There's a variety of volcanic activity and resultant landforms and landscapes.
Geochemistry provides some evidence for hot spots...
1. Basalts from hotspots are different from MOR basalt or Island Arcs = magma origin different source. 2. Oceanic crust appears to be subducted down to the core-mantle boundary; a major component of the plume material.
Seismic topography provides some evidence for hot spots...
1. P and S waves slow down through hotter, denser material. 2. 32 regions where P waves slow down are thought to be magma plumes. 3. S wave velocities indicated that the plumes extend to the Gutenberg discontinuity.
Volcano shape and the eruption volatility is largely determined by magma viscosity. 3 factors affect magma viscosity...
1. Temperature - higher temperature, lower density of magma and the easier it'll flow. 2. Dissolved gases - more dissolved gases, more fluid the magma. 3. Chemistry - higher silica content, more viscous magma.
Contrast the cooling times of Acidic and Basaltic lava.
> Cools faster and so doesn't travel far. > Takes a while to cool and solidify, flowing greater distance.
Contrast the ejections of Acidic and Basaltic lava.
> Explosive due to high pressure caused by subduction. Lava bombs, ash and dust > Effusive Lava and Steam
Contrast the eruption frequency of Acidic and Basaltic lava.
> Long dormant periods > Regular, sometimes continuous
Contrast the chemistry of Acidic and Basaltic lava.
> Rhyolite (more acidic); Andesite (less acidic) Silica Rich (66%+) - Low level of ferromagnesian minerals > Basalt Low Silica (44 - 52%) - Mostly Iron and Manganese.
Contrast the landforms created by Acidic and Basaltic lava.
> Steep-sided Strato-volcanoes; Caldera Island arcs > Gently sloping sides, Shield Volcanoes Lava plateau when eruption from multiple fissures
Contrast the viscosity and temperatures of Acidic and Basaltic lava.
> Very viscous magma - gas bubbles struggle to expand, resulting in pressure build up. 600 - 1000 C > Runny - due to the gas bubbles being free to expand as the magma rises. 1000 - 1200 C
Give examples of where Acidic and Basaltic lava is found.
> West Coast South America Mt. Saint Helen's > Hot Spots (Hawaii) Iceland
75% of all volcanic material is erupted at...
constructive margins, 80% of the world's volcanoes occur at destructive margins (considered the most hazardous).
Contrast where Acidic and Basaltic lava is found.
Acidic > Convergent (destructive) plate boundaries Basaltic > Mid-Ocean Ridges (Iceland) Divergent plate boundaries
On May 4th 2018, fresh eruptions sending fountains of lava 30m (100ft) out of the ground have taken place on...
Hawaii's Big Island, destroying several homes. The Civil Defence Agency said there were fissures on 3 streets and told residents to evacuate. It said there were deadly levels of SO2 gas and emergency crews wouldn't be able to help anyone affected. The new activity comes a day after Kilauea volcano erupted.
What are lava lakes?
Lava lakes are rare occurrences that form when lava collects in craters. They're signs that volcanoes are active but not erupting. There's enough lava present to fill a crater floor, but not enough to create a lava flow from the crater.
Where is most magma produced?
Plate margins produce the most magma and so this is where most volcanoes occur.
Give an example of a famous lava tube.
The most famous one of which is 'Thurston Lava Tube', formed 500 - 600 years ago on Hawaii.
Landscape change over time - In Hawaii's case, as the atolls and seamounts are moved N.W. by the Pacific Plate, they move out of the warm waters needed for...
a reefs survival, towards the Kuril-Kamchatka Trench, a destructive subduction zone off the coast of the Kamchatka Peninsula in east Russia. So, the atolls die and subside, forming flat-topped seamounts called guyots. The Pacific Plate is eventually subducted and destroyed at this trench along with the supported seamounts and guyouts.
What is the Gutenberg discontinuity?
boundary dividing outer core and mantle, 2,900 km down
Volcanoes at constructive margins, e.g. along the Mid-Atlantic Ridge, are both central vent and fissure. They erupt frequently but not often violently and are associated with ash and lava flows. At destructive plate margins, the plates are...
colliding. As one plate is subducted, intense pressures and heat cause melting of the rocks and sediments, potentially forming an acidic magma chamber. Very viscous and resistant to flow, there's often a huge build-up of pressure, resulting in very violent and dangerous eruptions involving ash and pyroclastics.
What is a magma plume?
column of extra-hot magma, consisting 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 a partial melting which arises from a drop-in pressure (decompression melting).
The lava on Hawaii flows frequently and freely. It's not a major hazard but can be locally...
destructive. The Kilauea eruption which lasted from 1983 - 2001 destroyed 13km of highway and around 180 buildings, including Kalapana village.
Magma plumes originate at the Gutenberg discontinuity. Heat from the core is conducted to the mantle and heated portions of the lower mantle becomes less dense and buoyant and rise as...
diapirs, which'll become the mantle plumes. Build-up of magma below the ocean surface will break through structurally weak areas and cause up doming, creating a basalt floods.
What are Fire Fountains?
emissions of incandescent lava which can reach heights of 10s to 100s of ft. Hawaiian eruptions tend to start with these, which may begin from a series of fissures, creating a 'curtain of fire'. This line of fountaining occurs roughly continuously for hours and then becomes focused on one vent.
What is a hot spot?
fixed area on the crust above a magma plume.
