Mauna Kea
Mauna Kea ( or , ) is a dormant volcano on the island of Hawaii. Its peak is above sea level, making it the highest point in the state of Hawaii. Most of the mountain is under water, and when measured from its oceanic base, Mauna Kea is the tallest mountain in the world measuring over . 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 due to construction of cinder cones, decentralization of its rift zones, glaciation on its peak, and weathering by the prevailing trade winds. Mauna Kea last erupted 6,000 to 4,000 years ago and is now considered dormant. The peak is only about higher than its more massive neighbor, Mauna Loa.
Lōʻihi Seamount
Radiometric dating was used to determine the age of rock samples from Lōʻihi. The Hawaii Center for Volcanology tested samples recovered by various expeditions, notably the 1978 expedition, which provided 17 dredge samples. Most of the samples were found to be of recent origin; the oldest dated rock is around 300,000 years old. Following the 1996 event, some young breccia was also collected. Based on the samples, scientists estimate Lōʻihi is about 400,000 years old. The rock accumulates at an average rate of per year near the base, and near the summit. If the data model from other volcanoes such as Kīlauea holds true for Lōʻihi, 40% of the volcano's mass formed within the last 100,000 years. Assuming a linear growth rate, Lōihi is 250,000 years old. However, as with all hotspot volcanoes, Lōihi's level of activity has increased with time; therefore, it would take at least 400,000 years for such a volcano to reach Lōihi's mass. As Hawaiian volcanoes drift northwest at a rate of about a year, Lōʻihi was southeast of its current position at the time of its initial eruption.