Dating tephra layers
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Thin layers only a few millimetres thick may be preserved, whereas they are rapidly eroded on dry land.
For example, cores from lakes near Hamilton revealed at least 46 tephra layers 2 to 120 millimetres thick, from seven North Island volcanoes (Taupō, Okataina, Tūhua, Taranaki, Tongariro, Ngāuruhoe and Ruapehu) over the last 20,000 years.
Some of the most complete tephra sequences have been found in lakes and bogs.Recently, 134 tephra layers, one nearly one metre thick, were found in cores from Leg 181 of the international Ocean Drilling Programme, around 700 kilometres east of the North Island.The layers record repeated large explosive eruptions from the Coromandel volcanic zone (Coromandel Peninsula–Tauranga area) from 2 to 12 million years ago, and then in the Taupō Volcanic Zone from about 2 million years ago.Once identified by geochemical analysis, a tephra layer provides a marker bed for an ‘instant’ in time, that instant being the time of eruption that produced the layer.
In New Zealand and elsewhere many studies have used tephra layers as a dating tool, a science called tephrochronology.
Similar studies from Auckland showed that scores of thin tephras from the same North Island volcanoes, including at least 43 tephras from Taranaki volcano, have rained out over the area for more than 70,000 years.