Basalt is the dark, heavy volcanic rock that makes up most of the world's oceanic crust. Some of it erupts on land, too, but to a first approximation basalt is an oceanic rock. Compared to the familiar granite of the continents, basalt ("ba-SALT") is darker, denser and finer grained. It's dark and dense because it's richer in the dark, heavy minerals bearing magnesium and iron (that is, more mafic) and poorer in silicon- and aluminum-bearing minerals. It's finer grained because it cools quickly, near or on the Earth's surface, and contains only very small crystals.
Most of the world's basalt erupts quietly in the deep sea, along the mid-ocean ridges—the spreading zones of plate tectonics. Lesser amounts erupt on volcanic ocean islands, above subduction zones, and in occasional large outbursts elsewhere.
Basalt is a type of lava that the rocks of the mantle make when they start to melt. 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 is in the source rock moves 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 midocean 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." However, the oceanic crust is constantly recycled into the mantle by plate tectonics. 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 volcanoes. 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. For now it's simplest just to say that both OIBs and LIPs have mantle source rocks that are more fertile than the typical MORB and leave things there.
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. But under favorable circumstances basalts can coexist with these felsic melts and erupt among them, for instance in the Great Basin of the western United States.
Where to See Basalt
The best places to see OIBs are Hawaii and Iceland, but almost any volcanic island will also do.
The best places to see LIPs are the Columbia Plateau of the northwestern United States, the Deccan region of western India and the Karoo of South Africa. Dissected remnants of a very large LIP occur along both sides of the Atlantic Ocean, too, if you know where to look. (See some of them at largeigneousprovinces.org.)
Ophiolites are found throughout the world's great mountain chains, but particularly well known ones are in Oman, Cyprus and California.
Small basalt volcanoes occur within volcanic provinces worldwide.
My gallery of basalt pictures includes basalts from OIB, LIP, ophiolite and continental settings.