At least internally, it was hotter than today. Why was this? A few reasons: 1 First, less time had passed since the planet was assembled from an unthinkable number of meteorite impacts, events that instantaneously transformed a very high kinetic energy into heat energy. Remember that [link to numerical dating discussion here] half of a population of radioactive atoms fall apart in the first half-life, followed by a quarter of them in the second half-life, followed by an eighth in the half-life after that, and so on.
Totaling all known radioactive isotope sources suggests something like five times as many radioactive decay events in the Archean as today. So there was a much higher level of heat production in the first eons of Earth history than there was in subsequent eons. The result was that the mantle, and the crust it produced, were both much hotter than they are today. Callan Bentley GigaPan. Though komatiite is a volcanic rock and thus we would expect a mostly fine-grained aphanitic texture , these large crystals of spinifex olivine are a common sight in komatiite outcrops, particularly near the top of the lava flows.
Apparently, as the ultramafic lava lost heat, it began to crystallize most rapidly from the top down. Crystals of olivine nucleated on the chilly upper surface of the flow and then grew downward into its molten interior, hanging into the molten rock like chandeliers on the ceiling.
Meanwhile, chunkier olivine crystals nucleated within the flow, free floating and untethered. These settled to the floor of the komatiite flow, building up a cumulate layer there. Granite is a coarse-grained, felsic igneous rock. Its coarse grained phaneritic texture indicates it cooled slowly, which implies it cooled underground — that is to say, it is intrusive.
Granite is a common rock of the modern continental crust. There are granite-like rocks in Archean cratons, too. Technically, these are not precisely granite, but instead a suite of three similar plutonic rocks: tonalite, trondjhemite, and granodiorite. Here is an example of one such unit: an outcrop of the Kaap Valley Tonalite, a 3. Explore it and and count how many different textural or compositional aspects to the pluton you can find:.
Those minerals are nonferromagnesian and silica-rich, and the result is a felsic magma. Then as now, those magmas cooled slowly underground, developing a coarse-grained texture. Interestingly, the oldest Archean sedimentary rocks are not silicliclastic. There are no sandstones. There are no conglomerates. There are no shales. This implies, quite strongly, that there was no land exposed above sea level — no rock exposures available for weathering and erosion.
But underwater, there were still chemical sedimentary rocks forming due to mineral precipitation from the ocean. Among the oldest Archean sedimentary rocks is chert. It is thick and covers a wide area, and records an extended period of chemical precipitation of silica from seawater, sometime around 3. Examine this boulder of the Buck Reef Chert, and consider its alternating black and white layers, subsequent faulting, veining, and pressure solution, and a modern surface veneer of lichens.
They are chemical sedimentary rocks composed of alternating layers of iron-rich oxides usually hematite or magnetite and silica precipitates chert or jasper. Specifically, BIFs imply a world without widespread free oxygen. But oxygen is a highly reactive element, and it likes to form bonds with other atoms. One possibility, the straightforward one, is that these are primary sedimentary layers: bedding, in other words.
In that case, a rhythmic alteration in sediment source is required. However, another possibility is that the original sedimentation was more or less homogeneous, and only later during diagenesis did the silica-rich and iron-rich components separate out from each other into layers. Later in the Archean, enough land had risen above sea level that significant quantities of siliciclastic sediments were produced: mud, sand, and pebbles.
Turbidity currents flowed into deep-water marine basins of the Archean, adjacent to significant sources of clastic sediment such as proto-continental terranes. Then, as now, these turbidity currents transported a slurry of sediments of different sizes. As the current slowed, its capacity dropped, and sedimentary grains settled out, in order of their weight. The biggest ones tended to be the heaviest ones, and settled out first, followed by progressively more and more lightweight grains which tended to be smaller.
