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Archean Felsic Volcanic Rocks
Archean felsic volcanic rocks are felsic volcanic rocks that were formed in the Archean Eon (4 to 2.5 billion years ago). The term "felsic" means that the rocks have silica content of 62–78%. Given that the Earth formed at ~4.5 billion year ago, Archean felsic volcanic rocks provide clues on the Earth's first volcanic activities on the Earth's surface started 500 million years after the Earth's formation. As the Archean Earth was hotter than the present, formation of felsic volcanic rocks may differ from the modern plate tectonics. Archean felsic volcanic rocks are distributed only in the preserved Archean greenstone belts, where deformed sequences of volcanic-sedimentary rocks are common. Felsic volcanic rocks are rare in the early Earth and only contribute to less 20% of rocks in the Archean greenstone belts worldwide. Nonetheless, mafic volcanic rocks (such as basalt and komatiite, silicate content <52%) occupy about 50% in the greenstone belts. [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Archean Felsic Volcanism-01
The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under an ocean deeper than today's ocean. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The earliest known life started in the Archean. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen. Etymology and changes in classification The word ''Archean'' comes from the Greek word (), meaning 'beginning, origin'. It was first used in 1872, when it meant 'of the earliest geological age'. Before the Hadean Eon was recognized, the Archean sp ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mineralogy
Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical) properties of minerals and mineralized artifacts. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization. History Early writing on mineralogy, especially on gemstones, comes from ancient Babylonia, the ancient Greco-Roman world, ancient and medieval China, and Sanskrit texts from ancient India and the ancient Islamic world. Books on the subject included the ''Naturalis Historia'' of Pliny the Elder, which not only described many different minerals but also explained many of their properties, and Kitab al Jawahir (Book of Precious Stones) by Persian scientist Al-Biruni. The German Renaissance specialist Georgius Agricola wrote works such as '' De re metallica'' (''On Metals'', 1556) and ''De Natura Fossilium'' ( ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as ''physical'' or ''mechanical'' erosion; this contrasts with ''chemical'' erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres. Agents of erosion include rainfall; bedrock wear in rivers; coastal erosion by the sea and waves; glacial plucking, abrasion, and scour; areal flooding; wind abrasion; groundwater processes; and mass movement processes in steep landscapes like landslides and debris flows. The rates at which such processes act control how fast a surface is eroded. Typically, physical erosion procee ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Magma Chamber
A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it upwards. If the magma finds a path to the surface, then the result will be a volcanic eruption; consequently, many volcanoes are situated over magma chambers. These chambers are hard to detect deep within the Earth, and therefore most of those known are close to the surface, commonly between 1 km and 10 km down. Dynamics of magma chambers Magma rises through cracks from beneath and across the crust because it is less dense than the surrounding rock. When the magma cannot find a path upwards it pools into a magma chamber. These chambers are commonly built up over time, by successive horizontal or vertical magma injections. Influx of new magma causes reaction of pre-existing crystals and the pressure in the chamber to increase. Th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ultramafic Rock
Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The Earth's mantle is composed of ultramafic rocks. Ultrabasic is a more inclusive term that includes igneous rocks with low silica content that may not be extremely enriched in Fe and Mg, such as carbonatites and ultrapotassic igneous rocks. Intrusive ultramafic rocks Intrusive ultramafic rocks are often found in large, layered ultramafic intrusions where differentiated rock types often occur in layers. Such cumulate rock types do not represent the chemistry of the magma from which they crystallized. The ultramafic intrusives include the dunites, peridotites and pyroxenites. Other rare varieties include troctolite which has a greater percentag ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Eastern Pilbara Craton
