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The Rock Cycle IGNEOUS ROCKS C A B Earth’s Changing Face: The basics of plate tectonics A C SEDIMENTARY ROCKS A B METAMORPHIC ROCKS MELTING OF PROTOLITH AND COOLING OF MAGMA B UPLIFT AND EROSION OF PROTOLITH, TRANSPORT AND DEPOSITION OF GRAINS/PRECIPITATES C DEEP BURIAL, HEATING AND COMPRESSION OF PROTOLITH, SOLID STATE ALTERATIONS The changing face of the Dynamic Earth, as well as the Rock Cycle are primarily products of the process known as “Plate Tectonics”. The Dynamic Earth Earth is a very dynamic planet, both externally and internally (but of course, all geological processes that we can actually observe are at the surface). This process explains large-scale movements of portions of the Earth’s crust over geological time. It is due to this dynamic nature of Earth that the three rock classes (Igneous, Sedimentary and Metamorphic) exist at all! Evidence for the presence of continents at considerably different latitudes and longitudes in the distant past (continental drift), the opening and closing of ocean basins, and the formation of mountains is easily rationalized under such a model, as is the concentration of earthquake and volcanic activity in certain portions of the crust. At the same time, it is important to note that the Earth is also the ultimate recycler – any of the three rock classes can be changed into any of the other rock classes. These tranformations are commonly illustrated as a ROCK CYCLE. The processes which drive the Rock Cycle, namely uplift, sea level change, deep burial and melting of crustal rocks are also directly tied to plate tectonic activities. 1 2 First Principles of Plate Tectonics The Earth’s lithosphere is not a continuous sheet, but rather, is broken into multiple plates. These rigid, solid plates rest upon the underlying semi-fluid asthenosphere. The Mechanics of Plate Tectonics and Tectonic Forces Due to their lower density, the lithospheric plates essentially float on the denser asthenosphere, and will shift laterally when acted upon by tectonic forces. A Brief Reminder: Asthenosphere and Lithosphere Another Food Analogy: Scum on Soup If you have ever made Cream of Mushroom Soup or soups of a similar consistency, you will probably have noticed a relatively thick scum forming on top which usually breaks up into smaller pieces once you have heated it for a while. This may be a reasonably good model for understanding plate tectonic processes, assuming that the pieces of scum act in a similar manner to tectonic plates and the underlying liquid soup approximates the behaviour of the asthenosphere. Of all the material in the mantle, the asthenosphere flows most readily. If you remember the analogy of the jelly sandwich “Jelly between the bread”, the asthenosphere relates to the jelly, whereas the overlying lithosphere and underlyihg mesosphere correspond to the two slices of bread. Remember that the lithosphere consist of both the uppermost, brittle part of the mantle (lithospheric mantle) and the crust. Scum (Lithosphere) Plates of Scum Liquid Soup (Asthenosphere) 3 4 Tectonic Forces: Convection Model Slab-Pull Model Convection involves the movement of currents (usually circular) within fluids (liquids and gasses) as a result of density and/or temperature gradients within the fluid. The effects of convection are clearly seen in soup when material bubbles to to surface in certain areas and is seen to be carried downward in other locations. One might visualize plate movement as being controlled by convection, as convection currents in the asthenosphere drag the scum (lithospheric) plates) along. Scum slab (plate) Scum slab (plate) Convection cell This would cause the old, outer edge of a plate to sink in a typical subduction zone setting. In effect, the slab of scum (lithosphere) could be pulled by the sinking, leading edge of the plate (which is subducted when it meets an adjacent slab). A trench marks the area where the plate begins its descent beneath the adjacent plate) Scum slab (plate) Scum slab (plate) Upward flow (new scum created) old scum destroyed As a lithospheric plate cools, it thickens a bit, but more importantly, it increases greatly in density (thermally dense). trench Scum slab (plate) old scum destroyed Convection cell Downward flow Downward flow “Slab pull” old scum destroyed Liquid soup (asthenosphere) Ridge-Push Model Liquid soup (asthenosphere) Scum slab (plate) “Slab pull” old scum destroyed It appears that the neither convection nor “ridge push” are strong enough on their own to move entire plates. However, it is probable that convection and “ridge push” contribute to plate movement to at least a minor degree (note that all models are compatible). Otherwise it may be difficult to explain processes such as obduction which involve