Econstudentlog

Rocks: A very short introduction

I liked the book. Below I have added some sample observations from the book, as well as a collection of links to various topics covered/mentioned in the book.

“To make a variety of rocks, there needs to be a variety of minerals. The Earth has shown a capacity for making an increasing variety of minerals throughout its existence. Life has helped in this [but] [e]ven a dead planet […] can evolve a fine array of minerals and rocks. This is done simply by stretching out the composition of the original homogeneous magma. […] Such stretching of composition would have happened as the magma ocean of the earliest […] Earth cooled and began to solidify at the surface, forming the first crust of this new planet — and the starting point, one might say, of our planet’s rock cycle. When magma cools sufficiently to start to solidify, the first crystals that form do not have the same composition as the overall magma. In a magma of ‘primordial Earth’ type, the first common mineral to form was probably olivine, an iron-and-magnesium-rich silicate. This is a dense mineral, and so it tends to sink. As a consequence the remaining magma becomes richer in elements such as calcium and aluminium. From this, at temperatures of around 1,000°C, the mineral plagioclase feldspar would then crystallize, in a calcium-rich variety termed anorthite. This mineral, being significantly less dense than olivine, would tend to rise to the top of the cooling magma. On the Moon, itself cooling and solidifying after its fiery birth, layers of anorthite crystals several kilometres thick built up as the rock — anorthosite — of that body’s primordial crust. This anorthosite now forms the Moon’s ancient highlands, subsequently pulverized by countless meteorite impacts. This rock type can be found on Earth, too, particularly within ancient terrains. […] Was the Earth’s first surface rock also anorthosite? Probably—but we do not know for sure, as the Earth, a thoroughly active planet throughout its existence, has consumed and obliterated nearly all of the crust that formed in the first several hundred million years of its existence, in a mysterious interval of time that we now call the Hadean Eon. […] The earliest rocks that we know of date from the succeeding Archean Eon.”

“Where plates are pulled apart, then pressure is released at depth, above the ever-opening tectonic rift, for instance beneath the mid-ocean ridge that runs down the centre of the Atlantic Ocean. The pressure release from this crustal stretching triggers decompression melting in the rocks at depth. These deep rocks — peridotite — are dense, being rich in the iron- and magnesium-bearing mineral olivine. Heated to the point at which melting just begins, so that the melt fraction makes up only a few percentage points of the total, those melt droplets are enriched in silica and aluminium relative to the original peridotite. The melt will have a composition such that, when it cools and crystallizes, it will largely be made up of crystals of plagioclase feldspar together with pyroxene. Add a little more silica and quartz begins to appear. With less silica, olivine crystallizes instead of quartz.

The resulting rock is basalt. If there was anything like a universal rock of rocky planet surfaces, it is basalt. On Earth it makes up almost all of the ocean floor bedrock — in other words, the ocean crust, that is, the surface layer, some 10 km thick. Below, there is a boundary called the Mohorovičič Discontinuity (or ‘Moho’ for short)[…]. The Moho separates the crust from the dense peridotitic mantle rock that makes up the bulk of the lithosphere. […] Basalt makes up most of the surface of Venus, Mercury, and Mars […]. On the Moon, the ‘mare’ (‘seas’) are not of water but of basalt. Basalt, or something like it, will certainly be present in large amounts on the surfaces of rocky exoplanets, once we are able to bring them into close enough focus to work out their geology. […] At any one time, ocean floor basalts are the most common rock type on our planet’s surface. But any individual piece of ocean floor is, geologically, only temporary. It is the fate of almost all ocean crust — islands, plateaux, and all — to be destroyed within ocean trenches, sliding down into the Earth along subduction zones, to be recycled within the mantle. From that destruction […] there arise the rocks that make up the most durable component of the Earth’s surface: the continents.”

