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Magmatism and Magmatic Rocks
Published in Aurèle Parriaux, Geology, 2018
In addition to these four types, phreatomagmatic eruptions, which are generally highly explosive, involve the interaction of magma and water. They can be submarine or sub-lacustrine (Fig. 6.31) eruptions or can result from the interaction of magma and an aquifer. Submarine eruptions that take place mainly along the mid-oceanic ridges are similar to the Hawaiian type since the magma solidifies as pillow lava (Fig 6.32). This shape is due to the fact that the outside of the lava mass solidifies in contact with water while the center is still melted and continues to flow.
Magmatism and Magmatic Rocks
Published in Aurèle Parriaux, Geology, 2018
In addition to these four types, phreatomagmatic eruptions, which are generally highly explosive, involve the interaction of magma and water. They can be submarine or sublacustrine (Fig. 6.33) eruptions or can result from the interaction of magma and an aquifer. Submarine eruptions that take place mainly along the mid-oceanic ridges are similar to the Hawaiian type since the magma solidifies as pillow lava (Fig 6.34). This shape is due to the fact that the outside of the lava mass solidifies in contact with water while the center is still melted and continues to flow.
Matangkaka manganese deposit, Ambitle Island, Feni Island Group, Papua New Guinea: a Quaternary epithermal stratabound manganese oxide deposit
Published in Australian Journal of Earth Sciences, 2022
The Danmagal Tephra is a compound unit consisting of multiple fall, flow and surge deposits from numerous small explosive events. Fall units comprise weakly stratified deposits, with mantle bedding and accretionary lapilli bearing beds. Exposures of mantle-bedded fall deposits crop out on the Kabang track south of the crossing of Matangkaka No. 2 tributary and along main Matangkaka Creek (Figure 4a, b). Other exposures of mantle-bedded fall deposits are present at numerous localities in the upper Nanum Valley, including along the track between drillholes CC21 and CC22 and on the track west of ‘Five Ways’ (Lindley, 2015, figure 10). Lindley (2015) also figured accretionary lapilli deposits in the headwaters of the Nanum Valley. Fall deposits are interbedded with massive pyroclastic flow deposits consisting of creamy coloured, poorly sorted trachyitic ash, lapilli and blocks, with rare mafic accessory lithics (Figure 4e). The high degree of fragmentation in both fall and flow deposits suggests they may be the product of phreatomagmatic eruptions. It is not uncommon for basal units of the tephra to include large logs and charcoal fragments. Logs and charcoal fragments have been observed by Lindley and Tamu (1994) in middle Ola Creek and several locations in lower Matangkaka Creek. Logs in Ola Creek are up to 20 cm diameter and have a north–south orientation. In Matangkaka Creek a haphazard collection of logs is oriented 040° M (Figure 4e). The orientation of these logs is consistent with flows originating from Ambitle Crater. Surge deposits have been identified in basal units of the tephra member in lower Matangkaka Creek (Figure 4d). Surge deposits are fine-grained and display low-angle crossbedding, pinching and swelling and dune forms. The deposits are overlain by a pyroclastic flow unit.