South Arch volcanic field facts for kids
Quick facts for kids South Arch volcanic field |
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 South Arch volcanic field
Location in Pacific Ocean
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| Highest point | |
| Elevation | 10 m (33 ft) Above sea floor |
| Dimensions | |
| Length | 35 km (22 mi) |
| Width | 50 km (31 mi) |
| Geography | |
| Location | 200 kilometres (120 mi) south from Hawaiʻi Island |
| Country | United States |
| State | Hawaii |
| Geology | |
| Mountain type | Volcanic field |
| Type of rock | Basaltic rock |
The South Arch volcanic field is a group of underwater volcanoes located south of Hawaiʻi Island. It has been active in the last 10,000 years. This volcanic area covers about 35 by 50 kilometres (22 mi × 31 mi) and is found very deep, at about 4,950 metres (16,240 ft) below the ocean surface.
Even though it's near the famous Hawaii hotspot, the South Arch volcanic field isn't a typical volcano that grows bigger over time. Instead, scientists think it formed because the huge weight of the growing Hawaiian volcanoes made the oceanic crust (the Earth's outer layer under the ocean) bend and crack. These cracks then allowed magma (molten rock) to rise up from deep inside the Earth.
Contents
What Does the South Arch Volcanic Field Look Like?
This volcanic field is about 200 kilometres (120 mi) south of Hawaiʻi Island. It sits at a depth of around 4,950 metres (16,240 ft) (that's almost 5 kilometers!) under the sea. The field is made up of many lava flows, which are several meters thick.
These lava flows are surrounded by a seafloor covered in sediment (like sand and mud). To the west and southwest, there are older, smaller underwater mountains called seamounts from the Cretaceous period.
Features of the Lava Flows
When scientists use special sonar to map the seafloor, new lava flows look bright. Older flows, which are covered by sediments, appear darker. Scientists have seen cool features on the young lava flows, like:
- Lava coils (like ropes)
- Lava rubble (broken pieces)
- Lava tumuli (small mounds)
- Pillow lavas (rounded shapes that form underwater)
- Polygons (cracked patterns)
- Sheet flows (smooth, flat areas)
The lava flows have smoothed out the seafloor where they erupted. In one spot, a lava flow front rises about 10 metres (33 ft) from the seafloor. The lava that erupted here was likely very runny and came out quickly. There's no sign of a large magma chamber (a big pool of molten rock) under the South Arch volcanic field.
How the South Arch Volcanic Field Formed
The South Arch volcanic field is located on top of the Hawaiian Arch. This Arch is an area of the seafloor around Hawaii that has bent downwards because of the huge weight of the Hawaiian volcanoes. This bending causes the Earth's crust to crack, allowing magma to push its way up to the surface and create new volcanoes.
Older seamounts in the area might have helped start the volcanic activity here. This kind of "off-center" volcanism has also been seen near other hotspots around the world.
Other Volcanic Fields Nearby
The South Arch volcanic field isn't the only one on the Hawaiian Arch. There's also the much larger North Arch volcanic field north of Oahu. Plus, there are small lava flows near the Pensacola Seamount southwest of Hawaiʻi Island.
Together, these volcanic fields cover a larger area than the islands of Hawaii themselves. They show that Hawaiian volcanism affects an area over 500 kilometres (310 mi) wide! Scientists call these "peripheral lavas" because they are on the edges of the main Hawaiian hotspot activity. The South Arch volcanic field is "upstream" of the Hawaiian hotspot, meaning it's where the hotspot's influence begins.
What the Rocks Are Made Of
Scientists have collected basaltic rocks from the South Arch volcanic field. These rocks are a type of alkali basalt and basanite. They contain small crystals called phenocrysts of different minerals like clinopyroxene, olivine, and plagioclase.
The lava flows are covered by thin layers of manganese crust, less than 0.15 millimetres (0.0059 in) thick. The rocks often show signs of changing over time, like forming a type of glass called palagonite.
The chemical makeup of these lavas is similar to other Hawaiian volcanoes at certain stages of their life. For example, they are like the "rejuvenated stage volcanics" on Kauai or the "preshield volcanics" at Loihi. They also share a high amount of helium-3 and carbon dioxide with the North Arch volcanic field.
Scientists believe the magmas here might have formed from a mix of molten rock from the Hawaiian hotspot and other components usually found at mid-ocean ridges. The magmas of the South Arch volcanic field also seem to have had a lot of water in them. This might be because the Earth's mantle (the layer below the crust) melted early and contained a lot of water.
When Did it Erupt and How Do We Study It?
The South Arch volcanic field seems to have younger lava flows in the center, surrounded by older flows. By looking at how thick the manganese crusts, palagonite, and sediment layers are on the lava, scientists can guess their age.
- Younger flows are thought to be between 1,000 and 10,000 years old.
- Older flows might be between 10,000 and 100,000 years old.
Another estimate for the age of some flows is 14,000–20,000 years ago. It's possible that magma came to the surface mainly where the lithosphere (the Earth's rigid outer layer) was already weak or cracked. This could explain why the vents (openings where lava comes out) of the South Arch volcanic field are found near older Cretaceous seamounts.
The South Arch volcanic field was first found in 1986 using a special sonar system called GLORIA sidescan sonar. Rocks were collected from it in 1988. Another study happened in 2001 using the Kaikō remotely operated underwater vehicle (ROV). Overall, scientists have explored this field using methods like:
- Dredging (scooping up samples from the seafloor)
- Remotely operated underwater vehicles (ROVs)
- Submersibles (small submarines for exploration)
