The Emperor Seamount Chain and the Hawaiian Ridge.

The most interesting geology in the world is typically found at the borders of plates; from the peaks of the Himalaya to the depths of the Mariana Trench, the extremes are typically found where huge slabs of rock interact. There are, however, a number of features which are hardly related to the motion and action of these plates at all; for example, the volcanic islands, which are related to huge upwelling hotspots in the Earth’s mantle.

Perhaps the most well known of these volcanic islands are within the US state of Hawaii (In this text I use Hawaii to refer to the state as opposed to the Big Island, Hawai’i). From looking at the ocean bathymetry around Hawaii it is very noticeable that there exists a long chain of seamounts towards the north west of the islands in varying states of erosion – these are the seamounts of the Hawaiian ridge. Further in the north west this chain diverges northwards, forming the Emperor Seamount Chain.

Google Data SIO, NOAA, US Navy, NGA, GEBCO, Data LDEO-Columbia, NSF, NOAA. 20°12'50''N 157°00'10''W 1,434.2km
Image of Hawaiian Archipelago (Google Earth)

These volcanic islands, and the seamounts behind them, are the result of a large hotspot below the Pacific plate. This hotspot means that the geothermal gradient below the oceanic crust is shallower than it would usually be, and so there is thinning of the lithosphere. This increased temperature leads to melting of the mantle, which becomes less dense in the melt regions and rises up. The melt rises until it reaches a level of buoyancy, at which point it can pool and solidify. In some regions, however, this magma continues to rise until it reaches the surface. At this point it is able to pour out, and form layers of very shallowly dipping basalt – producing a shield volcano.

Hypothetical Side Profile of a Shield Volcano
Hypothetical Side Profile of a Shield Volcano

In the Hawaiian archipelago these shield volcanoes have grown to be massive. The largest of these at the moment is Mauna Kea, towering a collossal 10,200m above the seafloor (4,207m above sea level). Only two of these volcanoes are currently active – Mauna Loa, and Kilauea.

The formation of new volcanoes is generally a result of the motion of the Pacific plate as opposed to that of the hotspot. For the past 40 million years the hotspot has likely been relatively stationary, while the Pacific plate has moved north west – so the chain of volcanoes has moved south east. Once the volcano moves away from the hotspot it becomes extinct, and the rate of growth is outweighed by the rate of erosion – and so it erodes away leaving a small seamount. Over 40 million years ago, it is believed that the motion of the hotspot southwards combined with the continued motion of the Pacific plate led to the different direction of motion of the Emperor Seamount Chain – the hotspot becoming stationary 40 million years ago led to the change in direction.

Google Data SIO, NOAA, US Navy, NGA, GEBCO, Data LDEO-Columbia, NSF, NOAA
The Emperor Seamount Chain and Hawaiian Ridge from Space. The prominent change in direction is visible (Google Earth)

As a result of the low viscosity of basaltic lava (resulting from the low silica and low crystal content) the lava flows are able to travel far and wide. It is this wide spread of lava which leads to the shield like profile of these volcanoes. On the islands which currently have active volcanism, a range of different lava flow style can exist. The most common of these are the a’a and pahoehoe lava. a’a lavas are characterised by a high volume flow rate with large channels containing thick flow units of high viscosity. On the extremities of the flow this lava begins to crystallise, and as this skin is broken up it leads to ‘clinkers’ which can fall off and produce an underflow to the lava flow.

A’a Lava Flow on Kilauea (USGS)

Pahoehoe lavas, conversely, have low volume flow rates and a low front flow velocity. These have much lower viscosity, and as a result of the lower velocity can build up a skin which is non disrupted, and is able to form lava channels through which the lava can flow relatively unhindered. When expressed on the surface these lavas show a ropey texture, on the order of 0.2-2m thick.

Cooled Pahoehoe flow showing ropey texture (USGS)
Cooled Pahoehoe flow showing ropey texture (USGS)

Many of the islands formed in Hawaii archipelago have large volumes. Mauna Kea, for instance, is approximately 3,200km3, and this massive volume leads to a large weight pressing down on the oceanic lithosphere. This loading is ‘compensated’ by the deformation of the elastic lithosphere, leading to flexure. It is this flexure which produces a moat around the islands, with raised ridges on the extremities. This can be observed in the following satellite image.

Effect of Loading on Bathymetry. Directly around the load there is a moat, surrounded by an elevated area. On a long wavelength this is elevated above the typical oceanic crust due to the hotspot.
Bathymetry around part of Hawaii. (School of Ocean and Earth Science and Technology, Hawaii -
Bathymetry around part of Hawaii. (School of Ocean and Earth Science and Technology, Hawaii – Shows depression immediately around island (dark blue)

The manner in which these islands have formed has had a large impact on the range of wildlife which can exist there. Hawaii (and the other Polynesian islands) are typically many hundreds if not thousands of miles away from neighbouring landmasses, which combined with the short period of time they’ve existed there for has limited the range of organisms which have been able to reach and establish themselves there. This leads to a unique ecology present on the island, which is primarily dominated by flying and marine animals.

Monk Seal Mum and Baby, showing folds of skin (NOAA, found on Marine Conservation Institute Website)

A number of marine animals live in the seas around the Hawaiian islands – including, for example, Humpback Whales, Spinner Dolphins, and the northern relative of the Elephant Seals found around Patagonia – the Northern Elephant Seals. Perhaps one of the most interesting of these marine animals are the Hawaiian Monk Seals, as it is one of only two mammals found natively in Hawaii. These are dark gray seals, named Monk Seals after the folded skin covering their heads. These seals feed on lobsters, eels, and fish which live on coral reefs around the atolls and islands, and can grow to 170-230kg – they are sexually dimorphic, such that females are typically larger than males.

The life habits of these seals mean that they are very susceptible to disturbance by human activity. Typically they can only be found on remote islands which have very little human impact, as fisheries and other predators compete with them for food. The impacts of climate change on the coral reefs from which they obtain most of their food are likely to also impact these seals.

Hawaiian Hoary Bat. (C. Pinzari, found on USGS website)

The other native mammal found on Hawaii is the Hawaiian Hoary Bat. This is a small bat with a wingspan of around 30cm which weighs about 14-18g. They have brown-gray coats, and white tinged ears. Hawaiian Hoary Bats typically live from sea level to ~2200m, however they have been observed higher than this. It is likely that they roost amount trees near forests on various Hawaiian Islands.

These bats are one of many flying animals in Hawaii, others of which include the Laysan Albatross (a large white and black seabird which feeds far from its breeding colonies) and the Hawaiian Noddy (a medium sized abundant tern).

Since humans arrived on these islands they have brought with them a range of species from outside, such as the Axis Deer (native to the Indian subcontinent, which has been introduced to the island of Lanai) and the Feral Wallaby (originally having escaped from a zoo). Humans have impacted the islands in other ways too – large amounts of the coral present have undergone bleaching, as a result of increased ocean acidification and waste runoff.

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