While arc volcanoes tend to get most of the public recognition due to their explosive behavior and the intermittent human catastrophes that result, they are a relatively minor component of the Earth's overall volcanic budget. Rifts, or spreading regions, are zones of massive volcanism, much of which occurs with little fanfare because the eruptions are under the oceans. Indeed, the entire, global-encircling chain of mountains along the mid-ocean ridge system is a vast chain of volcanoes, built up by the up-welling molten material that fills in the gap in the spreading oceanic crust and lithosphere. The present day rift system extends the entire length of the Atlantic, through the Indian Sea, where it bifurcates into a chain extending up into the Red Sea and then southward into the East African rift system (a continental rift that connects up to spreading oceanic ridges) and a separate chain that connects over into the East Pacific rise system.
Thousands of kilometers long, the mid-ocean ridge volcanic system accounts for 90% of annual volcanic activity. Of all subaerial volcanoes, only 15% are rift volcanoes, such as Mount Kilamanjaro, in the East African rift, or the volcanoes of Iceland, but these do contribute much to continental volcanism. However, the great volume of rift volcanism is in the Mid-Ocean ridge system, with 2.5 square kilometers of new seafloor area being produced each year, and a corresponding 12 cubic kilometers of magma injected into the spreading rifts. The average separation rate (two-way spreading from the ridge) of Earth's ocean rifts is about 4-5 cm/yr, so you can imagine a fissure that wide opening each year along the zipper-like extent of the entire mid-ocean ridge system, filling with hot magma that cools to form new oceanic crust. Some ridge systems spread faster, with the Pacific rise having a rate of 9 cm/yr, while others spread slower, with the mid-Atlantic having a separation rate of about 2-3 cm/yr. This process of forming new ocean floor has produced over 60% of the Earth's surface, and completely repaves this region every 200 million years or so.
On the basis of a few direct observations of submarine eruptions, and many more detections of eruptions from the underwater hydroacoustic system operated by the U.S. Navy to monitor submarine and ship activity, there are an estimated 20 eruptions each year on the mid-ocean ridge system. These eruptions tend not to be explosive, and involve "effusive" eruptions of runny magma. In part this is the result of eruption under a thick layer of water, some 3 km deep, the pressure of which keeps gasses and fluids in solution with the molten rock rather than letting the gas separate out and build-up pressure.
With so many submarine eruptions, it is no surprise that much of ocean water chemistry is the result of interactions of erupted products with the overlying water. Gases and fluids brought to the surface have in fact produced the atmosphere and the oceans, and continue to contribute to the fluid envelopes of the Earth.
A cross-section through a mid-ocean ridge system would show a complex layered structure in the upper few kilometers under the water layer. There is commonly a shallow magma chamber, perhaps a few kilometers in diameter down 2-5 kilometers under the center of the ridge. Above this chamber there is a sequence of vertical tabular dikes, or intrusions of molten rock along vertically oriented cracks. The sheeted dike sequence is overlain by pillow lavas, a layer of bumpy extruded rocks that cool quickly in contact with the ocean water and fill up from below with magma prior to cooling or bursting to make pillow upon pillow. Below the magma chamber there are horizontal layers of rock which result from settling out of refractory crystals in the magma chamber.
So, why is there magma in this region? What has caused the localized conditions enabling rock to melt? The general explanation is that due to the rifting process, the hot interior of the Earth can ascend more rapidly than would normally be the case. As the hot material rises, it encounters decreasing pressures. Material can melt if it rises fast enough by a process called decompression melting. The basic idea is that as pressure increases on a substance it has to be heated to larger and larger temperatures in order to melt. Thus, the melting curve of rocks increases with depth in the Earth, even for uniform rock composition. We also know that the temperature increases with depth, although for most of the mantle the temperature does not increase to higher than the melting temperature. But if we take very hot rock, which under high pressure would still be solid (sub-solidus), and we abruptly lower the pressure (bring it to shallow depth) without letting the rock cool off, the decrease in melting temperature due to decreasing pressure may allow the rock to melt. Thus, rapidly upwelling solid mantle (moving by solid state deformation or convection) can intersect the melting curve and partially melt under the ridge. This melt separates from the solid matrix and accumulates in magma chambers or large cavities filled with molten rock. Because the melt chemically differentiates (remember, every rock involves multiple minerals, each of which has a different melting temperature: heavy, refractory crystals don't melt and separate out as solid dregs, while the material that is above its melting curve tends to rise because it is less dense than the residue) the molten component that rises to the surface is quite different in overall composition than the original rock that melted. This melt fills the gap as the plates spread, and the cooling rock is incorporated into the growing plates on either side of the rift.
