Chapter 6 Volcaniclastic processesocean.tongji.edu.cn/space/zhifei/files/2018/11/Chapter-6...marine settings, important deposits result from ash fallout that is influenced by dominant

Marine Sedimentology Frontiers, Prof. Zhifei Liu

2018/11/16

School of Ocean and Earth Science, Tongji University 1

Chapter 6Volcaniclastic processes and deposits in the deep-sea

Volcanic activity at ocean-spreading centers, large oceanic islands, and especially subduction zones, leads to eruptive and non-eruptive processes that generate large volumes of volcaniclastic sediments in the deep-sea.

They record valuable information about sedimentary processes and geodynamic setting through their chemical composition and faciesarchitecture.

Marine volcaniclasticformations can be the host of ore-mineral deposits with economic interest.


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Types of volcaniclastic particles: pyroclastic, hydroclastic, autoclastic, alloclastic, and epiclastic. They are mainly distinguished on the basis of morphological characteristics.

1. Volcaniclastic materials: the evidence of volcanic activity

The smaller products of highly explosive activity of viscous magma include micropumice, bubble-wall shards, crystals with adhering glass, and lithic fragments.


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The majority of deep-sea volcaniclastics is formed either sub-aerially or in shallow water and transported to the deep-sea by two primary processes: fallout or some type of sediment gravity flow.

2. Transport and deposition of volcaniclastics to the deep-sea


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Volcanic influence on sedimentation in the deep-sea can be direct or indirect, and sedimentary record reflects temporal changes in clastic supply rates and pathways.

In general, volcanism and its related deposits are highly discontinuous through time. Direct volcaniclastic sedimentation is contemporaneous with eruptive periods (syn-eruptive periods). In marine settings, important deposits result from ash fallout that is influenced by dominant winds.

Indirect volcaniclastic-sedimentation results from the reworking of previously deposited volcanic material. It occurs both during syn-eruptive and inter-eruptive periods.

Two of the most significant deep-water volcaniclastic depositional areas are found adjacent to large oceanic islands, such as Hawaii, and in backarc basins bordering active oceanic island arcs.

3. Volcaniclastic contribution to marine sedimentation


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U1431

Greenish black volcaniclasticbreccia with sandstone and claystoneinterbeds.

U1431黄岩

珍贝

宪南

宪北

涨中

石星


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Microprobe and SEM studies on minerals (i.e. biotite, plagioclase, and hornblende) from the YTT suggest that biotite is phenocrystic while hornblende and someplagioclase are xenocrystic, implying that the latter xenocrysts were incorporated into the youngest Toba magma before the eruption.


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Mid-ocean ridge volcanism: volcanic activity is effusive along fissures with emplacement of tholeiitic basal; fragmentation can lead to formation of basaltic hyaloclastite or breccia; explosive volcanism occurs very rarely in the deepwater environment.

Seamounts: pillowed lava flows, hyaloclastite gravity flows are common; volcaniclastic processes and facies on seamounts appear to be very similar to those that occur along mid-ocean ridges.

Large oceanic islands: high magma flux from deep-seated mantle plumes, e.g., Hawaiian and Canary islands; capable of producing widespread volcaniclastic deposits;

Subduction zone: produces a much larger spectrum of magma compositions; more evolved and volatile-rich magmas more abundant; multiple sources (i.e., volcanoes) of volcaniclasticmaterial arranged in a curvilinear fashion adjacent to back- and forearc basins, and trenches.

4. Volcaniclastic sedimentation in various deep-sea environments


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Schematic drawing of the eruption to produce the hyaloclastite deposit on the seamount summit near the East Pacific Rise.


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Sources and processes of volcaniclastic sedimentation during the evolutionof an oceanic island arc with backarc basin formation.

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Chapter 6 Volcaniclastic processesocean.tongji.edu.cn/space/zhifei/files/2018/11/Chapter-6...marine settings, important deposits result from ash fallout that is influenced by dominant