Hotspots and Hawaii

What percent of volcanoes occur at hotspots?

5%

What are the eruptions like in Hawaii?

Eruptions are characterised by the outpouring of lava. Sometimes lava lakes and lava fountains may also be present. Eruptions are rarely explosive and fissure eruptions are evident in Hawaii (where lava erupts from several points or vents simultaneously). The lava on Hawaii flows frequently and freely. It isn't considered a major hazard but it may be locally destructive. The lava on Hawaii is basaltic and flows can fall under two categories: -Pahoehoe: 'flat swirly swirly' (lava flows with smooth, rope like surfaces) -A,a: flows with jagged, rough edges

What kind of activity occurs at hotspots (volcanic, seismic?)

Hotspots experience seismic as well as volcanic activity. Volcanic earthquakes are generated by magma movement and tectonic earthquakes occur because of structural weaknesses within the crust below.

Which volcanic hazards are present in Hawaii?

Lava lakes (rare occurrences that for, when lava collects in craters). They are signs that the volcano is active but not erupting. There is enough lava to fill the crater floor but not enough to create a lava flow which extends from the crater. Lava fountains- emissions of incandescent lava which can reach heights of tens to hundreds of feet. During fire fountain episodes, the lava is rapidly cooled when in contact with the air, forming dark clouds of cinders. The cooled material is often very fine and can be carried many miles away from the vent. Vog- Eruptions can give off tonnes of sulfur dioxide and when this reacts with moisture in the air it forms sulphuric acid droplets known as vog, which can cause respiratory problems and damage vegetation. This can travel over 100 miles depending on wind patterns. Laze (lava haze)- formed when lava flows reach the sea (as they often do in Hawaii) and create steam full of hydrochloric acid as the hot lava meets the cool sea water.

What are magma plumes?

Long-lived areas of high levels of heat flow within the mantle. Plumes consist of an upwelling long, thin conduit and a bulbous head which spreads at the base of the lithosphere and produces huge volumes of magma, owing to partial melting which arises from a drop in pressure (decompression melting). It is believed that heat from the core is passed to the mantle by conduction and heated portions of the lower mantle become less dense and more buoyant and rise as diapirs (which will become the mantle plumes).

What is an example of an oceanic hotspot?

The Hawaiian Islands were formed entirely of volcanic origin in the Pacific Ocean more than 3,200km from the nearest plate boundary. (but there are continental hotspots too!).

How is a chain of islands created?

The hotspot stays in the same place but because the plate above it moves over time this creates a chain of islands (if moving quickly or a cluster if rotating slowly around the hotspot- the Canary Islands), with the oldest being the furthest away from the hotspot and the youngest, active volcanic islands located above the hotspot. E.g. The 'Big Island' of Hawaii is the only volcanically active island in the chain.

What happens if a hotspot lies beneath a continental plate?

The huge volumes of magma will accumulate layer on layer and the flood basalts may cover thousands of square kilometres. An example is yellowstone.

What happens to the older islands in an island chain?

The older islands become part of the plate and are moved away from the heat source. Some islands will be eroded by waves and form flat-topped sea mounts called guyots. The older, extinct Hawaiian islands extend to the north-west of the main island as the plate moves in this direction at 10cm/year.

Where is the origin of magma plumes?

The origin of these plumes is thought to be at the Gutenberg Discontinuity. This is the boundary that divides the outer core and the mantle at a depth of 2,900km.

What are the alternative hotspot theories?

The upwelling of mantle plumes are commonly referred to as the cause of hotspots but the mechanisms by which hotspots are created are still under debate. -It has been suggested that hotspots may be created due to a thinning of the lithosphere when it experiences local extension (tensional forces). -It has also been put forward that due to episodes of oceanic subduction in the past, there may be higher concentrations of water in some parts of the mantle than others and that hotspots may exist due to the presence of this water, which drives the hotspot activity by a process called HYDRATION MELTING.

What are the characteristics of the Hawaiian island chain?

There are 8 main islands in the Hawaiian island chain. The hotspot is believed to be around 200 miles across, with narrow channels feeding the different volcanoes. The hotspot has existed for at least 70m years. The Hawaii-Emperor seamount chain veers North-westwards away from the 'Big Island' of Hawaii (6000km long). A sharp bend in the chain appears approximately 2,200 miles north-west of Hawaii, which shows that there was a change in plate direction around 43-45 million years ago. The Hawaiian islands are built from a series of enormous shield volcanoes, which is typical of a hotspot location. Basalt lava is in great supply at hotspots. Basalt lava is low in silica which means it has low viscosity, flows relatively easily and can spread over significant distances. Successive lava flows form gentle, effusive eruptions and build up over time to form a shield volcano, which has a relatively wide base and gentle slopes, usually with a gradient less than 10 degrees (the Hawaiian volcanoes typically have slopes of 2-3 degrees).

How are vocanic hotpsots created?

When 'plumes' of magma rise through the Earth's mantle. This build up of magma beneath the surface will eventually create volcanic activity on the ocean floor as it breaks through the structurally weak areas.