The Gardner Pinnacles are two barren rock outcrops surrounded by a reef and located in the Northwestern Hawaiian Islands at. The volcano is northwest of Honolulu and from French Frigate Shoals. The total area of the two small islets—remnants of an ancient volcano that is the world's largest shield volcano—is. The highest peak is. The surrounding reef has an area in excess of. The Gardner Pinnacles were first discovered on June 2, 1820, by the AmericanwhalerMaro, commanded by Captain Joseph Allen. The Gardner Pinnacles are home to the GiantOpihi, Hawaiian Limpet known as the ‘opihi ko‘ele, which is not found anywhere else in the world outside the Hawaiian Islands. Numerous insects live on the island. Since this is part of Papahānaumokuākea Marine National Monumentwildlife refuge, Gardner Pinnacles is off limits to even the military, although they made an unauthorized landing in 1961 or 1962 and blew off the tip of the outcrop to create an emergency helicopter landing spot for the Hawaiian HIRAN project, an effort to determine location of area islands with great precision for navigational purposes. To this day the tip has not been replaced, and debris from the blast can be found scattered throughout the island. In the Hawaiian Archipelago, adjacent islands/reefs are French Frigate Shoals to the southeast, and Maro Reef to the northwest.
Geology
According to a 2020 report in Earth and Planetary Science Letters, Pūhāhonu contains approximately 150,000 cubic kilometers of rock, based on a 2014 sonar survey. This would make it Earth's largest single volcano. Only about one-third of that volume is exposed above the sea floor. The rest is buried beneath a ring of debris, broken coral, and other material that has eroded from the peak. By comparison, from sea floor to peak, Mauna Loa, on Hawaii’s Big Island, is the tallest shield volcano on Earth. But it is nowhere near as massive as Pūhāhonu; a 2013 study estimates Mauna Loa’s volume at 83,000 cubic kilometers. Pūhāhonu is so heavy, researchers note, that it has caused Earth’s crust nearby—and thus the volcano itself—to sink hundreds of meters over millions of years. The Tamu Massif, a 4-kilometer-tall volcanic feature the size of the British Isles on the sea floor east of Japan, contains almost 7 million cubic kilometers of material and was once thought to be the world’s largest shield volcano. But Tamu Massif is now believed to have formed along a midocean ridge rather than over a single source of magma. That makes Pūhāhonu the largest known shield volcano on Earth.
Name
The Hawaiian name, Pūhāhonu, means 'turtle surfacing for air', from pūhā 'to breathe at the surface' and honu 'turtle'.
