when is the last time mauna kea erupted
6,000 to 4,000 years ago.
Mauna Kea
Mauna Kea last erupted 6,000 to 4,000 years ago and is now considered dormant. 
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Mauna Kea [SEP] Introduction
Mauna Kea last erupted 6,000 to 4,000 years ago and is now considered dormant. The peak is about higher than Mauna Loa, its more massive neighbor. In Hawaiian religion, the peaks of the island of Hawaii are sacred. An ancient law allowed only high-ranking aliʻi to visit its peak. Ancient Hawaiians living on the slopes of Mauna Kea relied on its extensive forests for food, and quarried the dense volcano-glacial basalts on its flanks for tool production. When Europeans arrived in the late 18th century, settlers introduced cattle, sheep and game animals, many of which became feral and began to damage the volcano's ecological balance.