“Basaltic magmas are a common starting point for many other kinds of igneous rocks, through the mechanism of fractional crystallization […]. Remove the early-formed crystals from the melt, and the remaining melt will evolve chemically, usually in the direction of increasing proportions of silica and aluminium, and decreasing amounts of iron and magnesium. These magmas will therefore produce intermediate rocks such as andesites and diorites in the finely and coarsely crystalline varieties, respectively; and then more evolved silica-rich rocks such as rhyolites (fine), microgranites (medium), and granites (coarse). […] Granites themselves can evolve a little further, especially at the late stages of crystallization of large bodies of granite magma. The final magmas are often water-rich ones that contain many of the incompatible elements (such as thorium, uranium, and lithium), so called because they are difficult to fit within the molecular frameworks of the common igneous minerals. From these final ‘sweated-out’ magmas there can crystallize a coarsely crystalline rock known as pegmatite — famous because it contains a wide variety of minerals (of the ~4,500 minerals officially recognized on Earth […] some 500 have been recognized in pegmatites).”

“The less oxygen there is [at the area of deposition], the more the organic matter is preserved into the rock record, and it is where the seawater itself, by the sea floor, has little or no oxygen that some of the great carbon stores form. As animals cannot live in these conditions, organic-rich mud can accumulate quietly and undisturbed, layer by layer, here and there entombing the skeleton of some larger planktonic organism that has fallen in from the sunlit, oxygenated waters high above. It is these kinds of sediments that […] generate[d] the oil and gas that currently power our civilization. […] If sedimentary layers have not been buried too deeply, they can remain as soft muds or loose sands for millions of years — sometimes even for hundreds of millions of years. However, most buried sedimentary layers, sooner or later, harden and turn into rock, under the combined effects of increasing heat and pressure (as they become buried ever deeper under subsequent layers of sediment) and of changes in chemical environment. […] As rocks become buried ever deeper, they become progressively changed. At some stage, they begin to change their character and depart from the condition of sedimentary strata. At this point, usually beginning several kilometres below the surface, buried igneous rocks begin to transform too. The process of metamorphism has started, and may progress until those original strata become quite unrecognizable.”

“Frozen water is a mineral, and this mineral can make up a rock, both on Earth and, very commonly, on distant planets, moons, and comets […]. On Earth today, there are large deposits of ice strata on the cold polar regions of Antarctica and Greenland, with smaller amounts in mountain glaciers […]. These ice strata, the compressed remains of annual snowfalls, have simply piled up, one above the other, over time; on Antarctica, they reach almost 5 km in thickness and at their base are about a million years old. […] The ice cannot pile up for ever, however: as the pressure builds up it begins to behave plastically and to slowly flow downslope, eventually melting or, on reaching the sea, breaking off as icebergs. As the ice mass moves, it scrapes away at the underlying rock and soil, shearing these together to form a mixed deposit of mud, sand, pebbles, and characteristic striated (ice-scratched) cobbles and boulders […] termed a glacial till. Glacial tills, if found in the ancient rock record (where, hardened, they are referred to as tillites), are a sure clue to the former presence of ice.”

“At first approximation, the mantle is made of solid rock and is not […] a seething mass of magma that the fragile crust threatens to founder into. This solidity is maintained despite temperatures that, towards the base of the mantle, are of the order of 3,000°C — temperatures that would very easily melt rock at the surface. It is the immense pressures deep in the Earth, increasing more or less in step with temperature, that keep the mantle rock in solid form. In more detail, the solid rock of the mantle may include greater or lesser (but usually lesser) amounts of melted material, which locally can gather to produce magma chambers […] Nevertheless, the mantle rock is not solid in the sense that we might imagine at the surface: it is mobile, and much of it is slowly moving plastically, taking long journeys that, over many millions of years, may encompass the entire thickness of the mantle (the kinds of speeds estimated are comparable to those at which tectonic plates move, of a few centimetres a year). These are the movements that drive plate tectonics and that, in turn, are driven by the variation in temperature (and therefore density) from the contact region with the hot core, to the cooler regions of the upper mantle.”

