Application of tephra volume models to ejecta volumes from subsurface explosion experiments
Deposit volume is a critical factor for reconstructing an explosive eruption. Volume estimate models typically used for large Plinian deposits have been adapted and improved repeatedly over the last few decades. Less work has been done to refine a method for estimating the volume from smaller deposits produced by discrete phreatic and phreatomagmatic explosions. The characterization of the volume and distribution of deposits is required to quantify the physical hazards presented by different explosion types and develop appropriate models of future eruptions. Six classic tephra volume models were assessed using a dataset from subsurface explosion experiments. The models typically did a poor job modelling the volume of proximal deposits as a component of total deposit volume of discrete explosion deposits. Models reproduced medial and distal deposit volumes with greater success, particularly the Exponential model and a more recent Linear Regression model. It is therefore recommended, when possible, to use digital elevation models produced from GPS or laser-based methods to characterize proximal deposits separately and to use tephra volume estimates for medial and distal deposits. Additionally, this dataset enabled the comparison of ejecta volumes with crater diameters and highlighted that this relationship only holds for simple crater scenarios without any lateral vent migration, collapse or erosion of the crater under study. The assessment and improvement of these methods are required to ensure accurate deposit volumes as they serve as one of the most important inputs to hazard assessments and numerical models.
Relationship between crater size and ejecta volume of recent magmatic and phreato‐magmatic eruptions: Implications for energy partitioning
Relationships between crater diameter (D in meters) and ejecta volume (V in cubic meters) of recent magmatic and phreato‐magmatic eruptions are expressed as D=0.11 V0.42, and D=0.97 V0.36, respectively. Crater diameters of phreato‐magmatic eruptions are ca. 2–5 times larger than those of magmatic eruptions of similar ejecta volume, suggesting that magma‐water interaction in phreato‐magmatic eruptions generated mechanical energies of crater formation 1–2 orders of magnitude larger than equivalent‐volume magmatic eruptions. The conversion ratio of thermal energy to kinetic energy in phreato‐magmatic eruptions is estimated to be 0.7–10 percent, which is in accord with the presently available data of laboratory experiments on melt‐water interaction [Wohletz and McQueen, 1984].
Unbedded diatreme deposits reveal maar-diatreme-forming eruptive processes: Standing Rocks West, Hopi Buttes, Navajo Nation, USA
Diatremes provide partial records of how dyke-delivered magma, periodically interacting with water, produces the largest known cylindrical conduit structures of any volcano type. We address how pre-eruptive country rock is disrupted and redistributed to form the diatreme structure during an eruption by establishing the internal architecture of the unbedded diatreme at Standing Rocks West, and the volumes and sources of wall-rock within the diatreme and in a complementary tephra ring at Teshim, both in the Hopi Buttes volcanic field. The unbedded diatreme is dominantly exposed as a massif comprising multiple subvertical columns, interpreted as ephemeral-conduit deposits of well-mixed, poorly sorted, composite and juvenile pyroclast-rich deposits that truncate marginal layered deposits, plus a peripheral heterolithic country rock breccia. Wall-rock clasts are scarce in the massif and were mainly sourced from ~75 to 245 m depth below the paleosurface. By contrast, the tephra ring has abundant wall-rock fragments, dominantly from the upper 75 m of the pre-eruptive sedimentary sequence. The diatreme structure is interpreted to have been formed by many small-volume explosions fed by a low flux of basaltic magma. Late stage explosive activity was rooted mostly within pyroclastic debris at shallow to intermediate depths within the diatreme structure. This resulted in damped and shifting intra-diatreme explosions and jets that facilitated gradual mixing, recycling and remobilization of debris in the diatreme, with incremental addition of juvenile material and possibly a local rise in the crater floor.