Hi everyone! :)

1) I was told that shield volcanoes (like these in Hawaii) are the safest ones, is it because the lava, or simatic minerals, erupted from these volcanoes flows very slowly?

2) In location of hot spots, "plumes MELT the lithosphere allowing the lava to flow onto the surface without a violent eruption" (quote from a text book). What I think is that magma must melt the lithosphere before it erupts, whether a violent eruption or a gentle one, or else the magma can't get onto the surface, true? That's why I don't understand what the quoting is referring, why by melting the lithosphere above a hot spot, the eruption is less violent, as the quote states?

Another question I would like to ask is, why are the islands (e.g. Hawaii and its neighboring islands) formed by a hot spot separated by some distance? It should form a continuous island instead of individual islands if magma is constantly coming out...

1) It's safe to say that you can live within a close vincinity (within tens of miles) from the center of a shield volcano.

However, it's misleading to say that the lava flows slowly. They can flow faster than you can run, especially when they're concentrated in small channels at steep angles. In comparison to other lavas, this is also wrong: basaltic lava has the least viscosity, as measured by the content of silica, compared with the andesitic and granitic lavas; the latter two often don't flow at all since they clump up and are so chunky from their viscosity.

2) This is only true in the case of oceanic plumes such as Hawaii, Reunion, etc. However, you can have calderas such as Yellowstone (discussed many times here) in the middle of continents situated on hot spots as well, and those are extremely explosive.

Your last question is a good think tank. I don't really know the answer, but I'll still take a stab.
I think the spaces between the Hawaiian islands suggests that the hot spot activity is either one of two things: 1.cyclic in intensity or 2. Regional in pressure build-up. The first is pretty self-explanatory. The second one means that the hot spot probably affects a somewhat broad area that vent out only thru Hawaii (the island). When the Pacific plate moves NWW, the region can still vent the pressure fairly well if the island is still in the region of the "spot." When the pressure mounts too much after the island moves off the spot, it creates another island. But certainly it can create multiple pressure vents at the same time such as Loihi. And this is true for the Galapagos islands as well.

1) It's safe to say that you can live within a close vincinity (within tens of miles) from the center of a shield volcano.

However, it's misleading to say that the lava flows slowly. They can flow faster than you can run, especially when they're concentrated in small channels at steep angles. In comparison to other lavas, this is also wrong: basaltic lava has the least viscosity, as measured by the content of silica, compared with the andesitic and granitic lavas; the latter two often don't flow at all since they clump up and are so chunky from their viscosity.

2) This is only true in the case of oceanic plumes such as Hawaii, Reunion, etc. However, you can have calderas such as Yellowstone (discussed many times here) in the middle of continents situated on hot spots as well, and those are extremely explosive.

Your last question is a good think tank. I don't really know the answer, but I'll still take a stab.
I think the spaces between the Hawaiian islands suggests that the hot spot activity is either one of two things: 1.cyclic in intensity or 2. Regional in pressure build-up. The first is pretty self-explanatory. The second one means that the hot spot probably affects a somewhat broad area that vent out only thru Hawaii (the island). When the Pacific plate moves NWW, the region can still vent the pressure fairly well if the island is still in the region of the "spot." When the pressure mounts too much after the island moves off the spot, it creates another island. But certainly it can create multiple pressure vents at the same time such as Loihi. And this is true for the Galapagos islands as well.

I believe there is another shield volcanic island forming right now about 30 miles south of the big Island and should start poking thru the surface of the ocean maybe 5,000 years or so from now. I forget the name of it. It sounds like there will be some new Hawaiian beach front property available south of the big Island maybe in a few thousand years from now.

May_wentee :cool:

1) So shield volcanoes generally produce less explosive eruptions! :)

2) So oceanic hot spots comes up, slowly melting the lithosphere, produces less violent eruptions, while continental hot spots and other volcanic mountains violently break the lithosphere which produces explosive eruptions! :)

Hot spots are "pockets of magma" that rises far from a plate boundary! Does this mean that usually there is no magma coming out over a hot spot, but it comes out suddenly in huge amounts (pockets of magma), which over time created separate Hawaiian Islands?

1) It's safe to say that you can live within a close vincinity (within tens of miles) from the center of a shield volcano.

However, it's misleading to say that the lava flows slowly. They can flow faster than you can run, especially when they're concentrated in small channels at steep angles. In comparison to other lavas, this is also wrong: basaltic lava has the least viscosity, as measured by the content of silica, compared with the andesitic and granitic lavas; the latter two often don't flow at all since they clump up and are so chunky from their viscosity.

