Here I try to chat about and list some definitions (as I understand them at least) associated with some loose geological terminology revealed by various groupings of minerals and rocks used by geologists.
Firstly, there are processes whereby groups of minerals are formed, and these are given rock names. Secondly, there names given to groups of rocks, which require an interpretation about the way they formed. Thirdly, there are groups of groups of rocks, which stretch the bounds of what we can say reliably.
(1) Groups of minerals
As an intro 'exsolution' is where phases (usually 2-3) crystallize at the expense of a host crystal and form as lamellae. The exsolved products as a whole is the same composition as host. The geometry of exsolved phases is typically crystallographically controlled. e.g. Perthite: 'Rock' name for exsolved feldpars.
Symplectite: phases (usually 3 or more) crystallize as a corona around host crystal. Composition may vary slightly, involving originally adjacent phases.
Kelyphite: Type of symplectite formed around garnet, typically from kimberlite where it reacts with volcanic liquids to form fine-grained intergrown serpentine+phlogopite+oxides
Myrmekite: Type of symplectite formed within K-feldspar, cosnisting of intergrown quartz and plagioclase.
There are also other kind-of-symplectites which are alteration products of particular minerals involving partial oxidation and/or hydration:
Pinite: Hydrous alteration of cordierite yielding intergrown muscovite+illite+smectite+other clay minerals±oxides
Bastite: is a textural term for platy replacement along cleavage planes in orthopyroxene and rarely clinopyroxene.
Leucoxene: Oxidation of titanium-bearing minerals yielding intergrown rutile+clay minerals+Fe-oxides
Iddingsite: Oxidation ± hydration of olivine yielding intergrown smectite+hematite±Fe-hydroxides
Saussurite: Oxidation + hydration of plagioclase yielding intergrown epidote+sericite±scapolite
Cancrinite: Single mineral formed as as result of hydration and cabronation of nepheline....countless other examples of hydration which forms single replacement minerals ...
(2) Groups of Rocks
On the other end of the spectrum there are many (often debatably used) rock names referring to groups of rocks. Typically the useage is process-oriented and hence interpretation rather than observation is implied:
Ophiolite: sequence of mafic-ultramafic volcanic and intrusive rocks produced as a result of sea-floor spreading, forming typical oceanic crust. This is then thrust onto continental crust and hence preserved resulting in 'ophiolite'.
Migmatite: literally 'mixed-rock' formed of host, melt and restite components. Strictly, melt is derived from in-situ melting of the host but term is also used generally for melts which are injected into hosts.
Amphibolite: a metamorphic facies describing all rocks that have been taken to medium pressure and temperature. Also, specifically referring to a metabasalt composed primarily of amphibole and plagioclase. Granulite, eclogite, blueschist also refer to facies.
Meteorite: wide compositional suite of extra-terrestrial rocks which have landed on earth.
Boninite/Komatiite/Kimberlite: (others): Suites of volcanic rocks defined by their geochemical composition and may easily be confused with similar rocks formed from very different tectonic processes.
Cataclasite/breccia/mylonite: fault/shear zone rocks of particular grain size and texture distributions.
Turbidite: sequence of greywackes and mudstones interlayered (may be formed of repeated 'Bouma sequences') as a result of turbidity currents in the middle depths of oceans.
Hyaloclastite: Volcaniclastic rock consisting of transported broken glass.
Biomicrite: Limestone made of shells set in carbonate mud.
Olistostrome/Olistolith/Melange: useage implying landslide/tectonic activity yielding a mixing of rocks.
(3) Groups of groups of rocks
Groups of groups of rocks are even more strange and interpretive leaving geoloists questioning such things as plate tectonics, plumes and long geological timescales. Some of these are:
Orogens
Continental/oceanic arcs
Continental/oceanic plateaux
Rifts
Coninental margins
Impactites/impact structures
Terranes (continent/microcontinent fragments)
Greenstone belt
Basin
Showing posts with label Random Musings. Show all posts
Showing posts with label Random Musings. Show all posts
2013-07-16
Reading: Mountains of the Mind - a thought dump
Having just finished the brilliant "Captain James Cook: A Biography" by Richard Hough, I am now most of the way through "Mountains of the Mind" by Robert MacFarlane. This combination has been immensely inspiring and enriching to me as a geologist, artist, antipodean dweller...in no particular order.
Cook sailed the seven seas on amazing voyages and saw things that no others had seen. Completion of the task of charting the coastlines of all the continents (except Antarctica but he was close and circumnavigated it for the first time). The first nourished global perspective if you like having gotten close to completing the map of the earth. As adaptable as our minds may be, it is this planetary situation that shapes us the most and its manifestations are nearly all that we interact with. This is something so funamental that we are obliged (perhaps) to have it reflected in it our own nature.