the thrusting of more dense oceanic crust on top of less dense continental crust (at least in some cases), and the rifting of continents. “Ridge Push” Scum slab (plate) Scum slab (plate) Injection of liquid soup (magma) between spreading plates (new scum created) old scum destroyed Liquid soup (magma) passively infills space between spreading plates (new scum created) So Which Model is Correct ? Note that a ridge is formed at the site of spreading because the newly solidified material is still warm (thermally buoyant) and floats higher than the older, cooler crustal material farther away from the site of spreading…hence the term “ridge-push” Scum slab (plate) Scum slab (plate) At the present time, it is thought that the majority of plate movement is due to slab-pull. Alternatively, perhaps plates are actively pushed from sites of spreading as new liquid soup (magma) is injected and solidifies. Scum slab (plate) trench Scum slab (plate) old scum destroyed Liquid soup (asthenosphere) 5 Types of Plate Boundaries 6 Differentiation of Oceanic and Continental Lithosphere Plate movement is manifested in two principal types of plate boundaries: 1. A divergent boundary is a boundary between two plates that are moving apart (diverging). New lithosphere is formed at divergent boundaries via seafloor spreading. As already alluded to, the lithosphere underlying the oceans and that underlying the continents differ in density. 2. A convergent boundary is a boundary between two plates that are moving toward one another (converging). Lithosphere is destroyed at convergent boundaries via subduction. Convergent boundary sub Convergent boundary Divergent boundary n ctio du Lithosphere is destroyed here New lithosphere forms here (seafloor spreading) As we will see, this is why we have ocean basins and continents in the first place! sub duc tion Significant differences in other characteristics are also noted. Lithosphere is destroyed here Creation and Destruction of Lithosphere Oceanic lithosphere versus Continental Lithosphere Features on the ocean floor show sites of plate divergence and plate convergence. The mantle part of the lithosphere (lithospheric mantle) is fairly uniform in composition throughout, being composed of ultramafic rocks. Divergent boundaries are represented by mid-ocean ridges (e.g. East Pacific Rise, Mid-Atlantic Ridge) in later stages, and rift valleys (East Africa Rift) in early stages of their development. However, the crustal part of the lithosphere varies in composition. Two types of crust are recognized: Oceanic crust (mafic crust which underlies the oceans). Convergent boundaries are represented by trenches associated with subduction zones (e.g., Peru-Chile trench). Continental crust (intermediate to felsic crust that underlies the continents). 7 8 Effects of lithospheric density and thickness Effects of Lithospheric Thickness: Modelling using water and blocks of wood. Oceanic Crust Average Density: 3.0 g/cm3 Average Thickness: 7 km Maximum Age: 180 million years Continental Crust Average Density:2.7 g/cm3 Average Thickness: 35-40 Maximum Age: 4.0 billion years The top of thick block of a buoyant material will stand higher in the water column than a thin block of the same material. However, the proportion of the block standing above and below the surface of the medium in which it is floating is constant. This concept is termed ISOSTASY (“equal standing”). For example the top surface of thick block of wood stands higher above water level than that of thin block of wood. However, the ratio of material above and below water level is the same for all of these blocks. Note: Average Density of Lithospheric Mantle is 3.3 g/cm3 Both oceanic and continental crust are welded to lithospheric mantle (the hard, brittle, uppermost part of mantle to form lithospheric plates). Bottom line: oceanic lithosphere (oceanic crust + lithospheric mantle) is heavy and thin whereas continental lithosphere (continental crust + lithospheric mantle) is light and thick! Likewise, the great thickness of continental lithosphere allows it to stand very high above the surface of the asthenosphere. Oceanic lithosphere, tending to be very thin, stands low. Ocean basins and continents manifest the combined effects of density and isostasy. Summary Effects of Density: Due to its lower overall density, a block of continental lithosphere will sit higher in the asthenosphere than a block of oceanic lithosphere of the same dimensions Oceanic crust: 3.0 g/cm3 Lithospheric mantle ~ 3.3 g/cm3 The combined effects of higher density and lesser thickness cause oceanic lithosphere to sit lower in the asthenosphere, forming relatively depressed areas of the crust (ocean basins). Continental crust: 2.7 g/cm3 Lithospheric mantle ~ 3.3 g/cm3 The combined effects of lower density and greater thickness cause continental lithosphere to sit higher on the asthenosphere, producing relatively elevated areas of the crust (continental land masses). Asthenosphere (semi-fluid part of mantle) This serves well to explain (in part) why ocean basins form areas of relative depression of the Earth’s crust, while continental plates form areas of relative elevation with respect to the centre of the Earth. There is, however, another factor involved. 