Geology Reference
In-Depth Information
Basalt comes in a variety of flavors, but two essential silicate minerals dominate them
all. One key mineral is plagioclase feldspar, by far the most important aluminum-bearing
mineral in terrestrial planets and moons and Earth's commonest crustal mineral. My MIT
professor Dave Wones once advised me that if I was ever shown a mystery rock and
quizzed as to its mineralogy, I should just say “plagioclase,” and I'd be right 90 percent of
the time. The second essential mineralogical ingredient of basalt is pyroxene, the common
chain silicate also found in peridotite. Pyroxene is one of a handful of common minerals
that can incorporate all of the big six elements (and many more less common elements, as
well).
To understand the origins of plagioclase and pyroxene, the two essential mineral in-
gredients of basalt, consider the strange freezing and melting habits of rocks. Four and a
half billion years ago, as Earth's magma ocean cooled, olivine formed first, then a little
bit of anorthite, and finally a lot of pyroxene. The resulting magnesium silicate rock was
peridotite, which formed much of the upper mantle. As masses of peridotite formed and
sank, they were reheated and partially remelted.
Oureverydayexperiencewithmeltingsuggeststhatthechangefromsolidtoliquidtakes
place at one specific temperature. Water ice melts at 32 degrees, most household candle
wax at about 130 degrees, and dense metal lead at 621 degrees Fahrenheit. But rock melt-
ingisn'tsosimple;mostrocksdon'tmeltentirelyatonetemperature.Ifyouheatperidotite
to about 2,000 degrees Fahrenheit, the first melt will appear. (It will appear sooner if the
peridotite is rich in volatile water and carbon dioxide.) The composition of those first mi-
croscopic droplets differs dramatically from that of the bulk of peridotite rock. The initial
melt has a lot more calcium and aluminum, a little more iron and silicon, and a lot less
magnesium.Thisinitialliquidisalsoalotlessdensethanitsperidotitehost.Consequently,
even a 5 percent melting of peridotite in the mantle generates a lot of magma that gathers
along mineral grain boundaries, collects in fissures and pockets, and rises toward the sur-
face—magma that will eventually become basalt. Over billions of years of Earth history,
the partial melting of peridotite has generated hundreds of millions of cubic miles of basalt
magma.
Molten basalt comes to planetary surfaces in two complementary ways. The more spec-
tacular is through volcanic eruptions like those in Hawaii and Iceland, with fiery magma
fountains and riverlike flows. Such dramatic eruptions are a consequence ofwater and oth-
er volatiles, which remain locked in the silicate liquid at the high pressures more than a
mile down, but which transform explosively to gas near the surface. Such explosive vol-
canism can eject ash and toxic gases high into the stratosphere and can hurl car-size vol-
canic “bombs” more than a mile outward to smush the surrounding countryside.
Layer by layer, these basalt eruptions of lava and ash can build mountains several miles
tall and cover thousands of square miles in black rock. This type of basaltic lava flow and