2) This is only true in the case of oceanic plumes such as Hawaii, Reunion, etc. However, you can have calderas such as Yellowstone (discussed many times here) in the middle of continents situated on hot spots as well, and those are extremely explosive.

Your last question is a good think tank. I don't really know the answer, but I'll still take a stab.
I think the spaces between the Hawaiian islands suggests that the hot spot activity is either one of two things: 1.cyclic in intensity or 2. Regional in pressure build-up. The first is pretty self-explanatory. The second one means that the hot spot probably affects a somewhat broad area that vent out only thru Hawaii (the island). When the Pacific plate moves NWW, the region can still vent the pressure fairly well if the island is still in the region of the "spot." When the pressure mounts too much after the island moves off the spot, it creates another island. But certainly it can create multiple pressure vents at the same time such as Loihi. And this is true for the Galapagos islands as well.

I think that's the name... and they say it won't poke out for at least a few million years.

Does this mean that usually there is no magma coming out over a hot spot, but it comes out suddenly in huge amounts (pockets of magma), which over time created separate Hawaiian Islands?

Honestly, I don't really know. Does anyone else want to contribute? I would like to hear how.

I think that's the name... and they say it won't poke out for at least a few million years.

You're close to correct. I was off a little....I looked it up and they (the scientists) say that Loihi rises at approx. 6,000 feet above the ocean floor and at it's present rate of ascension, will poke it's head above the water near the big island in about 1 million years. So it's never to early to start planning to buy some new Hawaiian beach front property. See your local Hawaiian real estate agent today!

http://www.scarborough.k12.me.us/wis/teachers/dtewhey/webquest/nature/hawaiian_islands.htm

1) What actually make shield volcanoes relatively safe, compared to the 2 other types of volcanoes? Is it simply because shield volcanoes produce less violent eruptions?

Besides, are simatic minerals and lava meaning the same thing? (does anyone know?)

2) Ocean hot spots allow magma to melt the lithosphere. For continental hot spot volcanoes and volcanoes along Ring of Fire (like Cascade Range), does magma usually break the lithosphere to erupt instead of melting it, creating an explosive eruption?

I actually don't know what simatic minerals are either.

But a quick Yahoo! search reveals something on the order of 'igneous rock with ferromagnesian mineral composition.' Which would render the synonymity to lava false, since lava is just a generic term describing liquid rock on the surface.

2) I don't really think the Cascades count as hot spots, since hot spots rise directly from some sort of mantle plume. The Cascades are associated with Juan de Fuca subduction. However both cases are still pretty similar: most of the time they just rise in the mantle and melt the crust progressively upwards in pockets of magma, creating plutons or volcanic chambers. But I'm not really sure on that either; correct me if I'm wrong.

You're close to correct. I was off a little....I looked it up and they (the scientists) say that Loihi rises at approx. 6,000 feet above the ocean floor and at it's present rate of ascension, will poke it's head above the water near the big island in about 1 million years. So it's never to early to start planning to buy some new Hawaiian beach front property. See your local Hawaiian real estate agent today!

http://www.scarborough.k12.me.us/wis/teachers/dtewhey/webquest/nature/hawaiian_islands.htm

Lol. You can already buy some pretty damn cheap land smack dab near the volcano today at extremely low prices - because it's almost continuously overrun by lava ROFL

The Wikipedia gives an explanation of the stages of the volcanic buildups in the Hawaiian fissure or ridge, but there seems to be a debate that has been going on now for about thirty years as to whether this socalled "hot spot" is due to thermal convection currents and circulation within the mantle below the lithosphere, or whether it is due to a direct upwelling of a mantle plume originating at the mantle-core boundary. To make matters even worse, to arrive at a more accurate explanation, nowadays the word "hotspot" and "mantle plume" are being used interchangeably. In any case, this hotspot is quite large and remains stationary below the lithosphere while the Pacific plate and the above volcanoes move west northwest above it.

