering [ , , - ].

Based on eye-witnessed eruptions and geological records, the crater diameter as a morphometric signature for maar-diatreme and tuff ring volcanoes is the result of complex interplay between the eruptions and the substrate. The dominant processes, such as many, small-volume explosions with various energies migrating within the conduit system vertically and horizontally, as well as gradual mass wasting depending on the physical properties of rock strength, are what control the final crater diameter.

On the other hand, the substrate beneath the volcano also plays an important role in defining crater morphology, as highlighted for terrestrial volcanic craters [ , , ], as well as extraterrestrial impact craters [ ]. In different substrates, different types of processes are responsible for the mass wasting.

For example, an unconsolidated substrate tends to be less stable due to explosion shock waves that may liquefy water-rich sediments, and induce grain flow and slumping, enlarging the crater [ ]. On the other hand, in a hard rock environment, the explosions and associated shock waves tend to fracture the country rock, depending on its strength, leading to rock falls and sliding of large chucks from the crater rim [ ]. The crater walls in these two contrasting environments show different slope angles [ , ]. These differences in the behaviour of the substrate in volcanic explosions may cause some morphological variations in the ejecta distribution and the final morphology of the crater Figure 8.

Morphology of volcanic crater and diatreme generated by phreatomagmatic eruptions in soft and hard substrate environments. The crater diameter is an important morphometric parameter in volcanic landform recognition; however, the final value of the crater diameter is the result of a complex series of processes, usually involving syn-eruptive, mass wasting processes of the crater walls, e.

This makes the direct interpretation of crater diameter values more complicated than predicted by simple chemical and nuclear cratering experiments [ , ].

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Thus, the morphometric data of a fresh maar or tuff ring volcano contain cumulative information about the eruption energy, the location and depth of shallow explosion loci, as well as the stability of the country rock and associated mass wasting. The largest eruption dominated model may only be suitable to express energy relationships without the effects of mass wasting from the crater walls.

This probably exists in only a few limited sites. For example, these eruptions should take place from a small-volume of magma, limiting the duration of volcanic activity and reducing the possibility of development of a diatreme underneath Figure 8. Additionally, these eruptions should be in a consolidated hard rock environment, with high rock strength and stability.

The following model for the interpretation of crater diameter and morphology data can be applied for phreatomagmatic volcanoes Figure 8. This model integrates both conceptual models for crater and edifice growth, but the majority of craters experience complex development instead of the dominance of the largest explosion event. The likelihood of the largest explosion event dominated morphology is limited to simple, short-lived eruptions due to limited magma supply or vent migration in a hard rock environment Figure 8.

If the magma supply lasts, the development of a diatreme underneath starts that has a further effect on the size, geometry and morphology of the crater of the resultant volcano. The crater diameter is often a function of basal edifice diameter W co or height H co , creating ratios which are commonly used in landform recognition in extraterrestrial environments [ , , ]. W co is often difficult to measure because of the subjectivity in boundary delimitation [ ].

Any break in slope could also be smoothed away by post-eruptive erosional processes. This small elevation difference from the surrounding landscape gives rise to some accuracy issues, particularly regarding the establishment of the edifice height. To demonstrate the limitations e.

Both volcanoes above were used to establish the average morphometric signature of an Earth analogue phreatomagmatic volcano [e. The early morphometric parameters were measured from topographic maps having coarse contour line intervals e. Their eruption history, including volume, facies architecture and morphology, are completely different.

The present crater floor of Pukaki volcano is well under the syn-eruptive surface, thus it is a maar volcano sensu stricto following Lorenz [ ]. This was formed by a magma-water interaction driven phreatomagmatic eruption from a small volume of magma of 0.

Volcanic Processes: Mechanisms in Material Transport

The present facies architecture of this volcano seems quite simple e. On the other hand, Crater Hill has a larger eruptive volume of 0. Later eruption formed an additional scoria cone and associated lava flow that filled the crater with lava up to m in thickness [ 80 , ]. Consequently, Crater Hill is an architecturally complex volcano with complex eruption history, i. The important implication of the examples above and the usual complex pattern and processed involved on the establishment of the final geomorphology e.

In other words, parameters to express morphometric signature cannot be compared between volcanoes with different eruption histories i. Comparison without knowledge of the detailed eruption history could be misleading.

Furthermore, the morphometric signature properties used in extraterrestrial volcano recognition for crater-type volcanoes should be reviewed, using a volcanological constraint on the reference volcano selection. Cone-type monogenetic volcanoes, such as spatter- Figure 2 , scoria or cinder; Figure 3 and tuff cones Figure 5 , are typically built up by proximal accumulation of tephra from low to medium 0.

Deposition from localized pyroclastic density currents is possible, mostly in the case of tuff cones [ , , ] and rarely in the case of scoria cones from violent Strombolian eruptions [ ].

Volcanic Processes: Mechanisms in Material Transport

On a fresh edifice, where no post-eruptive surface modification has taken place, these morphometric parameters are related to the primary attributes of eruption dynamics and syn-eruptive sedimentary processes. However, there are potentially two valid models to explain their dominant construction mechanisms, including a ballistic emplacement with drag forces and fallout from turbulent, momentum-driven jets at the gas-thrust region [ 84 , , , - ]. In both models, the angle of repose requires loose, dry media. This criterion is rarely fulfilled in the case of a tuff cone [e.

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In these cases the ejected fragments have high water-contents that block the free avalanching of particles upon landing [ , , , , ]. This is inconsistent with other magmatic cone-type volcanoes; the growth processes of tuff and littoral cones are not discussed in further detail here.

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The ballistics model with and without drag for scoria cone growth was proposed as a result of eye-witness accounts of eruptions of the NE crater at Mt. Etna in Sicily, Italy [ ]. These studies also showed that the typical particle diameter is cm-scale or less instead of dm-scale [ , ], which cannot be derived purely from impact breakage of clasts upon landing [ , ].

Especially during paroxysmal activity at Stromboli [ ] or more energetic violent Strombolian activity [ 83 , ], the dominance of fine particles in the depositional records contradicts the ballistics emplacement model for the cones. To solve the debate about cone growth, the jet fallout model was proposed [ ], based on the fact that there is a considerably high proportion of fines in cone-building pyroclastic deposits [ 60 , , ]. This proposed behaviour is similar to the proximal sedimentation from convective plumes, forming cones with similar geometry to scoria cones, e.

The fines content does not fulfil the criteria of pure ballistic trajectory, thus turbulent, momentum-driven eruption jets Figure 9A should be part of the cone growth mechanisms [ , , ]. As documented above, scoria cones demonstrate a wider range of granulometric characteristics [ ] than previously thought [ ]. Such switching from classical lapilli-dominated to ash- or block-dominated cone architectures reflects syn-eruptive reorganization of conduit-scale processes, including.

The latter case may or may not cause a change in eruption style e. This switching has significant consequences on pyroclast transport as well. The higher efficiency of magma fragmentation and production of finer pyroclasts e. A buoyant eruption column is created when the time of heat transfer is shorter than the residence time of fragments in the lowermost gas-thrust region [ , , ]. Thus, particle transport shifts from momentum- to buoyancy-driven modes [ - ]. Based on numerical simulations, these changes in the way pyroclasts are transported are consistent with modelled sedimentation trends from jet fallout as a function of vent distance [ ].

This is in agreement with the trend predicted by the ballistic emplacement model [ ]. In the case of scoria cone growth the final morphology is not only dependent on the mode of pyroclast transportation via air, but also on significant post-emplacement redeposition.

Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport Volcanic Processes: Mechanisms in Material Transport
Volcanic Processes: Mechanisms in Material Transport

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