Alan Stahler: California’s a new-comer
Cliffs to the east, ocean to the west, California One — the Pacific Coast Highway — traces the edge of the North American continent.
Were this highway to have been built some 300 million years ago, it would have been, not California One, but Nevada State Route One.
California, 300 million years ago, did not exist.
Mapmakers in the 1500s noticed (as we all noticed, in grade school) that South America and Africa look like pieces of a jigsaw puzzle.
For good reason: Earth’s surface really is a giant jigsaw puzzle.
The puzzle pieces are giant rafts of rock that drift slowly, this way and that.
All the continents, including North America, sit atop such rock rafts.
The rafts often collide.
Three hundred million years ago, all of Earth’s rock rafts were jammed together in one gigantic pile-up: the supercontinent of Pangaea (pronounced pan-JEE-uh or pan-GUY-uh, as you like).
South America and Africa, North America and Europe, all nestled together. The puzzle was complete.
Roughly 200 million years ago, the puzzle broke apart.
Africa and South America, still joined to each other, drifted one way; North America and Eurasia, also conjoined, drifted another.
Sometime after that, Africa drifted away from South America, North America from Eurasia.
The rock rafts, with continents aboard, still drift, at about the rate your fingernails grow.
The Atlantic coasts of North and South America mark the trailing edges of the rafts as they drift apart.
Their leading edges are marked by their Pacific coasts. It’s at their leading edges that the rafts collide.
Collisions between rafts are dramatic. Some 40 million years ago, the raft bearing India collided with the Asian raft.
Like pushing on a rug, the collision squeezed and folded — still squeezes and folds — the rocks between, thrusting the Himalayas skyward, and suturing India to Asia.
Over the past hundreds of millions of years, the leading edge of North America collided with rafts bearing large islands.
The islands sutured onto Nevada, thus assembling California.
Sometimes, a rock raft drove sea floor into North America.
When continent collides with seafloor, they (usually) don’t suture; rather, the seafloor, made of denser rock, slides beneath the continent.
It’s hot down there, beneath a continent. And, beneath all that rock, pressure is intense.
Many seafloor rocks hold water in their crystals.
As the seafloor dives downward, heat and pressure re-arrange the atoms in the rocks, forming new crystals, crystals with no room for water.
Released from the rock, the water seeps upward, into the rock above.
Throw salt on ice and the ice will melt, even if the ice is colder than its normal melting point.
Water does the same to rock.
As water from the cooked seafloor seeps into the rock above, the rock melts to form magma.
The magma burbles upward; collects in huge chambers beneath the surface; and erupts in great volcanoes.
Some magma never erupts; it remains in its chamber, cools, and freezes. Insulated by miles of rock above, it freezes slowly, giving crystal grains time to grow large, forming rock in the granite family (“grain-ite”).
Rain and snow, wind and weather, over millions of years, eat away at the miles of rock above the magma chambers, until the frozen rock —some still capped by the last remains of the rock above — sees daylight.
As the frozen rock cooled, below ground, it contracted, and cracked.
Cracks offered an escape route for atoms and molecules that couldn’t fit into the crystals of the young rock — molecules of water, for instance, which seeped — still hot – through the cracks.
Dissolved in the water were other atoms and molecules that had been rejected by the growing crystals.
As the water cooled, these dissolved substances themselves crystallized in the cracks, creating veins of quartz, and, within the quartz, gold.
Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages – kid to adult – and may be reached at firstname.lastname@example.org
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