"The assembly line that forms the volcanoes is driven by a "hot spot," or plume of hot material, deep within the Earth that partially melts to produce magma as it rises beneath the Pacific Plate. As the plate moves west-northwest, each volcano moves with it from its place of origin above the hot spot. The age and orientation of the volcanic chain records the direction and rate of movement of the Pacific Plate. The pronounced 43-million-year-old bend between the Hawaiian Ridge and the Emperor Seamount Chain marks a dramatic change in plate direction."
http://hvo.wr.usgs.gov/volcanowatch/1995/95_09_08.html

Among other factors, it is this pronounced bend leading to two seperate fissure zones that is the most challenging aspect to finding a precise and accurate working hypothesis for the formation of the volcanoes of Hawaii:

"The Hawaiian chain apparently consists of two strands of volcanoes located along distinct but parallel curving pathways. Multiple volcanoes line up to form each strand....The alignment of the Hawaiian Islands reflected localized volcanic activity along segments of a major fissure zone on the ocean floor. This “great fissure” origin for the islands served as a working hypothesis for subsequent studies until the mid-20th century....Then it was pointed out that the time-progressive volcanism along the Hawaiian chain could be explained by the lithosphere moving across a stationary hot spot in the mantle. This led to the theory of deep mantle plumes when Morgan [1971; 1972] proposed that a) this hot spot was continually supplied by a plume from the deep mantle, and b) there are approximately 20 such plumes in the mantle. Fixity relative to one-another, time-progressive volcanic tracks, and a high rate of volcanism were considered to be the primary characteristics of volcanic regions fuelled by deep-mantle plumes.

The Hawaiian - Emperor system appears superficially to fit the fixed deep mantle plume hypothesis well....The most compelling and widely quoted evidence today is still geometric considerations such as fixity, perceived parallelism with other volcanic chains and the regular time progression of volcanism, along with the high melt productivity. Lnear time-progression of volcanism and high magmatic productivity can be explained by other mechanisms such as propagating cracks and high mantle fertility. Nevertheless, for about 20 years there has been no serious challenge to the fluid-dynamic, deep thermal mantle plume hypothesis for the origin of the Emperor and Hawaiian islands and seamounts. There are, however, a substantial number of aspects of the chains that are not predicted by the plume hypothesis and fit it poorly. These must give clues to alternative genesis models.....

Aspects of the Emperor and Hawaiian chains unpredicted by the plume hypothesis:
1) The “bend” did not result from a change in direction of Pacific plate motion....In addition to the change in trend, the locus of volcanism moved south by ~ 800 km relative to the geomagnetic and biofacies reference frames while the Emperor Seamount chain formed.
2) The Emperor chain began near a ridge.
3) There is no Emperor/Hawaiian “plume head."
4) The magmatic rate is highly variable.
5) There is no heatflow anomaly: Lithosphere underlain by a thermal plume is expected to be thinner and to have higher heat flow than the average for lithosphere of the same age elsewhere. Heatflow across the Hawaiian swell, however, shows no significant anomaly.
6) Mantle temperature is elevated by up to ~ 200°C: Petrology can be used to infer the temperature where surface-erupted melts were last in equilibrium with the mantle source, and Tp, the mantle potential temperature. The results for Hawaii are variable, and whether or not a high temperature anomaly is inferred depends on what “average” mantle temperature is considered to be. “Average” mantle temperature is mostly studied at mid-ocean ridges (MORs), since this is where magma of mantle origin is mostly erupted. The temperatures required for plumes to rise are debated. Peridotic plumes from the core-mantle boundary would require temperature anomalies of the order of 600 K.... [However] Combinations of geophysical and geochemical arguments (e.g., “garnet signature” in rare-Earth elements, olivine-liquid geothermometers have been used to infer temperatures beneath Hawaii of [only] 150 - 200 K higher than MORs.
7) The melt originates from the shallow asthenosphere [upper ~200km region of the 2,900 km mantle].
8) The basalt geochemistry [of the Hawaiian volcanoes] does not require a mantle plume, or a deep or hot source.
9) Seismology has not detected a plume.

Any satisfactory theory for Hawaiian volcanism must explain (or rationalize) the:
- change in migration direction of the melting locus at the bend,
association of the great bend with the Mendocino fracture zone,
- change in migration rate at the bend,
- apparent commencement of the volcanic chain near a ridge,
- absence of a “plume head”,
- large variations in magmatic production, and a current magmatic rate about 3 times greater than the next most productive hotspots,
- absence of a significant heat flow anomaly,
- absence of lithospheric thinning,
- absence of a strong high-temperature signal in the erupted basalts,
- production of very large volumes of magma even though the depth to the top of the melting column is exceptionally large compared with MORs,
- spatial and temporal variation in the composition of erupted lavas on a variety of scales, remote location of Hawaii, near the center of a very large plate,
- seismic whole-mantle mantle structure that is apparently normal compared with the Pacific ocean elsewhere,
- occurrence of a bathymetric swell (a moat and “arch”) along the eastern two-thirds of the Hawaiian chain and wrapping around its southeastern end, with alkalic basaltic volcanism occurring at some places along it."