In most cases, it is believed that sea-floor spreading is actually a rather passive process, meaning that the plates are not forced apart by the upwellings, but rather the upwelling occurs because the plates are pulled apart primarily by the old subducting slabs at the other edge of the plate. In some cases, though, upwellings can break apart continents, as is happening in eastern Africa, and as broke South America off from Africa. The upwellings then evolve into spreading ridge systems like the mid-Atlantic rise. The ridges themselves are defined as ridges because they are hot regions which tend to be buoyant. As the ocean crust and lithosphere increase in age (in the perpendicular direction to the ridge) the rock cools, contracts (densifies) and rides lower on the surface of the Earth. There is thus a systematic increase in ocean depth with distance from the mid-ocean ridges, caused by cooling of the oceanic lithospheric plate. This persists up to the point where the lithosphere is cold enough to become gravitationally unstable, whereupon it sinks into the mantle on its own, pulling the trailing plate of younger age downward behind it.
So, how do we know about the deep structure of mid-ocean ridges? There are recent efforts to go explore these ridges in situ, using ships that conduct seismic experiments. Using underwater explosions or air bursts, P waves are sent out from source and bounced off of the subsurface layers, ultimately being recorded on seismometers towed in very long cables behind ships. Another source of information is the chemistry of the rocks themselves, which reveal the nature of the melting, differentiation, and ascent processes. More direct evidence is often provided by rare instances of exposed oceanic crust that was upthrust onto continents (often during continental collisions) rather than being subducted as is the fate of most oceanic crust and lithosphere. There are a few places where old oceanic crust is found exposed on the continents, in formations that are called ophiolites. This includes sites in Cyprus, the Western U.S., Oman, Greece, Papua New Guinea and Noumea. These regions show the pillow lava layer, the sheeted dike complex, and the deeper cumulate layers produced by refractory materials that have separated in the magma chamber. Very consistent structures are found in different regions suggesting that the process of oceanic crustal formation is fairly ubiquitous for all mid-ocean ridge systems.
If we go out to any oceanic crust and pick up a rock, it will be made of basalt. This is a type of rock that is about 50% silica, with relatively enriched magnesium, iron and calcium compared to most continental rocks. Typically a fresh (unweathered) basalt is pretty dark or black, due to the presence of the heavy metals. Basalts are the typical rock type that will result when any chunk of the mantle is melted and the lighter material separated and then cooled. Since most of the Earth's surface is covered by basaltic ocean floor, it is the most common rock found on the surface of the Earth.
In contrast to andesitic rocks that are erupted in arc volcanoes, molten basalt is a low viscosity, runny magma type. This is because basalt is relatively low in silica (which makes andesite magmas sticky). As a result, basalt builds up very broad volcanoes called shield volcanoes, with layer upon layer of flows and underground injections. The viscosity is about the same as that of chunky soups, so steep-sided volcanic edifices cannot be built up by basaltic eruptions. This runny nature of basalt magmas is also a factor in preventing rift volcanoes from building up large pressures for explosive eruptions. The volcanoes tend not to plug themselves up with sticky rock nearly as effectively as in arc volcanoes. Combined with eruption under water, which causes the gasses and other volatiles to remain in solution in the rock, effusive eruptions result.
As new oceanic crust is formed by cooling of pillow basalts and deeper dikes, it is strongly fractured by joints and faults, some of which are caused by earthquake deformation due to the crustal extension, and some of which are the result of thermal contractions associated with cooling. Water in the ocean can seep down into these cracks and as it penetrates into the new oceanic crust it is heated up. The hot water then interacts with the rocks, leaching minerals and elements out of the rock. The hot water increases in temperature and then rises, carrying this bounty of leached elements with it, and jets out onto the ocean floor in vents. Some of these vents, first discovered by submarine dives about 20 years ago, are so enriched in heavy materials such as copper, zinc, lead and sulfides that the hot steam and water produces 'black smokers' or vents of darkened hot water and steam 'erupting' at the base of the ocean. Much of the material leached from the rocks settles out onto the ocean floor (producing economic concentrations of some materials such as manganese nodules), but some is mixed into the ocean, controlling the ocean chemistry.
In addition to the vast upwelling regions along ocean ridge systems, there are other major volcanic centers that are removed from plate boundaries, or only incidentally associated with them. This includes major mid-plate ocean island volcanoes such as in Hawaii, Tahiti, or Fiji. In the Pacific Ocean in particular, there are long chains of islands that trend NW-SE, with older, more eroded islands toward the northwest. It is believed that many of the island chains have been produced by the northwesterly motion of the Pacific plate over several relatively fixed locations of melting below the plate. These 'hotspots' appear to remain fixed relative to the plate because their origin is deeper than the plate itself, with up-wellings that may come from as deep as the lowermost mantle. This causes us to view hotspot volcanoes as somewhat distinct from the rift and arc volcanoes which are clearly related to the creation and destruction of oceanic lithosphere.