The Eastern Pilbara Craton is the eastern portion of the Pilbara Craton located in Western Australia. This region contains variably metamorphosed mafic and ultramafic greenstone belt rocks, intrusive granitic dome structures, and volcanic sedimentary rocks. These greenstone belts worldwide are thought to be the remnants of ancient volcanic belts, and are subject to much debate in today's scientific community. Areas such as Isua and Barberton which have similar lithologies and ages as Pilbara have been argued to be subduction accretion arcs, while others suggest that they are the result of vertical tectonics. This debate is crucial to investigating when/how plate tectonics began on Earth. The Pilbara Craton along with the Kaapvaal Craton are the only remaining areas of the Earth with pristine 3.6–2.5 Ga crust. The extremely old and rare nature of this crustal region makes it a valuable resource in the understanding of the evolution of the Archean Earth. Description The Easte ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Warrawoona Group
The Warrawoona Group is a geological unit in Western Australia containing putative fossils of cyanobacteria cells. Dated 3.465 Ga, these microstructures, found in Archean chert, are considered to be the oldest known geological record of life on Earth. Description The fossils in this group were discovered by Arthur Hugh Hickman in 1983 in Warrawoona, , a region on the Pilbara craton in the northern part of Pilbara province. Whether or not the fossils are authentic was disputed in the past, as abiotic processes could not be ruled out. Currently the fossils are thought to be of biological origin, however there is no conclusive evidence of fossilized organisms in the formation, and whether the lines in the rock are fossilized stromatolites. The rocks also include felsic volcanic rocks. See also * North Pole, WA for similar issues * Origin of life * Timetable of the Precambrian * Geology of Australia The geology of Australia includes virtually all known rock types, s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Volcano-sedimentary Sequence
A volcano-sedimentary sequence is a stratigraphic sequence derived from the alternation and combination of volcanic and sedimentary events. The volcanic material of these sequences may include lava flows and tephra or reworked volcanic material, for example basaltic sand or pebbles. Areas surrounding volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...s are common settings for volcano-sedimentary sequences. Edvard G Svendsen is a famous Norwegian geologist who first discovered these sequences in the deepest depths of Portswood. Volcanic rocks Stratigraphy Sedimentary rocks {{Petrology-stub ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Supracrustal Rock
Supracrustal rocks (''supra'' (Latin for "above")) are Rock (geology), rocks that were Deposition (geology), deposited on the existing basement rocks of the crust (geology), crust, hence the name. They may be further Metamorphism, metamorphosed from both sedimentary (rock), sedimentary and volcanic rocks. The oldest minerals on Earth are Detritus (geology), detrital zircon grains from the Yilgarn Craton in the Mesoarchaean (3.2–2.8 Ga) Jack Hills, Western Australia dated to up to 4.4 Ga, meaning that granitic continental crust and probably supracrustal rocks formed during the Hadean, within 200 million years of Earth accretion. Most Hadean rocks were probably recycled into the mantle before the end of the eon, however, and such pre-4.0 Ga mineral inclusions, the only traces from the earliest rock formation on Earth, are rare. The Acasta Gneiss, Acasta orthogneisses of the Slave Craton, Canada, are regarded to be the oldest rocks on Earth, dated to 4.06 Ga, in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Granitoid
A granitoid is a generic term for a diverse category of coarse-grained igneous rocks that consist predominantly of quartz, plagioclase, and alkali feldspar. Granitoids range from plagioclase-rich tonalites to alkali-rich syenites and from quartz-poor monzonites to quartz-rich quartzolites. As only two of the three defining mineral groups (quartz, plagioclase, and alkali feldspar) need to be present for the rock to be called a granitoid, foid-bearing rocks, which predominantly contain feldspars but no quartz, are also granitoids. The terms ''granite'' and ''granitic rock'' are often used interchangeably for granitoids; however, granite is just one particular type of granitoid. Granitoids are diverse; no classification system for granitoids can give a complete and unique characterization of the origin, compositional evolution, and geodynamic environment for the genesis of a granitoid. Accordingly, multiple granitoid classification systems have been developed such as those based o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Craton
A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle. Terminology The term ''craton'' is used to distinguish the stable portion of the continental crust from regions that are more geologically active and unstable. Cratons are composed of two layers: A continental ''shield'', in which the basement rock crops out at the surface ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