The result was a graded bed: coarse at the bottom, fine at the top. In shallower water, in river deltas and nearshore settings, there were better-sorted, more mature sand deposits, dominated by quartz sand. These are essentially the siliciclastic version of stromatolites. Next, consider an example from the Pongola Supergroup in Africa, as viewed on the bedding plane, and compared with a modern analogue.
Many MISS show soft-sediment and soft-mat! MISS show us that microbial mats were a key feature in Archean sedimentary systems, helping to bind sediment in place.
They also show us that at least one aspect of the biology on Earth has been consistent for billions of years. Some Archean sediments are very similar to modern day conglomerates — full of cobbles and pebbles and sand, that were deposited by fast-moving river currents. Based on their size, sorting, and the rounding of the cobbles in them, we think they accumulated in modestly-sized wrench basins formed in releasing bends along the edges of transpressional embryonic orogens.
The clasts that make up these conglomerates tend to be volcanic in origin, as one might expect for a hot, young, immature planet. As small proto-continental masses were colliding with one another along what we today call greenstone belts, dilational wrench basins opened up, ready to receive the sloughed-off clastic detritus of the primordial mountains.
Some Archean conglomerates contain sand grains and pebbles of pyrite. These detrital pyrites are primary sedimentary grains, not some later manifestation of metamorphism. The fact that pyrite grains could tumble along, getting physically rounded but not chemically weathered is a surefire signal that the atmosphere of the Archean Earth was decidedly oxygen-poor. Another unusual aspect of really old sedimentary rocks is that they record a fair number of violent meteorite impact related rocks.
But during the early Archean, even though the Late Heavy Bombardment had concluded, impacts were still fairly frequent events. Fortunately, we can now begin to read the story of their impact because unlike the Hadean, a rock record still persists from the Archean.
Greenschist contains clots and lenses of blue quartz and abundant sulfide. Unit does not crop out and is known only from subsurface borings and artificial exposures. Southeast Humboldt County and northwest Lander County. Locally includes rocks of Silurian and Devonian age. Includes units such as Valmy Formation of north-central Nevada and some rocks mapped as Palmetto Formation in northern part of Esmeralda County and adjacent parts of Mineral and Nye Counties. New York. Condrey Mountain Schist Triassic?
Potassium-argon age on muscovite from unit is about Ma Lanphere and others, and on a whole rock sample is about Ma Suppe and Armstrong, , indicating a Late Jurassic metamorphic age. Protolith is probably Triassic and Paleozoic in age. Highly sheared graywacke, mudstone, siltstone, and shale with lenses and pods of sheared greenstone, limestone, chert, blueschist, and serpentine.
Identified as melange by some investigators. Complexly folded, locally highly foliated and recrystallized undifferentiated sedimentary and volcanic rocks that in places are lithologically similar to Jurassic and Triassic rocks in the Aldrich Mountains of the Blue Mountains province and in other places resemble Elkhorn Ridge Argillite, Clover Creek Greenstone, and Burnt River Schist Gilluly, Rhode Island.
Blackstone Group - greenstone, amphibolite, serpentinite Late Proterozoic? Blackstone Group - greenstone, amphibolite, serpentinite - Dark-green, massive to foliated metamorphosed gabbroic and basaltic rock interlayered with epiclastic rock; local pillow-structures preserved.
Consists primarily of epidote, actinolite, chlorite, and plagioclase. Includes rock mapped formerly as Hunting Hill Greenstone. South Carolina. Latimer complex of Griffin Paleozoic or Neoproterozoic Latimer complex of Griffin : metamorphosed mafic-ultramafic complex consisting mainly of mafic rocks including amphibolite, metagabbro, and greenstone metabasalt.
South Dakota. Metabasalt Proterozoic Paleoproterozoic Dark-green amphibolite, actinolite schist, and greenstone. Interflow units consists of graphitic schist, chert, and carbonate- and silicate-facies iron-formation.
