Mauna Kea is about a million years old, and has thus passed the most active shield stage of life hundreds of thousands of years ago. In its current post-shield state, its lava is more viscous, resulting in a steeper profile. Late volcanism has also given it a much rougher appearance than its neighboring volcanoes; contributing factors include the construction of cinder cones, the decentralization of its rift zones, the glaciation on its peak, and the weathering effects of the prevailing trade winds. Mauna Kea last erupted 6,000 to 4,000 years ago and is now considered dormant.
In Hawaiian mythology, 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 mountain's ecological balance. Mauna Kea can be ecologically divided into three sections: an alpine climate at its summit, a Sophora chrysophylla–Myoporum sandwicense (or māmane–naio) forest on its flanks, and an Acacia koa–Metrosideros polymorpha (or koa–ʻōhiʻa) forest, now mostly cleared by the former sugar industry, at its base. In recent years, concern over the vulnerability of the native species has led to court cases that have forced the Hawai'i Department of Land and Natural Resources to eradicate all feral species on the mountain.
With its high elevation, dry environment, and stable airflow, Mauna Kea's summit is one of the best sites in the world for astronomical observation. Since the creation of an access road in 1964, thirteen telescopes funded by eleven countries have been constructed at the summit. The Mauna Kea Observatories are used for scientific research across the electromagnetic spectrum and comprise the largest such facility in the world. Their construction on a landscape considered sacred by Native Hawaiians continues to be a topic of debate.