“The outer core will not transmit certain types of seismic waves, which indicates that it is molten. […] Even farther into the interior, at the heart of the Earth, this metal magma becomes rock once more, albeit a rock that is mostly crystalline iron and nickel. However, it was not always so. The core used to be liquid throughout and then, some time ago, it began to crystallize into iron-nickel rock. Quite when this happened has been widely debated, with estimates ranging from over three billion years ago to about half a billion years ago. The inner core has now grown to something like 2,400 km across. Even allowing for the huge spans of geological time involved, this implies estimated rates of solidification that are impressive in real time — of some thousands of tons of molten metal crystallizing into solid form per second.”

“Rocks are made out of minerals, and those minerals are not a constant of the universe. A little like biological organisms, they have evolved and diversified through time. As the minerals have evolved, so have the rocks that they make up. […] The pattern of evolution of minerals was vividly outlined by Robert Hazen and his colleagues in what is now a classic paper published in 2008. They noted that in the depths of outer space, interstellar dust, as analysed by the astronomers’ spectroscopes, seems to be built of only about a dozen minerals […] Their component elements were forged in supernova explosions, and these minerals condensed among the matter and radiation that streamed out from these stellar outbursts. […] the number of minerals on the new Earth [shortly after formation was] about 500 (while the smaller, largely dry Moon has about 350). Plate tectonics began, with its attendant processes of subduction, mountain building, and metamorphism. The number of minerals rose to about 1,500 on a planet that may still have been biologically dead. […] The origin and spread of life at first did little to increase the number of mineral species, but once oxygen-producing photosynthesis started, then there was a great leap in mineral diversity as, for each mineral, various forms of oxide and hydroxide could crystallize. After this step, about two and a half billion years ago, there were over 4,000 minerals, most of them vanishingly rare. Since then, there may have been a slight increase in their numbers, associated with such events as the appearance and radiation of metazoan animals and plants […] Humans have begun to modify the chemistry and mineralogy of the Earth’s surface, and this has included the manufacture of many new types of mineral. […] Human-made minerals are produced in laboratories and factories around the world, with many new forms appearing every year. […] Materials sciences databases now being compiled suggest that more than 50,000 solid, inorganic, crystalline species have been created in the laboratory.”

Some links of interest:

Rock. Presolar grains. Silicate minerals. Silicon–oxygen tetrahedron. Quartz. Olivine. Feldspar. Mica. Jean-Baptiste Biot. Meteoritics. Achondrite/Chondrite/Chondrule. Carbonaceous chondrite. Iron–nickel alloy. Widmanstätten pattern. Giant-impact hypothesis (in the book this is not framed as a hypothesis nor is it explicitly referred to as the GIH; it’s just taken to be the correct account of what happened back then – US). Alfred Wegener. Arthur Holmes. Plate tectonics. Lithosphere. Asthenosphere. Fractional Melting (couldn’t find a wiki link about this exact topic; the MIT link is quite technical – sorry). Hotspot (geology). Fractional crystallization. Metastability. Devitrification. Porphyry (geology). Phenocryst. Thin section. Neptunism. Pyroclastic flow. Ignimbrite. Pumice. Igneous rock. Sedimentary rock. Weathering. Slab (geology). Clay minerals. Conglomerate (geology). BrecciaAeolian processes. Hummocky cross-stratification. Ralph Alger Bagnold. Montmorillonite. Limestone. Ooid. Carbonate platform. Turbidite. Desert varnish. Evaporite. Law of Superposition. Stratigraphy. Pressure solution. Compaction (geology). Recrystallization (geology). Cleavage (geology). Phyllite. Aluminosilicate. Gneiss. Rock cycle. Ultramafic rock. Serpentinite. Pressure-Temperature-time paths. Hornfels. Impactite. Ophiolite. Xenolith. Kimberlite. Transition zone (Earth). Mantle convection. Mantle plume. Core–mantle boundary. Post-perovskite. Earth’s inner core. Inge Lehmann. Stromatolites. Banded iron formations. Microbial mat. Quorum sensing. Cambrian explosion. Bioturbation. Biostratigraphy. Coral reef. Radiolaria. Carbonate compensation depth. Paleosol. Bone bed. Coprolite. Allan Hills 84001. Tharsis. Pedestal crater. Mineraloid. Concrete.

February 19, 2017 Posted by | biology, books, Geology | Leave a comment