Geology Reference
In-Depth Information
Convection, by which collections of hot atoms move thermal energy wholesale, is a bet-
terplanetary choice forcooling. Youexperience convection whenever youboilwater.Pour
water in a pan, turn up the heat, and wait. The process is slow at first, as the hot pan con-
tacts the cold water and transfers heat by conduction—wiggle by wiggle, metal atoms in
thepanjostlemoleculesofwater.Butsoonanothermechanismtakesover.Heatedvolumes
ofwateronthebottombegintoexpandandrisethroughthecooler,denseroverlyingwater,
transferring heat en masse to the surface. Simultaneously, the cooler, denser surface waters
sinktothehotbottom.Fasterandfastertheheatexchangeprogresses,withcolumnsofwa-
ter rising and sinking, until you reach a rolling boil. Through the convective cycling of hot
water up and cool water down, large volumes of water spread heat throughout the liquid in
a fast and efficient dance.
On the grand scale of Earth, convection appears over and over—in cooling offshore
breezes on a summer day, in grand ocean currents that sweep from the Equator to the
Arctic, in the turbulent lightning-laced fronts of thunderstorms, in boiling hot springs and
spurting geysers. And so it is in Earth's interior; cooler, denser magmas and rocks near the
surface sink, as hotter, less-dense magmas deep down rise to the surface. For all of Earth's
history, convection has been the primary driver of planetary cooling.
And then there's radiation, the third mechanism of heat transfer. Any hot object radiates
heat to its cooler surroundings in the form of infrared radiation that travels 186,000 miles
per second through a vacuum. This familiar form of energy, abundantly evident whenever
you relax and soak up the rays of the shining Sun, is similar in its behavior to waves of
visible light (though heat radiation has slightly longer wavelengths). Perhaps the most ob-
viousinfraredenergysourceistheSun,whichbathesEarthininfraredradiationthattravels
across the vacuum of space in about 8.3 minutes. An electric space heater, a toasty fire
in your fireplace, and an old hot water radiator are other familiar examples. Every warm-
er object radiates heat to its cooler surroundings. Your body is no exception. That's why
a crowded auditorium can get so uncomfortably warm—each person radiates heat like a
hundred-watt lightbulb—a fact one may easily verify by putting on a pair of night-vision
goggles, which make people and other animals that emit infrared radiation appear to glow
brightly in the dark.
The rate of heat transfer, whether by conduction, convection, or radiation, depends on
the difference in temperature between the hotter and colder objects. Conduction is swifter,
convection more vigorous, and radiation much more intense if the temperature differences
are large. Earth is a warm planet. Orbiting the Sun as it does in the coldness of space, it is
always radiating heat into the void. But the red-hot post-Theia Earth blasted its excess heat
energy into space at a rate unmatched in modern times. It literally glowed in the black void
of space.
