Privileged Planet: our rocks are different (Introduction)

by David Turell , Thursday, October 19, 2023, 17:12 (402 days ago) @ David Turell

We have rocks not all like the other planets in this solar system:

https://www.nationalgeographic.com/science/article/earth-geology-rare-rocks-solar-syste...

"In fact, our planet is something of a geological oddball given what we know about our solar system neighbors. Granite isn’t the only rock that’s ordinary on Earth but rare on other worlds. These missing rocks can add up to entire missing landscapes: beyond Earth, you’d have a tough time finding anything like the limestone columns of Hạ Long Bay, a stratovolcano like Mt. Fuji, or even quartz sand dunes.

"Scientists discuss the ordinary rocks found on Earth’s surface that are special, rare, or nonexistent elsewhere in the solar system—based on our current knowledge.

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"Igneous rocks, which form from magma (which is called lava when it erupts on the surface), aren’t anything special in our solar system. The surfaces of the moon, Mars, Mercury, Venus, and even Jupiter’s angry little moon Io are all overwhelmingly volcanic. But ordinary magma doesn’t usually make granite—it has to be recycled first.

"Fresh from the interior of a planet, magma usually forms a blackish rock called basalt—not granite, says planetary geoscientist Harry McSween of the University of Tennessee. Granite, which usually forms from magmas that cooled and partially re-melted over and over—the minerals like quartz in granite melt easily, so they liquefy and separate from the leftover solid as rocks start to melt. The resulting recycled magmas form many different types of granite and granite-like rocks, which make up most of the Earth's continental crust.

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“'Earth, with plate tectonics and water, has this easy way of making granite all the time,” says planetary scientist Matthew Siegler of the Planetary Science Institute in Tucson, Arizona, who helped spot the granite on the moon. “But none of the other planets have that.”

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"Sedimentary rocks like sandstones form when preexisting rocks break down into bits that pile up and stick together to form a new rock. Pressure helps this process along, but it’s not enough on its own—“you need water to help cement together the different particles that are coming together,” says Mars geologist Kirsten Siebach of Rice University.

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"Of course, the Earth’s plate tectonics and water are not the only things to set it apart. Our home planet is the only one to host life—and the rocks here show it.

"Take limestone, a chalky rock rich in minerals called carbonates. Limestone is common on Earth because life produces it en masse: the rock forms when shells and skeletons of ocean creatures, especially from coral reefs, pile up on the seafloor as the organisms die. When these remains cement together, they form enormous blocks of limestone.

"Life speeds up limestone formation so much that “even geologists sometimes think that limestone can only form with life,” says Siebach. But lifeless processes can make carbonate-rich rocks like limestone, too. The key ingredients are a bit of shallow, warm water that’s not too acidic and some carbon dioxide — both of which existed on Mars in the past.

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"Marble would be very strange to find in space, and not only because it starts off as limestone. It’s a metamorphic rock, which means it becomes something new under extreme heat and pressure without melting.

"On Earth, metamorphism usually happens slowly and deep underground. The heat and pressure at depth transform rocks and minerals, which is how graphite becomes diamond and limestone becomes marble. But on other worlds, it’s more typical to find metamorphic rocks forged in the split-second shock of a meteoroid impact.

“'The rocks are exposed to very high pressures and high temperatures, but only fleetingly,” McSween says.

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"Ultimately, if we could drill deep enough, we’d actually find metamorphic rocks on every planet, says McSween. But the Earth is special. Thanks to plate tectonics, rocks from the deep interior and the surface trade places all the time, substantially expanding our planet’s unusual geological repertoire.

"On Earth, there’s an efficient way “to get those rocks up to the surface and cycle surface rocks down to be metamorphosed,” McSween says. “And we really don't have any process that does that on other planets.'”

Comment: The Earth with its abundant water, floating continental plates on its surface, massive volcanic activity and active subduction evolved to this point over time with the help of living matter. Formed by natural chance contingencies or purposeful design? Very difficult to ignore design.