Then comes the Mountains of the Mind...a fantastic progression through the chapters is certainly to a geologist's liking. Cook managed the 2.5D conception and charting of the coastlines in the late 1700s. In the 1800s mountaineering and mapping gave us the third dimension as altitudes and contours were established in large swaths of the globe. During this century Hutton and Darwin gave us conception of great geological and evolutionary time, the until-then missing dimension.
The 1900s provided refinement of all these dimensions with advancement of the microscope, the telescope, radioactive dating, plate tectonics. The documantation of the full dimensionality of our landscapes is something like complete to a particular resolution with fine-scale details and active change being areas in need of constant attention. The Mountains of the Mind really takes the funtamentals of this 3-century-history and examines the way that this interaction between man and planet is ultimately the mirror for our own mind....the circumstance of our existence...the context for life...the interactions of the human condition with the physical. How can we communicate these conceptions about the land? Do we all have a non-verbalized (possibly subconscious) understanding of this that is deeply engrained?
The valleys and oceans of our thoughts can be tested, challenged, sculpted and crystallized against views into the magnificence of it all. Our knowledge of the earth is the fabulous backdrop to the further worlds of the imagination and provide all our metaphors for understanding. I am reminded of this connection very clearly in the words of many passages from Thus Spoke Zarathustra, talking of our feelings of being 'human...all too human' as we place ourselves in context on top of a mountain!
The Mountains of the Mind takes us on a journey through the history of mountaineering. From the evolution of thought from fear to awe to obligatory exploration. From the early conceptions of geological processes to great time. Ideas on ice(!). Human spirit, collaboration. Achieving undoubtedly great things. For me this was a great summary of how it is possibly to put the world together in ones mind independendly of education and socialization. I probably got a taste for this aged 9 on top of peaks of the lake district. It was the the longest day of the year and Dad took me up there to overnight on the cold craggy mountaintops where we huddled in rocky gully watching the sun rise through dramatic peaks at about 4am. Leaping forward to the present I'm still viewing things the same way.
Cook sailed the seven seas on amazing voyages and saw things that no others had seen. Completion of the task of charting the coastlines of all the continents (except Antarctica but he was close and circumnavigated it for the first time). The first nourished global perspective if you like having gotten close to completing the map of the earth. As adaptable as our minds may be, it is this planetary situation that shapes us the most and its manifestations are nearly all that we interact with. This is something so funamental that we are obliged (perhaps) to have it reflected in it our own nature.
Then comes the Mountains of the Mind...a fantastic progression through the chapters is certainly to a geologist's liking. Cook managed the 2.5D conception and charting of the coastlines in the late 1700s. In the 1800s mountaineering and mapping gave us the third dimension as altitudes and contours were established in large swaths of the globe. During this century Hutton and Darwin gave us conception of great geological and evolutionary time, the until-then missing dimension.
The 1900s provided refinement of all these dimensions with advancement of the microscope, the telescope, radioactive dating, plate tectonics. The documantation of the full dimensionality of our landscapes is something like complete to a particular resolution with fine-scale details and active change being areas in need of constant attention. The Mountains of the Mind really takes the funtamentals of this 3-century-history and examines the way that this interaction between man and planet is ultimately the mirror for our own mind....the circumstance of our existence...the context for life...the interactions of the human condition with the physical. How can we communicate these conceptions about the land? Do we all have a non-verbalized (possibly subconscious) understanding of this that is deeply engrained?
The valleys and oceans of our thoughts can be tested, challenged, sculpted and crystallized against views into the magnificence of it all. Our knowledge of the earth is the fabulous backdrop to the further worlds of the imagination and provide all our metaphors for understanding. I am reminded of this connection very clearly in the words of many passages from Thus Spoke Zarathustra, talking of our feelings of being 'human...all too human' as we place ourselves in context on top of a mountain!
The Mountains of the Mind takes us on a journey through the history of mountaineering. From the evolution of thought from fear to awe to obligatory exploration. From the early conceptions of geological processes to great time. Ideas on ice(!). Human spirit, collaboration. Achieving undoubtedly great things. For me this was a great summary of how it is possibly to put the world together in ones mind independendly of education and socialization. I probably got a taste for this aged 9 on top of peaks of the lake district. It was the the longest day of the year and Dad took me up there to overnight on the cold craggy mountaintops where we huddled in rocky gully watching the sun rise through dramatic peaks at about 4am. Leaping forward to the present I'm still viewing things the same way.
Subscribe to:
Posts (Atom)