geology of the big island of hawai‘i. island of hawai‘i
Post on 21-Dec-2015
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The Big Island of Hawai‘i• Youngest Hawaiian island• Five volcanoes above sea level: Kohala, Mauna Kea,
Hualalai, Mauna Loa, and Kilauea
• Two submarine volcanoes: Mahukona and Lo‘ihi
• Submarine rift zones• Giant submarine landslides• Pahala Ash
– a layer of weathered ash found all over the island
– may come from more than one volcano• Mauna Kea in the north
• Kilauea in the south
– has been used to try to correlate age of Big Island volcanoes
Kohala Volcano• Oldest of the Big Island volcanoes
• A single set of rift zones (two total)
• No exposed caldera--probably buried by postshield lavas
• Shield Stage rocks:– Pololo Member, 500 to 300 k.y.a. (k.y.a. = thousand years ago)
– tholeiitic basalt, exposed in valley walls
– alkalic basalts appear toward end of shield stage
• Postshield Stage rocks:– Hawi Member, 260 to 60 k.y.a.
– mostly mugearite, some trachyite (alkalic rocks more felsic than hawaiite)
• Amphitheater-headed valleys, such as Waipi‘o Valley– are drowned valleys; were cut when local sea level was lower
• Giant submarine landslides
Mauna Kea Volcano• Three rift zones (postshield stage)• Filled caldera• Shield Stage rocks
– Hamakua Member, 380 k.y.a. to ?• tholeiitic basalts grading upwards into alkalic rocks (alkalic basalts and
hawaiiites)• exposed in sea cliffs along the Hamākua Coast
• Postshield Stage rocks– Laupahoehoe Member, ? to 4 k.y.a– buries most of the shield stage rocks– mostly hawaiite– eruptions restricted to upper slopes of the volcano– most eruptions also formed large cinder cones– form a steep-sided “cap” on the surface of the old shield
volcano
• Had ice-age glaciers on its summit
Hualalai Volcano• end of Subaerial Shield Stage/start of Postshield Stage
– Hualalai Member, 13 k.y.a. to present (1801 A.D.)
– alkalic basalts, a few hawaiites, and a trachyte• no tholeiitic rocks are exposed at the surface!
– Lavas commonly contain pieces of country rocks called xenoliths• many are highly crystalline, phaneritic rocks
– represent pieces of an older, solidified magma chamber
– gabbro (plagioclase and pyroxene) and peridotites (mostly olivine and pyroxene)
• Pu‘u Wa‘awa‘a– a trachyte pumice cone; also contains obsidian
– produced a very thick lava flow
– produced from an isolated magma chamber?
• Since this volcano has been active at the same time as Mauna Loa, its lava flows are interbedded with Mauna Loa lava flows
Mauna Loa Volcano• Largest volcano on earth
• has a caldera and two main rift zones
• normal faulting, especially along southwest rift zone
• giant submarine landslides off its southwestern flank
• only has Subaerial Shield Stage rocks exposed– all rocks are tholeiitic basalts
– Ninole Member, ~540 k.y.a.• oldest exposed rocks on the island of Hawai‘i
• may represent an extinct earlier volcano or an early stage of Mauna Loa
• named for the Ninole Hills, former ridges between amphitheater-headed valleys that have been overrun by later Mauna Loa eruptions
– Kahuku Member (unknown age)• lavas that bury the Ninole Member
• separated from younger lavas by an erosional surface
– Ka‘u Member, 300 k.y.a. to present• produced by current eruptive activity
• covers the surface of most of the volcano
– may be nearing the end of main shield-building activity since it has less
frequent eruptions than Kilauea
Kilauea Volcano• Most active volcano on earth
– has been continuously erupting since 1983 from vents on the east rift zone
• has a caldera and two main rift zones– caldera bounded by normal fault scarps
– faulting produces benches in the caldera walls
– rift zones have small lava shields, spatter cones, and cinder cones
• slumping of southern (seaward) flank produces normal faulting– results from expansion of rift zones due to dike injection
– seaward flank has dropped more than 600 meters
– can produce large earthquakes and tsunamis
• only has Subaerial Shield Stage rocks exposed– all rocks are tholeiitic basalts
– Hilina Member, 100 to 31 k.y.a
– Pahala Ash, ~31 k.y.a.• contains Pele’s tears and Pele’s hair
– Puna Member, 30 k.y.a. to present