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The Deicke and Millbrig bentonite layers, specifically the potassium bentonite layer, K-bentonite, were formed from a volcanic eruption during the Taconic orogeny during the Late Ordovician on Laurentia, the craton of North America. Researchers are very interested in the eruptions that formed these bentonite layers because they are thought to be some of the largest volcanic eruptions in the last 600 million years of Earth history, and the resulting ash layer for each eruption individually was greater in volume than the Toba eruption. Bentonite is a type of clay that is formed from the weathering of volcanic ash deposits. Some researchers suggested that the ashes were from a volcanic arc that was on a convergent crust boundary. Researchers believe this because the trace element geochemistry of the bentonite shows that its source was a felsic calc-alkalic magmatic source, which is characteristic of volcanism from a continental crust destructive plate margin setting. ==Origins== The volcanoes that caused the eruptions are said to be from a magmatic arc on continental crust in a convergent tectonic system. The Taconic orogeny occurred when North America collided with either an island arc or microcontinent. Also according to researchers a few statements can be made about the eruptions produced from this system:〔 # Due to widespread ash deposits it can be inferred that there was more pyroclastic air fall debris than pyroclastic flow. A pyroclastic flow deposit would be much more localized to the eruption source. # The large scale and dispersion of the ash tells researchers that the eruptions must have been very large and also have occurred in a convergent tectonic setting. Researchers believe this because the trace element geochemistry of the bentonite shows that its source was a felsic calc-alkalic magmatic source. This type of source is characteristic of volcanism from a continental crust destructive plate margin setting. # The eruptions probably began with a Plinian or ultra-Plinian phase that produced a crystal-rich ash as a coignimbrite, air fall deposit # The sequence of events is part of the normal sequence for volcanic eruptions in which ignimbrite is generated. # In the area where the maximum bentonite thickness and grain size is recorded there is no evidence of nearby vent deposits, this means that these locations are at a minimum of several kilometers away from the volcano vents. The volcanic eruption that produced the Deicke K-bentonite bed which has been dated to 457.1 ±1.0, which was calculated using a concordant uranium-lead dating zircon fraction. The Deicke and Millbrig eruptions have a minimal estimation of covered and at least of pre-compaction bentonite that was accumulated over North America. Some researchers believe that these eruptions could have caused a global cooling event, lowering the Earth's surface temperature several degrees. Other researchers did not find enough data to support the claim of significant cooling based on their sample location. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Deicke and Millbrig bentonite layers」の詳細全文を読む スポンサード リンク
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