I wanted to understand some of the geologic oddities to better understand meteorites, which are more diverse than the native rocks of Earth.

A teaser: there is more water in our mantle than in our oceans — thanks to the properties of eclogite, the colorful rock on the right.

Eclogite is forged from some anomalous alchemy from down below our crust and makes the tectonic plates possible, and thus, differentiates Earth from the other planets. In short, the peculiar conversion of the black volcanic basalt of our sea floors into this colorful and denser mineral, makes subduction possible. The basalt that comes up from the mantle cools and slides over to the subduction zone over ~ 150 million years. Cool enough to start sinking, it is still lighter than the mantle, and for it to cycle back down, it has to become much more dense, and it does, 30 miles down, becoming eclogite which then sinks further into the mantle, which itself circulates as a viscous fluid down to our molten core, on a billion year roundtrip to circle back to the crust again.

“Without eclogite, our plate-tectonic system would grind to a halt” (37) “Only Earth has developed the habit of subduction, which has helped keep the planet on an even keel for eons. Subduction keeps the interior and exterior of earth in communication with each other, returning not only the solid ocean crust but also volatiles like water and carbon dioxide vented by volcanos back to the mantle. In contrast, other planets, like Mars with its fossil river valleys, have simply lost their volatiles to space over time, with nothing held in reserve.” (40)

Subduction brings water down to the magma, forming granite (which accumulates in the continental crust), and lowering the viscosity of the mantle, allowing it to flow convectively, keeping the plates in motion. Granite is unique to Earth in our solar system.

The mantle is 1,800 miles thick and makes up 84% of Earth’s volume.

“A xenolith is a chunk of rock that happened to be picked up by an ascending body of magma and subsequently preserved within it as the magma crystallized. They are emissaries from otherwise inaccessible realms, carrying messages encoded in their minerals. In fact, most of the rare rock samples we have of Earth’s mantle are xenoliths that were borne up by basaltic magmas.” (163)

And wow, I had no idea how recent many of the fundamental theories are for how the world works (e.g., plate tectonics in 1965). Here are some more interesting bits:

The meteorites called “chondrites were the universal ancestors of all rocks on Earth and other rocky planets and moons. They preserve a memory of the starting composition of these worlds, something that all other rocks have forgotten, particularly on Earth, where the process of differentiation has continued for 4.5 billion years and generated a prodigious diversity of rocks that fall far from the chondritic tree in their chemistry. The crust has been distilled from the mantle by partial melting — a kind of smelting process that can yield basaltic ocean crust in one step, but requires many iterations to generate granitic continental crust, unique to Earth. As products of all this differentiation, refining, and remixing, no native Earth rocks can tell us about the bulk composition of the planet. Luckily the chondrites — those ancient emissaries with astonishingly long memories — occasionally drop from the sky and recall the recipe in detail.” (25)

The geodynamo “has existed for more than 3.5 billion years but fluctuates daily. The magnetic field is our protective shield; it deflects not only the relentless solar wind, which would otherwise strip away Earth’s atmosphere over time, but also cosmic rays, which zing in from interstellar space with enough energy to damage living cells. It arises in Earth’s outer core, where the movement of liquid iron creates a giant, self-perpetuating electromagnet, the geodynamo.” (48) “Seafloor basalts provide a high-fidelity chronicle of polarity reversals for the past 170 million years (at which point the record is lost to subduction). The most recent flip occurred about 770,000 years ago, deep in the Ice Age.” (50) More recently, “the intensity of the field has been falling at a rate of about 6% per century.” (51)

Diamonds are not forever, despite the marketing. “Diamonds are metastable at Earth’s surface—well outside their natural thermodynamic habitat (100+ miles down in our mantle). Any diamond at atmospheric pressure is converting very slowly to graphite, in atomic scale layers, from the outside in” (76).

Coral reefs disappeared from Earth for 10 million years. “For us in the Anthropocene, the causes of the Permian extinction— including ocean acidification— are uncomfortably familiar, and the story of the post-Permian reef gap sounds like a classic good news/bad news joke. The good news is that the corals did in fact recover and flourish once again. The bad news is… it took a thousand times longer than the entire duration of human history.” (86)

“Temperature increases with depth, at a typical geothermal gradient of 75°F/mile.” (92)

Salt: “From Farsi for ‘mountain of salt,’ a namakier is perhaps more accurately described as a glacier of salt that can flow over the land surface as fast as inches per year, rivaling its icy counterparts for speed.” (95) Rock salt forms in lagoons and isolated basins with evaporation of the water. It is denser than the sediment and begin to sink; under pressure from above, the interstitial pore spaces collapse, and its density rises further. Once it is a block of crystalline salt, it resists further compaction and about a mile down, it becomes more buoyant than the rocks above it, and it has the wild property of being able to flow viscously in the sold state. To correct the density inversion, it bubbles up like hot wax in a lava lamp, forming salt domes and canopies that then trap petroleum beneath them. Salt domes are the major target of oil exploration in places like the Gulf of Mexico.

Pangaea: “From formation to final breakup, supercontinents have life cycles of 500-700 million years; Pangaea was preceded by Rodinia (at its peak about 1 billion years ago) and even the more ancient Nuna (1.8 billion years ago). Each of these would have had its complimentary superocean” (111)

“For most igneous rocks, crystal size in inversely related to the rate at which they cooled from a magma. Different minerals ‘freeze’ out of a magma at different temperatures— a phenomenon called ‘fractional crystallization.’ Minerals with the highest crystallization temperatures will nucleate first, invariably getting a head start (to form large crystals) on those that form at lower temperatures (the undifferentiated matrix).” (120)

“Zircon (ZrSiO4) which occurs primarily as a minor constituent of granite, is singularly resistant to both chemical breakdown and physical abrasion. In contrast to diamonds, zircons really are forever. Zircon also happens to be ideal for isotopic dating; at the time of crystallization, it accepts some Uranium in its atomic lattice. Over time, the two isotopes of Uranium, both radioactive, break down into their daughter isotopes of lead. Measuring the lead-to-uranium ratios in zircon crystal allows it to be dated with high precision. The most ancient earth objects ever found are tiny zircon crystals that survive as grains in an ancient sandstone but recall quite clearly their crystallization in a granitic magma 4.4 billion years ago. Zircon crystals also have the capacity for reincarnation. An old zircon crystal, after eons of dormancy, can grow new concentric layers when reheated by a magmatic or tectonic event. These resemble miniature tree rings — but may have formed in episodes of growth separated by hundreds of millions of years. A single zircon crystal can be a microcosm containing the tectonic history of a continent.” (167)