LECTURE 10
SEDIMENTARY AND METAMORPHIC ROCKS -- THE ROCK CYCLE
SEDIMENTARY ROCKS
Sedimentary rocks are always derived from preexisting rocks by weathering, then erosion and transportation followed by deposition. Actually, for sedimentary rocks, the table in lecture 1 pretty much summarizes sedimentary rocks. Weathering of just about any mineral yields three components -- quartz, clay and cations in solution. These components go on to make sandstones from the quart, shales from the clay and various chemically or biochemically derived rocks from the cations. This even explains why the oceans are salty and where marine etc. animals got the CaCO3 for their shells.
We can add a few details. First of all rocks can be mature or immature. Mature means the weathering has gone to completion and grains have become ground down and small. In effect all chemical rocks are mature because weathering has placed the cations in solution. So clastic rocks (made of materials like pebbles, sand and clay that travel in the bed or suspended loads) are the ones in which we can talk about degrees of maturity. We can make a classification of clastic sedimentary rocks as follows:
|
COARSE >2mm grains -- A millimeter is about a dime's thickness |
CONGLOMERATE grains rounded |
BRECCIA grains -not rounded sorting especially poor |
|
MEDIUM 1/16-2 mm |
SANDSTONE usually quartz - grains rounded |
arkose and greywacke - not rounded includes feldspar and rock fragments in addition to quartz, poor sorting so some clay or silt present |
|
FINE <1/256 mm |
SHALE grains. clay and fine grained quartz |
RARE but rock flour (see glaciation) |
Chemical (usually biochemical) sedimentary rocks all are derived from the dissolved load. So all cations (Ca, K, Na, Fe, Mg etc.) dissolved during weathering have various dissolved travels according to their different chemistries and biochemistries. Thus Ca usually ends up in some aquatic animal as its shell and when the animal dies Ca ends up (CaCO3) as limestone. Ca may also end up as an evaporitic mineral (gypsum CaSO4.2H2O or anhydrite CaSO4).
K may end up under very arid conditions as KCl (sylvite) a mineral formed as a rock in severely evaporated seas. Potassium (K), since it is about as abundant as Na in rocks would be expected to be about as abundant as Na in the oceans. However, K is strongly attracted to clay minerals and, though not a part of their composition, is adsorbed to their surface (stuck there by static electricity -- clays are small grains of minerals that, because they are small, have lots of surface area and thereby unsatisfied electrical charges. The Ks are all electrically charged and so this partially explains the attraction.. Na goes off to sea and, under evaporation conditions, does form rock salt deposits (halite = NaCl). Some is used by animals, but when they die it reenters the sea because it is in the animal soft parts (unlike Ca). Na essentially accumulates as NaCl in the oceans.
At one time, it was thought one could date the oceans by getting the average Na brought in the dissolved load and divide the total amount in the oceans by the annual amount. This division lead to about a hundred million years (remember for the future). However it was noticed that the amount of NaCl in rainfall near the oceans was greater than far away. It soon became learned that evaporation in oceans near wave crests put fine grained NaCl into the atmospheric circulation so it could be a part of any resulting rainfall. It is now thought that NaCl is removed from the ocean by this process about as fast as it reenters.
Fe if thoroughly oxidized (Fe+3 which forms Fe2O3) is fairly insoluble and stays in the "B" soil horizon until that horizon is eroded. The less oxidized Fe+2 which forms FeO (or in a mineral -- magnetite FeO.Fe2O3 is the same as Fe3O4). For now the point is there are lots of sedimentary iron deposits. Mg can go to form the rare evaporitic mineral magnesite (MgCO3) however, usually it substitutes in Calcite for half the Ca ions to change calcite to the new mineral dolomite (CaMg(CO3)2. This mineral (also a rock name) is characteristic of Chicago rocks. Lots of old limestone has been converted to dolomite. Ca and Mg have different sized ions (Mg is smaller). The reason why this substitution is possible is that calcite and dolomite are actually layered carbonates and the Mg comes to occupy the totality of every other layer of cations.
Anyway the common chemical (usually biochemical) sedimentary rock is limestone. Then there is its replacement, dolomite, and then other relatively rare ones follow. A few helpful descriptive terms are added.
|
CHEMICAL SEDIMENTARY ROCK |
MINERAL COMPOSITION |
USEFUL DESCRIPTIVE ADJECTIVES |
|
Limestone |
CaCO3 |
if sandy (arenaceous), if clayey (argillaceous) -- If the dead bodies of animals behave like grains of sand -- calcarenite |
|
Dolomite |
CaMg(CO3)2 |
see above, also note that geologists dislike dolomite for both rock and mineral thus sometimes called dolostone |
|
Evaporites |
NaCl, KCl, CaSO4.2H20, CaSO4 |
see above |
|
Iron Deposits |
Fe2O3.nH2O, Fe2O3, Fe3O4 |
see above |
|
Coal |
C |
see above |
One should note also in sedimentary rock that sea level is rarely steady it either rises (is transgressive) or sinks (is regressive). Imagine a shallow sea undergoing sedimentation. Sand will be near shore followed by fine clastics (which are easier to transport offshore) and finally followed by limestone beyond the clastic limit. If sea level rises the same condition will exist except that the original sand etc. will now be further offshore and will receive shale (and then as sea level rises) limestone on top of it. The result will be transgressive (they cross time) layers sandstone, fine clastics and limestone. As the seas go out this will be expressed upside down as regressive layers. Note that regression exposes the events to erosion whereas transgression covers the events. The result is there is much better representation of transgression in the rock record.