Thickness of individual flows ft , m. Dark-green sills of amphibolite, actinolite schist, greenstone, and serpentine. Thickness of sills variable, up to 1, ft m. Georgetown Intrusive Suite - Tonalite, quartz gabbro, quartz diorite, metapyroxenite, and hornblendite. Metaperidotite, Metapyroxenite, and Hornblende Metagabbro - Serpentinite is metaperidotite, Hornblende metagabbro. Metavolcanic and Metasedimentary Rocks - Greenstone or amphibole gneiss.
Mount Rogers Formation - Conglomerate, graywacke, laminated siltstone, and shale. Mount Rogers Formation - Greenstone with sedimentary interbeds. Virgilina Greenstone - Greenstone metabasalt. Ammonoosuc Volcanics Ordovician Ammonoosuc Volcanics - Fine-grained chloritic and biotitic gneiss and greenstone in areas north of Bellows Falls; biotite gneiss and amphibolite south of Bellows Falls. Southeastern Vermont. Hazens Notch Formation, Belvidere Mountain Amphibolite Member - Coarse- to fine-grained hornblende-epidote-albite rock; grades to epidote-chlorite-actinolite-albite greenstone where less metamorphosed.
Northern and Central Vermont. Hazens Notch Formation, Greenstone and Amphibolite - Chiefly albite-actinolite-chlorite-epidote greenstone; locally hornblende-epidote-chlorite-albite amphibolite. Hoosac Formation, Amphibolite and Greenstone - Amphibolite and actinolitic greenstone.
Southern and Central Vermont. Discontinuous lenses of metabasalt, informally referred to as Turkey Mountain metabasalt member of Hoosac Formation, actually occur at different stratigraphic positions extending through a stratigraphic distance of to m above base of Hoosac Formation.
From type area on Turkey Mountain in Saxtons River quad south to Massachusetts State line, basalts form at least three relatively persistent units Ratcliffe, Basalt mapped in northeast corner of this map, above Wilmington thrust system, correlates with type Turkey Mountain. As used here, Turkey Mountain metabasalt member consists of several laterally and vertically discontinuous, nonidentical flows and volcaniclastic deposits including, but not restricted to, type Turkey Mountain Member of Hoosac Formation as used by Doll and others and by Skehan They mapped the lower basalts as unnamed greenstones in Hoosac Formation.
Turkey Mountain metabasalt member consists of light-green to dark-green epidote-amphibole greenstones and amphibolite metabasalts. Metabasalt varies from massive to very well layered. Finely laminated, quartzose and epidotitic volcaniclastic beds several centimeters thick are interlayered with more massive, strongly foliated, black amphibolite. Where in contact with surrounding metasedimentary rocks, layering within metabasalt and volcaniclastic beds is concordant and gradational with enclosing metasediment.
Light-gray or yellowish-greenish-gray, well-laminated quartzite or, less commonly, gritty, pebbly conglomerate 0. Base of Turkey Mountain metabasalt member in contact with rusty muscovite-albite-biotite schist. Metabasalts probably originated as thin composite basalt lava flows that contained intercalated basaltic volcaniclastic rocks. Missisquoi Formation, Coburn Hill Volcanic Member - Actinolite-epidote-chlorite-albite greenstone and hornblende-albite-epidote amphibolite; includes pillow lavas.
Orfordville Formation, Post Pond Volcanics - Greenstone, green chloritic schist interbedded with schistose felsite, quartz-feldspar-sericite schist; fine-grained chloritic, biotitic gneiss, all west of Ammonoosuc fault; mainly amphibolite east of the Ammonoosuc fault. Orfordville Formation, Sunday Mountain Volcanics - Greenstone, chloritic schist, felsite, and quartz-feldspar-sericite schist.
Ottauquechee Formation, Greenstone and Amphibolite. The black phyllite contains a previously unreported sub-unit of gray carbonate schist. The Thatcher Brook Member named in an abstract by Armstrong and others, is a carbonaceous albitic schist with greenstones and ultramafics. These rocks have previously been included in the Ottauquechee but have never been differentiated from the black phyllite.