The Hawaiian Islands experience effusive activity in which eruptions are characterised by the outpouring of lava flows. Lava lakes and fire...
fountains may also be present. Explosive eruptions are rare. Fissure eruptions, where lava erupts from several points or vents simultaneously are evident in Hawaii.
Hot spots experience seismic and volcanic activity. Volcanic earthquakes are...
generated by magma movement and tectonic earthquakes occur because of structural weakness at the base of the volcanoes or in the crust below.
Upwelling plumes appear to form above "thermochemical piles" of...
hot, dense material at the Gutenberg-discontinuity (mantle-core boundary).
Some parts of the East African Rift System are...
hundreds of metres deep and thousands of metres wide.
Landscape change over time - It's harder to recognise older islands as mass collapse of the...
island's flanks and subaerial denudation of the original shield volcanoes occurs. Once shifted from the hot spot by plate movement, it becomes inactive and fringing reefs can develop at the fringes of the island, such as Oahu.
The East African Rift System is a series of rift valleys stretching from Mozambique to the Red Sea - about 4000 km. Over the past 30 million years, the crust has been stretched, creating tension in the local rocks. The resultant rifting, with magma forcing...
its way to the surface and creating a line of active volcanoes. It formed because the Nubian and Somalian plates are diverging.
Hawaiian lava flows typically advance a few miles an hour, but most move faster when encased by a...
lava tube. Prolonged flows of basaltic lava result in hard crust being formed on the surface of the lava flow. Creating a thermal insulating tunnel, through which the lava can flow and eventually drain off. Some tubes are many miles long and often tall and wide enough to easily drive a car through.
There's two extreme types of magma, Basaltic and Acidic. The more viscous the...
magma, the greater the potential for explosive eruptions. Viscosity is reduced by higher temperatures. Higher silica content makes magma more viscous.
In episode of rupturing, lava from curtains of fire is rapidly cooled when in contact with the air, forming dark clouds of cinders. The cooled material is fine and...
may be carried many miles from the vent. If the lava falls to the ground it may merge with other drops of fallen lava to form lava flows. If the flow reaches the sea, they can create steam full of HCl acid called Laze, as the hot lava meets the cool seawater.
The Hawaiian island chain lies at the centre of the Pacific plate, thousands of km from the nearest plate boundary. Its formation is due to the existence of a hot spot. As the Pacific plate has slowly moved...
northwest over the Hawaiian hot spot (10cm a year), vast amounts of basalt have accumulated on the ocean floor to produce the Hawaiian Islands.
Volcanoes are found around rift valleys. E.g. Mt. Kilimanjaro and Mt. Kenya (highest mountains in Africa) are volcanoes in the East African Rift System. Not all hot spot volcanism leads to...
shield volcanoes, El Teide on Tenerife in the Canary Islands is a strato-volcano which has erupted on several occasions since 1700 (last in 1909).
Landscape change over time - Some 30km to the S.E. of Hawaii, the next volcano in the chain, Loihi, is...
slowly rising from the ocean floor. Currently, this seamount's summit is 970m below sea level. Its position directly above the hot spot means that it'll continue to grow and eventually become an island.
Landscape change over time - As the volcanoes subside, reefs can grow upwards to keep up with this rate of...
subduction. The encircling reef is maintained, but now separated from the partially sunken island by a lagoon. Gradually, the island will sink below the ocean leaving a coral atoll, e.g. the Lighthouse Atoll in Belize.
Landscape change over time - The crust is buoyed up by the mantle plume's heat when situated over...
the hot spot, but as it moves away from the plume, it cools and subsides, and thus so do the islands located on top of it.
The active volcanic peaks of Mauna Loa and Mauna Kea on the Big Island reach over 4000m above sea level and rise more than 9000m from the ocean floor. Over millions of years, as the Pacific plate moves northwest away from...
the hot spot, the volcanoes lose their magma source and become extinct. So, on older Hawaiian Islands (e.g. Kauai), volcanism isn't active. There, weathering and erosion have broken down the volcanic rocks into deep, fertile soils and valleys carved by rivers.
What is magma (typically basaltic) produced by?
the partial melting of the mantle at divergent plate boundaries.
The volcanoes from diapirs eventually rupture the ocean water surface to form islands. The hot spot stays in...
the same place but the plate above moves, creating an island chain (oldest island furthest from the hot spot).
The Hawaiian Islands will subside (partly due to their own weight and subsidence of the oceanic plate) to form seamounts, atolls and guyots over time. The nature of Hawaii's volcanic activity determines the morphology of...
the shield volcanoes which create the islands and gives rise to the landforms, e.g. lava tubes, black sand beaches and also to the variety of tephra which can be found scattered on the volcano slopes.
Volcanic eruptions produce many landforms. This diversity relates to the eruptions type and location, the lava and variety of materials such as ash, pumice and gases produced. Once magma is ejected at...
the surface, its behaviour is affected by its viscosity, which is determined by its chemical composition and temperature. Eruptions are divided into two groups: explosive and effusive. Volcanoes are classified according to the type of eruption.
There are alternative hot spot theories - The idea of upwelling mantle plumes is commonly referred to as the cause of hot spots, the mechanism by which hot spots are created are still...
under debate. It has been suggested that hot spots may be created due to a thinning of the lithosphere when it experiences local extension.
There are alternative hot spot theories... It has been put forward that due to episodes of oceanic plate subduction in the past, there may be higher concentrations of...
water in some parts of the mantle than others and that hot spots may exist due to the presence of this water, which drives the hot spot activity by a process called hydration melting.
