Huber a strongly supports Lane's correlations. Longo , after extensive field mapping and sampling at Isle Royale and the Keweenaw, gives field observations and geochemical data that also strongly confirms the correlation of the Greenstone flow. This correlation means that the Greenstone flow is one of the earth's largest lava flows; according to Longo , it has an aggregate volume of km 3 mi 3 , comparable to the Roza flow of the Columbia River Flood basalts, which is estimated to be km 3 mi 3 by Swanson et al.
The areal extent of the Roza, 40, km 2 15, mi 2 , is much larger than the Greenstone flow, km 2 mi 2 , a comparison which results from the ponding of the Greenstone within the rift basin.
Thus, the solidification of the Greenstone flow is a kind of magma ocean experiment, the likes of which is rare on this planet. Isle Royale and much of the Keweenaw is mainly underlain by basaltic lava, the result of hundreds of successive eruptions from the Rift. Mostly this basalt made its way to the surface rapidly, but some was held in magma chambers and evolved before erupting. These types of lava flows are called "flood basalt" because they literally flood the land with lava.
The western United States has quite a bit of both types. Much of the Klamath Mountains of northern California and southern Oregon is broken-up pieces of to million year old seafloor crust the million year old Josephine Ophiolite is a good example , while most of eastern Washington and Oregon the Columbia River Basalt , and northeastern California the Modoc Plateau Basalt are underlain by flood basalt.
If you think of basalt as mantle juice, the way we talk about extracting oil from olives, then basalt is the first pressing of mantle material. The big difference is that whereas olives yield oil when put under pressure, midocean ridge basalt forms when pressure on the mantle is released. The upper part of the mantle consists of the rock peridotite , which is even more mafic than basalt, so much more so that it's called ultramafic.
Where the Earth's plates are pulled apart, at the mid-ocean ridges, the release of pressure on the peridotite makes it start to melt—the exact composition of the melt depends on many details, but in general it cools and separates into the minerals clinopyroxene and plagioclase , with smaller amounts of olivine , orthopyroxene, and magnetite.
Crucially, whatever water and carbon dioxide are in the source rock move into the melt too, helping keep it molten even at lower temperatures. The depleted peridotite left behind is dry and higher in olivine and orthopyroxene. Like nearly all substances, melted rock is less dense than solid rock. Once formed in the deep crust, basalt magma wants to rise, and at the center of the mid-ocean ridge, it oozes onto the seafloor, where it rapidly solidifies in the ice-cold water in the form of lava pillows.
Farther down, basalt that doesn't erupt hardens in dikes , stacked vertically like cards in a deck. These sheeted dike complexes make up the middle part of the oceanic crust, and at the bottom are larger magma pools that slowly crystallize into the plutonic rock gabbro. Midocean-ridge basalt is so important a part of Earth's geochemistry that specialists just call it "MORB.
Therefore MORB is rarely seen, even though it is the majority of the world's basalt. To study it we have to go down to the ocean floor with cameras, samplers, and submersibles. The basalt we're all familiar with comes not from the steady volcanism of the midocean ridges, but from more vigorous eruptive activity elsewhere that builds.
These places fall into three classes: the subduction zones, the ocean islands, and the large igneous provinces, huge lava fields that are called oceanic plateaus in the sea and continental flood basalts on land. Theorists are in two camps about the cause of ocean island basalts OIBs and large igneous provinces LIPs , one camp favoring rising plumes of material from deep in the mantle, the other favoring dynamic factors related to the plates.
Subduction brings MORB and water back into the mantle. These materials then rise, as melt or as fluids, into the depleted mantle above the subduction zone and fertilize it, activating fresh magmas that include basalt. If the basalts erupt in a spreading seafloor area a back-arc basin , they create pillow lavas and other MORB-like features.
These bodies of crustal rocks may later be preserved on land as ophiolites. If the basalts rise beneath a continent, they most often mix with less mafic that is, more felsic continental rocks and yield different sorts of lavas ranging from andesite to rhyolite.
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