Basalt is the result of partial melting of meteoritic material (Earth is made of a huge mass of meteorites!), so it forms on other terrestrial planets (which also accumulated from meteorites) as well as Earth, making basalt the “mother liquor” of volcanoes on terrestrial planets. It is found all over Earth, but especially under the oceans and in other areas where Earth’s crust is thin. It formed in the Isle Royale-Keweenaw region because of the Midcontinent Rift. Most of Earth’s surface is basalt lava, but basalt makes up only a small fraction of continents. Sometimes basalt is said to be “primitive” which means that it hasn’t evolved or changed since it first formed. Changes are likely as the melt migrates and interacts with rocks it passes through.
Keweenaw lavas are mainly basaltic: continental flood basalts with isotopic signatures close to bulk composition of Earth (Paces, 1988). Within the sequence of flows there are several cycles of evolution in subcrustal magma chambers. Overall the lavas become slightly more primitive with time. The ages are well established from U-Pb dating of zircons. Most of the great outpouring of rift lavas occurred in about 2 million years.
The figure below shows U-Pb dates on zircons from pegmatite zones of the Portage
Lake Volcanics, Keweenaw Peninsula (Paces and Miller, 1993).
Lane (1911) first recognized and described the mirror-image geological and lithological similarity of the PLV and the CHC on both sides of the Syncline (sketch, below), and further suggested that the great lava flow of the Keweenaw Peninsula (Greenstone Flow, left) and the large flow of Isle Royale are the same. Huber (1973a) strongly supports Lane's correlations. Longo (1984), 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 (1984), it has an aggregate volume of 1650 km3 (396 mi3), comparable to the Roza flow of the Columbia River Flood basalts, which is estimated to be 1500 km3 (360 mi3) by Swanson et al. (1975). The areal extent of the Roza, 40,000 km2 (15,450 mi2), is much larger than the Greenstone flow, 5000 km2 (1930 mi2), 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. Basalt is the most common composition of lava rocks that cool from magma, liquid rock that rises from the deep Earth at volcanoes. Today basalt is forming at many active rifts, including Iceland, the East African Rift Valley, the Red Sea and the Rio Grande Valley of New Mexico and Colorado.
Paces and Miller (1993) published a summary of age information on the igneous rocks of the Midcontinent rift (Fig 1, above), which shows that the PLV represents one of the last events of the approximately 25 million year period of rift magmatism.
Sketch from Lane, 1911
Bornhorst & Brandt, 2009
K Schulz USGS (modified after Nicholson et al., 1997)
Isle Royale North shore, looking SW
