Geology Reference
In-Depth Information
ash is extremely fine-grained and rich in glass, the consequence of liquid cooling so rap-
idlythatcrystalsdon'thavetimetoform.Theresultisafeaturelessblackcrustofhardened
lava. Other distinctive olivine basalts, which occur only when peridotite is partially melted
atrelativelyshallowdepthsoflessthantwentymiles,containafewlustrousolivinecrystals
that formed underground in the first stage of melt solidification. The green crystals decor-
ate the otherwise bland black rock.
It takes a lot of explosive force for magma to break through to the surface, so a signific-
ant fraction of basaltic magma never makes it aboveground. Rather, these red-hot liquids
are stuck far underground, where they cool more slowly and form inch-long, lathe-shaped
feldspar and pyroxene crystals in rocks called diabase or gabbro. Sometimes the magma
is injected into near-vertical cracks in subsurface rocks to form smooth-faced dikes. If the
host rock is soft and erodes away millions of years later, the result can be a long, straight
diabase wall that can look uncannily like a crumbling archaeological site. Alternatively, if
the magma is injected between flat-lying layers of sedimentary rocks, it can form a thick
blanketlike sill. The Palisades cliffs on the Hudson River, prominent just upstream from
New York City on the western shore of the Hudson River, are the result of one of a series
of basaltic sills that dip gently to the west to form parallel highlands (and some of the most
expensive real estate) in northern New Jersey and southern New York. Still other times the
liquid sits and cools in irregular magma chambers that can form miles underground and
that stretch for miles across. But whatever their ultimate geometry, diabase and gabbro are
really exactly like basalt.
With the inevitable formation of the basaltic crust, Earth for the first time enjoyed a
sturdy, solid surface that could float. Before the crust, when magma and peridotite alone
defined the planet's surface, no topographic feature could rise for long to a significant
height above the mean elevation. Red-hot peridotite mush is not nearly strong enough to
support a mountain. But tough basalt, with its relatively low density, is a different story.
The average density of basalt is more than 10 percent lower than that of peridotite. That
means a floating mass of basalt ten miles thick will project more than a mile above the
magma ocean. Rapidly accumulating volcanic cones could rise even higher, perhaps more
than two miles above the mean. As a consequence, Earth's blemished surface began to de-
velop some real character.
Hostile World
Seenfromspace—from thesafedistance oftheyoungMoon,forexample—Earth'sbasalt-
ic veneer appeared deep black with arcuate red cracks and local bright points where im-
mense fountaining volcanoes broke the surface. Jets of dirty white, ash-laden steam ob-
scured some of the most volatile-rich volcanic cones and adjacent portions of the globe.