9 Where subduction is the dominant plate tectonic process, ocean basins close, and adjacent continental plates move closer to one another (e.g. Pacific Ocean). Why does it matter that continental lithosphere is different from oceanic lithosphere ? • • 10 Oceanic lithosphere is constantly being formed and destroyed (by seafloor spreading and subduction). Therefore, ocean basins are transient features. Continental lithosphere is too buoyant to be destroyed by subduction – in effect, plates of continental lithosphere “go along for the ride” as oceanic lithosphere is created and destroyed. Therefore continents are relatively permanent features (although they can change configuration). Where seafloor spreading is the dominant plate tectonic process, oceanic basins increase in size and adjacent continents move farther apart (e.g. Atlantic Ocean) To better appreciate plate tectonic processes, we should look at plate boundaries in greater detail. 11 12 Mature Divergent Plate Boundaries: Mid-Ocean Ridges Mid-ocean ridges are the best-known form of divergent plate boundary and are the area where seafloor spreading occurs; this produces new oceanic lithosphere. For this reason, these are also known as “Constructive boundaries”. Red Sea-Gulf of Aden: An ocean basin in the making Upper crust of oceanic lithosphere is made of the volcanic rock basalt (aphanitic mafic rock, cooled at surface). East African Rift will probably stop spreading and become a “failed arm” Future ocean basin Lower crust of oceanic lithosphere is made of the plutonic rock gabbro (phaneritic mafic rock, cooled at depth) Incipient Divergent Plate Boundaries: Continental Rift Valleys Early evidence of seafloor spreading Considerable evidence for the mobility of the continents was first brough forth by Alfred Wegener (1880-1930). Largely due to his own curiousity, he made the following observations and conclusions which led to the development of the theory of “Contiental Drift”. Hot plume in mantle upwarps lithosphere of continent (thermal dome) 1. Jigsaw puzzle fit of continents (first noted by Wegener in 1910) Cracks develop in this dome generally in the form of a triple junction (incipient rift valley). Mafic magma is generated by decompression melting. Magma that cools at depth forms gabbro of lower crust. Magma that cools at surface forms basalt. Two of the arms continue to spread, forming oceanic lithosphere of an ocean basin. The remaining “failed arm” stops spreading and becomes filled with sediment (Bay of Fundy basin represents one such failed arms of the Atlantic). This jigsaw puzzle fit of continents suggested the former existence of a supercontinent (a continent composed of several or all of the known continents). 13 Early evidence of seafloor spreading 14 Early evidence of seafloor spreading 3. Distribution of old mountain belts and coal deposits. 2. Fossil evidence (noted by Alfred Wegener in 1915). Still, not all of his colleages were convinced! Wegener also brought forth strong evidence for the linkage and common formation of well-known mountain ranges on either side of the Atlantic Ocean, as well as continuity of coal deposits in South America and Africa. Old mountains belts (Appalachians and Caledonides) are now separated, but if the continents are fit together, these mountain chains form a continuous belt. Fossils of land organisms (of Permian-Triassic periods) such as the reptiles Mesosaurus and Lystrosaurus and the fern Glossopteris were distributed over several continents: how did they get from one continent to another? Some of Wegener’s opponents suggested land bridges that later subsided as the answer, but he showed that this was a preposterous and unnecessary conclusion. More recent evidence of seafloor spreading Symmetry of magnetic stripes (defined by polarity of magnetic minerals in basaltic rock of seafloor-first discovered in1940s) Symmetrical pattern of normal and reverse polarities on either side of a divergent boundary can only be explained if magnetic polarity fluctuates as new crust is continually being formed. On joining up the “jigsaw pieces” such that there was a continuous linkage of fossil occurrences, Wegener convincingly demonstrated that there was reasonably good evidence for the former connection of the continents as such. 15 16 More recent evidence of seafloor spreading The linear