Member is in fault contact with the silvery green schist of the Pinney Hollow Formation to the west. Age is Cambrian Ratcliff, in press. Pinnacle Formation, Tibbit Hill Volcanic Member - Albite-actinolite-chlorite-epidote greenstone; locally pillowed and vesicular. Pinney Hollow Formation, Chester Amphibolite Member - Thin-layered, ligniform amphibolite and hornblende schist; includes actinolitic greenstone and greenstone north of Windham.
Pinney Hollow Formation, Greenstone - Greenstone and actinolitic greenstone. Shaw Mountain Formation - Chiefly tan to brown weathered quartzose limestone and calcareous quartzite characterized by specks of limonite after ankerite; locally underlain by quartz conglomerate and overlain by blue fossiliferous crystalline limestone; greenstone and quartz-sericite schist.
Stowe Formation, greenstone and amphibolite - Epidote-albite-chlorite rocks contain actinolite and hornblende where more metamorphosed. Underhill Formation, Greenstone - varied composition including albite-chlorite-epidote-calcite and sericite-magnetite-chlorite-clinozoisite rocks. Waits River formation, Standing Pond Volcanic Member - Amphibolite, garnet amphibolite, coarse garnet schist with fasciculitic hornblende, and hornblende maculite; contains pillow lavas near St.
Johnsbury and passes eastward into actinolitic greenstone and greenstone south of Windsor. Carboniferous and Permian volcanic rocks Devonian to Permian; Triassic in Asotin County Predominantly altered andesite, basalt, and diabase with interbedded chert and argillite; includes some tuff, greenstone, and spilitic volcanic rocks; northern Cascade Mountains.
Mostly schistose greenstone, some agglomerate, and rarely lapilli; includes minor beds of limestone with associated argillite and graywacke; northwestern Stevens County.
Sedimentary and volcanic rocks, undivided. Cherty and slaty argillite, siltstone, graywacke, chert, greenstone, tuff, andesite, and spilitic volcanics. Altered basalt, pillow lavas, and flow breccia of inner volcanic belt of Olympic Peninsula; includes minor interbedded red limy argillite and associated manganese ore.
Mainly black to gray slate or slaty argillite, argillite, black to dark-gray siltstone in north-central Stevens County and grayish olive-green silty argillite in west-central Stevens County. Many occurrences of Early and Middle Ordovician graptolites; also rare conodonts. Conglomerate, graywacke, siltstone, argillite and interbedded fossiliferous limestone, greenstone, and minor angular conglomerate in northwestern Stevens and Ferry Counties.
Impure quartzite, sandstone, graywacke, greenstone, ribbon chert, chert breccia, and limestone in Snohomish County and on San Juan Island. Middle Permian rocks in northeastern Washington. Gray-brown, coarse, poorly sorted pebbles and cobbles of limestone, dolomite, reddish-brown quartzite, black slate or phyllite, and rarely granitic rocks in a gray sandy phyllite matrix; northeastern Pend Oreille County and southwestern Stevens County. Rocks become finer grained and more schistose and the unit becomes thicker toward the southwest, where there is included an isolated subunit which may be a tillite, consisting of cobbles, boulders, and blocks of argillite and carbonate rocks in a fine silty matrix.
Mostly phyllite with interbedded carbonate rocks, quartzite, and gritstone; some tufflike beds and conglomerate at the base. Mostly homogenous schistose greenstone; in places massive, mottled, and containing conspicuous calcite and epidote. Tuffaceous chlorite schist in upper part in northern Pend Oreille County. Amphibolite and plagioclase amphibolite in Little Pend Oreille Lakes district.
Massive to sheared or schistose greenstone with dark-green ovoid spots; agglomeratic and amygodaloidal in places; sheared pillows near Blue Creek, central Stevens County; minor intrusive phase and probable center of eruption west of Finch magnesite quarry; central to southwestern Stevens County. Predominantly sedimentary rocks.
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