The Earth’s Crust
The following passage is adapted from an article describing the origins of the ocean floor (© 2009 by Peter Kelemen).
- At the dark bottom of our cool
- oceans, 85 percent of the
- earth’s volcanic eruptions
- proceed virtually unnoticed.
- Though unseen, they are hardly
- insignificant. Submarine
- volcanoes generate the solid
- underpinnings of all the world’s
- oceans: massive slabs of rock
- seven kilometers thick.
- Geophysicists first began to
- appreciate the smoldering
- origins of the land under the
- sea, known formally as ocean
- crust, in the early 1960s.
- Sonar surveys revealed that
- volcanoes form nearly
- continuous ridges that wind
- around the globe like seams
- on a baseball. Later, the same
- scientists strove to explain
- what fuels these erupting
- mountain ranges, called
- mid-ocean ridges. Basic
- theories suggest that because
- ocean crust pulls apart along
- the ridges, hot material deep
- within the earth’s rocky
- interior must rise to fill the
- gap. But details of exactly
- where the lava originates and
- how it travels to the surface
- long remained a mystery.
- In recent years, mathematical
- models of the interaction
- between molten and solid
- rock have provided some
- answers, as have examinations
- of blocks of old seafloor now
- exposed on the continents.
- These insights made it
- possible to develop a detailed
- theory describing the birth of
- ocean crust. The process
- turns out to be quite different
- from the typical layperson’s
- idea, in which fiery magma
- fills an enormous chamber
- underneath a volcano, then
- rages upward along a jagged
- crack. Instead, the process
- begins dozens of kilometers
- under the seafloor, where
- tiny droplets of melted rock
- ooze through microscopic
- pores at a rate of about 10
- centimeters a year, about as
- fast as fingernails grow.
- Closer to the surface, the
- process speeds up, culminating
- with massive streams of lava
- pouring over the seafloor
- with the velocity of a speeding
- truck. Deciphering how liquid
- moves through solid rock deep
- underground not only explains
- how ocean crust emerges but
- also may elucidate the behavior
- of other fluid-transport
- networks, including the river
- systems that dissect the
- planet’s surface.
- Far below the mid-ocean ridge
- volcanoes and their countless
- layers of crust-forming lava
- is the mantle, a 3,200-
- kilometer-thick layer of
- scorching hot rock that forms
- the earth’s midsection and
- surrounds its metallic core.
- At the planet’s cool surface,
- upthrusted mantle rocks are
- dark green, but if you could
- see them in their rightful
- home, they would be glowing
- red- or even white-hot. The
- top of the mantle is about
- 1,300 degrees Celsius, and it
- gets about one degree hotter
- with each kilometer of depth.
- The weight of overlying rock
- means the pressure also
- increases with depth about
- 1,000 atmospheres for every
- three kilometers.
- Knowledge of the intense heat
- and pressure in the mantle led
- researchers to hypothesize in
- the late 1960s that ocean crust
- originates as tiny amounts of
- liquid rock known as melt,
- almost as though the solid
- rocks were “sweating.” Even
- a minuscule release of pressure
- (because of material rising
- from its original position)
- causes melt to form in
- microscopic pores deep within
- the mantle rock.
- Explaining how the rock sweat
- gets to the surface was more
- difficult. Melt is less dense
- than the mantle rocks in
- which it forms, so it will
- constantly try to migrate
- upward, toward regions of
- lower pressure. But what
- laboratory experiments
- revealed about the chemical
- composition of melt did not
- seem to match up with the
- composition of rock samples
- collected from the mid-ocean
- ridges, where erupted melt
- hardens.
- Using specialized equipment
- to heat and squeeze crystals
- from mantle rocks in the
- laboratory, investigators
- learned that the chemical
- composition of melt in the
- mantle varies depending on
- the depth at which it forms;
- the composition is controlled
- by an exchange of atoms
- between the melt and the
- minerals that make up the
- solid rock it passes through.
- The experiments revealed
- that as melt rises, it dissolves
- one kind of mineral,
- orthopyroxene, and
- precipitates, or leaves behind,
- another mineral, olivine.
- Researchers could thus infer
- that the higher in the mantle
- melt formed, the more
- orthopyroxene it would
- dissolve, and the more
- olivine it would leave behind.