arrangement and sequential development of hotspot volcanoes (volcanoes produced by movement of a tectonic plate over a stationary magma plume originating in the mantle). The Hawaiian Islands and Emperor Seamount Chain are prime examples of this. Types of Convergent Plate Boundaries Oceanic-oceanic convergence -subduction of oceanic lithosphere under another plate of oceanic lithosphere -magma produced by hydration melting rises to surface to produce a volcanic island arc (e.g. Japan) Hawaii (“the big island”) is the latest in the series of these volcanic islands. It’s position on the Pacific Plate is directly above the mantle plume at the current time. However, this will change as the plate continues to move. New islands will be formed to the SE of Hawaii, unless the plate changes its direction of motion. Oceanic-continental convergence -subduction of oceanic lithosphere under a plate of continental lithosphere -magma produced by hydration melting rises to surface to form a continental volcanic arc (e.g. Cascades with Mt. St. Helens) Continent-continent collision -where two pieces of continental lithosphere meet (intervening ocean becomes completely closed) -continental lithosphere can’t be subducted, so basically collides, shortens and thickens -Earth’s highest mountain belts produced in this way (e.g. Himalayas) Note: The heights of hotspot volcanoes decrease with increasing distance (and time) from the point of active volcanism (due to cooling and sinking of lithospheric material). Also note the “kink” in Hawaiian-Emperor volcanic island/seamount chain, indicating change in the direction of plate movement. Evidence of Subduction Convergent Plate Boundaries As you know, these are zones where lithospheric plates move toward one another and where oceanic lithosphere is consumed back into the mantle. For this reason, these are also known as “Destructive plate boundaries” • The process of crustal “destruction”, i.e. subduction, ensures that the Earth retains a constant volume. Otherwise, the Earth would be continually expanding in size-we know this isn’t happening. Existence of ocean trenches (deepest areas of the ocean) -a trench is the surface expression of a subduction zone where oceanic lithosphere begins its descent. Explosive volcanoes (hydration melting produces thick, viscous intermediate or felsic magma). Extrusive igneous rocks andesite or rhyolite (and lots of pyroclastic materials). Intrusive rocks include diorite and granite. 17 Other geologic consequences of plate tectonics 18 Another Minor Complication: Transform Plate Boundaries Siliciclastic sediments are derived from the erosion of mountains that ultimately owe their existence to uplift associated with the convergence of plates. If a mountain chain is close to the sea, a sedimentary “clastic wedge” can form through differential sediment transport (mainly by water) and the segregation of sedimentary grains. Transform plate boundaries occur where lithospheric plates move beside one another without significant collision or subduction. These are basically “offsets” that form within moving plates. No oceanic lithosphere is created or destroyed (these are also know as “strike-slip” boundaries). Poorly sorted conglomerates generally occur on land, close to the mountains. Better sorted sands are found close to the shoreline. Mud (silt + clay) is generally deposited offshore. Beyond the reach of mud (i.e. where water is clear), limestone can be deposited on a “carbonate platform”. Mountains (on land) Sea These boundaries occur most commonly in oceanic lithosphere of ocean basins lateral to divergent plate boundaries. They also occur in continental lithosphere (e.g. San Andreas fault). conglomerate sandstone siltstone/shale limestone No magma is generated in this type of boundary. Other geologic consequences of plate tectonics Implications of Plate Tectonics Mountains (without volcanoes in this case) So…Oceanic lithosphere is constantly being created at divergent plate boundaries, destroyed at convergent plate boundaries, and offset at transform plate boundaries slate schist gneiss Again, oceans are temporary features. In the past 600 million years, Atlantic has opened, closed and reopened (we are now witnessing only the latest opening event). The convergence of lithospheric (particularly continental) plates and resultant crustal thickening is the necessary prelude for regional metamorphism (metamorphic grade increases with depth). Contact metamorphism can also occur around chambers of magma generated by hydration melting). 19 20 Continental Drift: 750 Millon years ago to present Artist’s depiction of early stages of breakup of the supercontinent Rodinia about 750 Ma (million years before present). This supercontinent is believed to have formed about 1300 Ma. End of lecture 21