Effectively, the up-welling hot rock in a hotspot burns a hole through the overriding lithosphere, whether it is oceanic or continental, and the magma produced by the hot temperatures of the up-welling (originating by the same type of decompression melting as occurs under rifts) penetrates to the surface, producing a volcanic mountain superimposed on the plate. The motion of the plate translates the mountain in the plate motion direction, eventually decapitating the magma conduits that fed into the mountain, and shutting off the volcanic activity. At that time, the magma burns a new path up to the surface, creating a new volcano in the chain. This is much like passing a sheet of metal over a fixed blowtorch, creating a welded scar in the sheet due to passage by the heat source.
The most famous hot spot island chain is the Hawaiian islands, which extend all the way from the current hotspot site under the big island of Hawaii, out to the northwest past now extinct volcanic mountains of Maui, Oahu, Kauai, Midway, on to submarine atolls along the chain and on to submarine mountains along the Emperor Island chain, eventually leading to Kamchatka where the Pacific plate subducts under the Eurasian (in detail under the North American) plate. The age of the islands increases toward the northwest, and erosion of the exposed island and submarine landslides of the deeper underwater mountain whittle away the islands until they are below the water. While the three volcanoes on Hawaii (Mauna Loa, Mauna Kea, Kilauea) are all active volcanoes (Mauna Kea is dormant, possibly permanently, but probably not; many large telescopes have been built on its summit with the hope that it not erupt again soon....). A new island is building up on the southeastern flank of Hawaii (Loihi), and this may grow into the next island in the chain. Because the ocean depth is 4 km, the exposed mountains on the islands are really the crest of giant volcanoes, with Hawaii being the largest mountain on the Earth, if we define the structure relative to the surrounding sea floor level. This reflects huge outpourings of lava, but recognizing that Hawaii is just the youngest of many volcanoes produced by the same hotspot, we can begin to appreciate the magnitude of melting associated with this hotspot.
The large, sustained melting under an island chain requires a steady heat source persisting for many tens of millions of years (the oldest islands in the Hawaiian-Emperor chain are almost 80 million years old, and yet older ones appear to have been subducted away from the surface). Many Earth scientists believe that plumes of hot up-welling material are responsible, with a plume that may be 50-100 km cylinder of rising material extending possibly as deep as the core-mantle boundary. The rock that melts under hotspots has some distinctions from mid-ocean ridge basalts, but this is only true of the minor components. The main rock types of hotspots found in the oceans is again basalt (the typical produce of melting mantle rocks), so the whole island of Hawaii is a massive pile of basaltic lava flows. But minor chemical differences reflect the deep-seated origins of the upwelling material, which differs from mid-ocean ridge materials. The basalt flows on Hawaii have two distinct types. The Hawaiian names are Pahoehoe and Aa. Pahoehoe is like road tar, with fluid, but often ropy or rippled surfaces. Aa is more broken up and blocky, and the lava flow tens to be meters thick.
Since we think that hotspots are generally independent of the shallower mantle circulation directly involved in the production and destruction of oceanic lithosphere, we would expect up-welling plumes to produce some hotspots below continents as well. Indeed, there are similar 'hotspot' tracks burned into the continental rocks of all of the continents. In North America, the best example is the Yellowstone hotspot. The westerly motion of the North American plate has pushed the plate over a major source of heat now centered under Yellowstone (where there is active volcanism in the form of some rising magma, thermal hotsprings and geysers such as Old Faithful). The track of Yellowstone extends across Idaho and into southern Oregon, with older and older volcanic rocks as one moves westward.
While the up-welling of a plume under oceanic lithosphere causes melting that gives rise to basalts, which is because mantle rock is involved. However, up-wellings under continents require a much greater magma pathlength through the continental lithosphere and crust. This causes basaltic melts of mantle material to be contaminated by the host rocks, enriching the magmas in silica and volatiles. Thus, some hotspot regions on continents have had massive explosive eruptions, unlike the effusive flows founds on Hawaii. The Yellowstone hotspot track has several calderas along its length, where past massive explosions have occurred, dwarfing even the vast explosion that produced the Long Valley Caldera of eastern California. Thus, hotspots do have some catastrophic potential, but it is fairly limited due to the small percentage of hotspot volcanoes on the surface (5%).
The island of Iceland is actually a hotspot volcano located right on the mid-ocean rift in the northern Atlantic. The combined volcanism of the hotspot and sea-floor spreading have built up the island to above sea-level. Iceland has extensive volcanic activity along the rift that bisects it from south to north, and is extensively studied as a natural laboratory for what happens at mid-ocean ridges in general. But, there is the unusual juxtaposition of the hotspot that complicates any generalizations based on Iceland observations. For the most part hotspot volcanoes are located within the interiors of plates, and appear to have fixed deep origins, below the plates. Most have effusive basaltic eruptions, but there are some on continents with explosion capabilities.
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