Around 2 million years ago, underground volcanic eruptions created a number of mountains along the coast. This is due to another key event that shaped the bay’s topography. When the sea rises into these valleys, as it did after the most recent ice age ended around 10,000 years ago, the valleys become inlets enclosed by sheer cliffs, such as those around Yunotsu.Įven today, Yunotsu Bay is deep enough for large ships to enter. As tuff erodes - little by little, without large blocks breaking off - it tends to form deep valleys. Thick and sticky ash released by the underwater volcanic eruptions that tore the ground apart solidified into tuff, a relatively soft type of rock. The natural features that made Yunotsu such a fine harbor are the result of volcanic activity that began some 15 million years ago, when the Japanese archipelago was separated from continental Asia. These characteristics meant that many of the veins were both heavily laden and relatively easy to reach even with chisels and hammers, which miners used to dig into the mountainsides before the age of explosives. Sennoyama, they noticed that several of the silver veins they discovered were located close to the surface - a result of the hydrothermal water’s extensive flow through the brittle mountain. When prospectors in the 1500s started mining at Mt. As the liquid cooled and solidified due to the change in temperature and pressure, countless silver veins were left in the coarse rock. Sennoyama and transporting silver throughout the mountain. This metal-containing fluid then seeped upward through cracks in the bedrock, penetrating the porous Mt. The hydrothermal water’s heat released elements including silver and copper from the nearby rock and magma. Volcanic activity continued underneath the mountain, with magma heating groundwater from below and turning it into hydrothermal water, a fluid that has a temperature of more than 200☌ but does not boil away due to the high-pressure conditions deep underground. The newly formed peak was notably brittle, like a giant pile of sand. The mountain began to take shape some 1.5 million years ago, when a volcanic eruption caused a pileup of pyroclastic material such as extremely hot ash and chunks of partially solidified lava around the crater. Sennoyama, the ancient volcano around which the mine grew from the mid-1500s onward. It is no exaggeration to say that Iwami Ginzan became one of the world’s most productive silver mines thanks to the distinctive geological features of Mt.
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