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Transcript
Geology of the Big Island of Hawai‘i
Island of
Hawai‘i
The Big Island of Hawai‘i
• Youngest Hawaiian island
• Five volcanoes above sea level: Kohala, Mauna Kea,
Hualalai, Mauna Loa, and Kilauea
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•
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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
submarine
land slides
submarine
rift zones
submarine
land slides
submarine
rift zone
Lō‘ihi Seamount
Kohala
Mauna Kea
Hualālai
Mauna Loa
Kīlauea
distribution and
thickness of
Pāhala Ash
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
Kohala Volcano
cinder cones near the summit of Kohala Volcano
Waipi‘o Valley before the tsunami of 1946
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
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–
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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
grooves cut by the motion of glaciers
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
trachyte
lava flow
Pu‘u
Wa‘awa‘a
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
fault scarp
Nīnole
Hills
fault scarp
Mauna Kea
Mauna Loa
Kīlauea
Ninole Hills
Kealakeua Bay
fault
scarp
South Point
fault
scarp
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
motion of
south flank
fault
scarps
Mauna Loa
Kilauea
Caldera
Kilauea Caldera
fumaroles in Kilauea Caldera
Southwest Rift Zone
Southwest Rift Zone
Current Eruptive Activity
lava flows
fault
scarps