"The Emperor and Hawaiian Volcanic Chains: How well do they fit the plume hypothesis?," by G. R. Foulger1 & Don L. Anderson.
http://www.mantleplumes.org/Hawaii.html

Hi everyone! :)

1) I was told that shield volcanoes (like these in Hawaii) are the safest ones, is it because the lava, or simatic minerals, erupted from these volcanoes flows very slowly?

The magmas from Hawaii have some of the lowest silica contents in the world, and they can flow up to 30 miles per hour on a good slope--which will outrun a fast horse. I wouldn't suggest that you stand and watch the lava coming at you.

2) In location of hot spots, "plumes MELT the lithosphere allowing the lava to flow onto the surface without a violent eruption" (quote from a text book). What I think is that magma must melt the lithosphere before it erupts, whether a violent eruption or a gentle one, or else the magma can't get onto the surface, true? That's why I don't understand what the quoting is referring, why by melting the lithosphere above a hot spot, the eruption is less violent, as the quote states?

Basalt is very low in silica, therefore doesn't hold onto its gas content, which escapes easily. Therefore the eruption is a lot less violent.

Another question I would like to ask is, why are the islands (e.g. Hawaii and its neighboring islands) formed by a hot spot separated by some distance? It should form a continuous island instead of individual islands if magma is constantly coming out...

Yes, the magma is there all the time, but it has to find weaknesses in the rock of the oceanic crust in order to burst through and start forming an island.

Read my post above about the formation about the Hawaiian Island volcanoes, but the physical forces on Earth and the timing between eruptions will not allow you to have just one super-valconae that would result in just one Hawaiian Island. They are all a series of volcanoes that took place over a long stretched-out time period: thus seperate islands.

Yes, in relation to your posting - depending how fast you can run? - shield volcanoes in that aspect are amongst the safest because of the slow moving low-silica content of the lava.

The lithosphere in the ocean is much thinner than the lithosphere on continents; therefore, I would think it would be more prone to explosive eruptions in the ocean. I do not know. Could you tell me where you got your source that ""plumes MELT the lithosphere allowing the lava to flow onto the surface without a violent eruption"? Thanks.

Honestly, I don't really know. Does anyone else want to contribute? I would like to hear how.

There are ordinary plumes of magma, rising from the mantle to break through weak spots in the crust and form volcanoes; then there are superplumes, which create areas like the Siberean or Deccan Traps, and can produce thousands of cubic miles of new landscape.

1) So shield volcanoes generally produce less explosive eruptions! Hot spots are "pockets of magma" that rises far from a plate boundary! Does this mean that usually there is no magma coming out over a hot spot, but it comes out suddenly in huge amounts (pockets of magma), which over time created separate Hawaiian Islands?
Yes, shield volcanoes generally produce less explosive erosions.

A "hot spot" is magma, but there is a trend in modern literature that seems to imply that hot spots are the same as magma plumes that arise directly from the Earth's mantle/core boundary. They are not - but could be! There is a difference in convection currents between the upper and lower mantle and the lithosphere. There is ongoing research in this area but because there is a transition zone between the upper and lower mantle, it is thought that there are more than one circulatory regions. In other words, not many - if any? - magma plumes arise directing from the mantle/core boundary. So a hot spot can just be a result of plate tectonic pressure, heating, and compaction within a given area of that plate, without any continuous uprising force from directly below the lithosphere. This is why you can have shallow but wide dome mountains without any erruptions or folding. They are just a slight upsurge from that magmatic hot spot below.

1) So shield volcanoes generally produce less explosive eruptions! Hot spots are "pockets of magma" that rises far from a plate boundary! Does this mean that usually there is no magma coming out over a hot spot, but it comes out suddenly in huge amounts (pockets of magma), which over time created separate Hawaiian Islands?
Yes, shield volcanoes generally produce less explosive explosions.

A "hot spot" is magma, but there is a trend in modern literature that seems to imply that hot spots are the same as magma plumes that arise directly from the Earth's mantle/core boundary. They are not - but could be! There is a difference in convection currents between the upper and lower mantle and the lithosphere. There is ongoing research in this area but because there is a transition zone between the upper and lower mantle, it is thought that there are more than one circulatory regions. In other words, not many - if any? - magma plumes arise directing from the mantle/core boundary. So a hot spot can just be a result of plate tectonic pressure, heating, and compaction within a given area of that plate, without any continuous uprising force from directly below the lithosphere. This is why you can have shallow but wide dome mountains without any erruptions or folding. They are just a slight upsurge from that magmatic hot spot below.