All this may sound complex, but deep down weathering makes quartz and clay and dissolved materials. These go to make the sedimentary rocks.
METAMORPHIC ROCKS
Heating rocks under various pressures makes metamorphic rocks. Deformation (especially folds and faults) is commonly associated with metamorphism and perhaps should have been discussed first.
Metamorphic rocks were little understood by early geologists and little progress was made until 1890. Metamorphic rocks have layers of platey minerals formed during metamorphism and these arrangements much resembled sedimentary rock conditions and the understanding of the difference came slowly.
The formation of glacial ice is very much like the early stages of metamorphism. First snow recrystallizes and makes blobs of ice and then the pores are removed under pressure and the ice forms a crystalline texture -- like a three dimensional picture puzzle --- and all traces of the original shapes of the snow flakes are removed although the composition (except for expulsion of most gases) has not changed.
Pentti Eskola, a Finnish geologist, discovered that many metamorphic rocks of different mineralogical composition, had the same chemical compositions and he concluded they were all originally the same rock (basalt) that had been metamorphosed to different temperatures. He further discovered that he could map zones of these minerals and determine the direction from less to more metamorphosed rocks. The mineral facies are complex and can be here avoided.
However, the eventual idea came about that all rocks start changing as soon as they are formed. A sediment, for example will gradually get buried. If it were buried to 10,000 feet, then, because temperature increases about 1 degree Celsius per 100 feet, the sediments would have been heated about 100 degrees . Added to surface temperature that is greater than boiling temperature. Such minor heat (and oxidation or reduction etc.) happens to all sediments and would be called diagenesis. The further idea is that there are all gradations from fresh rock through diagenesis. (sometimes called lithification) through metamorphism to the eventual melting of the rock so that is difficult or impossible to distinguish it from an igneous rock.
Another feature develops gradually in the directional compression of rocks. That is axial plane cleavage. This is a planar tendency parallel to a slice dividing a fold in half (a plane at right angles to the applied pressure). It starts as a fracture or cleavage (not a true cleavage in the mineralogical sense). Then platey minerals (chlorite first then micas and then amphibole -- a linear mineral arranged in planes) form parallel to the axial plane cleavage. This is a platey tendency generally across the original bedding. However, as a terrain is further deformed, the bedding is squeezed more and more so that it comes to lie almost parallel to the axial plane development most of the time. Thorough search of a metamorphosed area will reveal the difference between the two directions at the crests of the folds. It should be further noted that axial plane cleavage and all that follows is an empirical idea (it is seen to happen, but without an underlying theory to support it).
Nonetheless, given degrees of metamorphism and axial plane thoughts we can make a general classification of metamorphic rocks that works in most situations and reflects a continuum of increasing metamorphism. First comes diagenesis., then slatey cleavage (slates have parallel surfaces that are axial plane cleavage), then schists (chlorite at a low level and then mica develop parallel to the axial plane cleavage. There are several levels to these schists but we will ignore most of them. A higher level is the amphibolite (a schist made of the pencils of amphibole arranged in layers). Then comes gneiss. In a gneiss. the bands of minerals are broad and the idea is the they are so hot that ions move considerable distance to make new minerals. Finally, the rock starts melting and we get a migmatite. A migmatite. is a complexly banded rock with dikes and sills that look igneous. In summary, the sequence is 1. diagenesis., 2. slate, 3. chlorite (green) schist, 4. mica schist, 5. amphibolite schist, 6. gneiss.., and 8. migmatite.
One doesn't want to forget contact metamorphism -- this intensifies toward the intrusion. Also, there are granite gneisses, presumably metamorphic granites although the bands could be flow structures. In a "pure" (nothing is truly pure) sandstone the grains recrystallize into a crystalline texture. They may retain hints of their old rounded grains (see lab and the statements about glaciation above). These recrystallized sandstones would be called quartzite. Later metamorphism (which would be indicated by the impurities) would reflect the scheme above. Similarly a limestone would recrystallize to a marble and then, as the heat increased, would lose its CO2 and the Ca would react with impurities (clay and sand) to make various aluminum silicates. Then this rock would follow the general scheme above as metamorphism increased.
Metamorphic rocks complete the rock cycle. Rocks form in various places and can all be metamorphosed or weathered and eroded. It was a grand discovery (not always completely right at first) of Hutton.
An interesting philosophical approach is to ask the question why don't metamorphic rocks revert (retrogress) back to their original condition as they are cooled and exposed (thereby removing heat and pressure). In chemistry we sort of expect such reversions although not in complex reactions like burning wood. Well first of all some retrogression does occur although not completely. However, deep retrogression is rare. In metamorphism the rock originally contained water and this is heated and reacts and the rest is driven away. Similar things happen to other volatiles (e.g., CO2). The result is that when cooling and pressure reduction occur, retrograde metamorphism can't occur much because the volatiles are gone. Also the reactions may take too long to occur during the cooling time.
Last modified July 8, 1997