Chapters 20 and 21: Plate Tectonics and Mountain Building
Early Theories of Orogenic Belts and Mountain Building
1. An orogeny is a term used to define the process of mountain building.
2. Figure 21.7: Orogenic Belts are regions of intense folding and faulting accompanied by granitic intrusions and metamorphism which result from mountain building processes.
3. Orogenic belts therefore denote areas of present or former mountain ranges and, as we shall later see, mark the collision sites of lithospheric plates.
4. Early theories of orogenic belts and mountain building (pre 1950s) did not as yet recognize the concepts of plate tectonics, seafloor spreading or lithospheric plates.
James Hall (1857)
1. In 1857, James Hall noted that mountain belts contained much thicker sedimentary sequences than found in adjacent areas of continents and that this strata was often deformed.
2. Hall reasoned that great loads of sediments must have depressed and bent the crust until it failed. Failure or collapse of the crust, in turn, caused the enclosed strata to crumple and thus form mountains as surface expressions of the deformed strata.
3. Some years later, J.D. Dana in 1873 coined the term geosynclines in referring to old belts of thick, deformed strata along the edges of continents which we now recognize as orogenic belts.
Vertical versus Horizontal Tectonics
1. The intense folding and faulting of strata observed by early scientists led to several hypotheses as to what caused the deformation.
2. James Hutton (late 1700's) argued that mountains resulted from vertical upheaval of crustal material. He envisioned the earth as a heat engine that caused frequent upheavals along the surface. This crustal upheaval and associated folding occurred as a result of emplacement of molten granite from the hot interior to the surface.
3. During the early 1900s, however, many geologists attributed folding and faulting in mountain belts to lateral compression, rather than vertical forces, but mistakenly thought that lateral compression was the result of a shrinking earth.
1. Serious discussion of large-scale continental displacements began in 1908 when F.B. Taylor, an American geologist, suggested that mountain belts represented the wrinkling of crust due to the drifting of continents.
2. Figure 20.1 & 21.1: Alfred Wegener (early 1900's) formulated the idea of drifting continents and continental drift. Much of his theory of continental drift emerged from his ability to fit together now separated continents to form a giant supercontinent. In doing so, he was able to match up major orogenic belts.
3. Alfred Wegener also postulated a mechanism by which subcrustal material yielded to certain forces resulting in crustal blocks flowing slowly across the upper mantle.
4. Figure 20.2: Beginning in 1921, A.L. Du Toit identified identical fossils on widely separated continents. He wondered how similar fossils could be dispersed over entire oceans and concluded that the now separated continents must have at one time been joined. He published his famous book Our Wandering Continents in 1937 in support of continental drift.
5. Figure 20.13: We now recognize continental drift as a viable theory.
1. Once the idea of continental drift took hold in the early part of the 20th century, scientists began looking for an internal mechanism by which to explain the movement of these giant land masses.
2. Figure 19.8: Arthur Holmes (1928) proposed thermal convection as the driving force for continental drift.
3. There was significant opposition to the drift theory continued until as late as 1960's when Holmess idea of convection finally took hold following the advent of plate tectonics.
4. Other mechanisms have since been proposed to explain contental drift as we shall see later.
Evidence for Continental Drift
1. Continents appear to fit together like a jigsaw puzzle
2. Orogenic belts extend across now widely separated continents
3. Similar fossils found on now widely separated continents (e.g. Mesosaurus and Glossopteris)
4. Coal deposits in Antarctica
5. Glacial deposits of similar age found in Africa, S. America, India and Australia
6. Fossils of warm water organisms found in frigid climates
7. Polar Wandering
1. Additional evidence for continental drift was supplied by studies of paleomagnetism in the rock record.
2. Figure 19.13 & 19.14: Physicists knew of rock magnetism as early as 1850 and recognized that certain magnetic minerals record the orientation of the earths magnetic field. These magnetic minerals may align during sedimentation where they become preferentially oriented within the earths magnetic field. Magnetic minerals can also become aligned in igneous rocks, especially basaltic lava flows, during crystallization.
3. It wasnt until the 1950s and 1960s, however, when the study of rock magnetism accelerated.
4. Figure 10.14: Measurements of paleomagnetism in ancient rocks indicate orientations different from that of the earths present magnetic field.
5. Since the Earth's magnetic pole has essentially remained in the same position throughout its history, changes in magnetic directions recorded in rocks of different ages must be due to the drifting of continents.
6. Figure 20.23: Paleomagnetic measurements of magnetically susceptible rocks of different ages can therefore determine how continents drifted through time.
1. Until the 1960s most geologists assumed that the crust beneath the ocean basins was very old, generally flat and fixed in place.
2, Figure 5.30: Extensive shipboard reflection surveys after WWII, however, revealed a seafloor exhibiting broad ridges, deep trenches, escarpments and countless submerged volcanoes called seamounts.
3. Figure 17.27: The most striking features of ocean floors are immense, volcanically active mid-ocean ridges, underwater mountain ranges that snake across the ocean basins.
4. The idea of seafloor spreading can be traced back to Arthur Holmes who in 1928 presented the hypothesis of convection in order to explain continental drift. Holmes, however, did not envision mid-ocean ridges per-say but speculated that ocean basins may have formed through the splitting apart of continents due to upward-flowing mantle.
5. By the 1960s, seismic imaging of the ocean floor had demonstrated the existence of vast submarine mountain ranges known as mid-ocean ridges.
6. Figure 19.8: It was Harry Hess, a geologist from Princton University, who in 1962 envisioned mid-ocean ridges as sites of rising thermal convection cells.
7. The hypothesis of sea-floor spreading postulates that mid ocean ridges are sites where oceanic crust rifts and moves apart laterally. According to Hess, convection cells flow upward, then laterally away from oceanic ridges while carrying oceanic crust along as if on a conveyor belt. His hypothesis explains the high heat flow measurements, numerous earthquakes and submarine volcanic eruptions of basalt that have since been observed along modern active mid-ocean ridges.
Rifts and Hot Mantle Plumes
1. Figure 5.30: : Other evidence for the movement of lithospheric plates comes from lines of submarine volcanoes called seamounts. Hot spots are responsible for the eruption of seamounts. Hot spots result when blobs of upwelling mantle called mantle plumes impinge upon the base of the overlying lithosphere. Since the hot spot is stationary, the line of seamounts results from the movement of oceanic lithosphere over the hot spot.
2. J. Tuzo Wilson (1960's-70's) determined that seamounts were symmetrically distributed as to relative age outward from mid-ocean ridges such that younger seamounts occurred closer to the ridge axis. The ages of seamounts were determined by measuring the thickness of seafloor sediments and noting the fossils recovered from drilling of deep-sea sediments.
3. Figure 20.7: J. Tuzo Wilson also worked out the mechanism of transform faults that offset segments of mid-ocean ridge, noting that active faulting only occurs between the ridge segments.
4. Figure 20.6: Oceanic crust, in turn, is destroyed by subduction along convergent (ocean-ocean or ocean-continent) plate boundaries.
Characteristics of Mid-Ocean Ridges and Ocean Basins
1. Figure 17.27: Mid-ocean ridges are submarine mountain ranges extending hundreds to thousands of kilometers in length.
(a) Sites of extensive basaltic eruptions where ocean floor is created.
(b) Characterized by high heat flow and shallow earthquakes.
(c) Figure 20.15: Ridge axes may contain numerous submarine hot springs where heated seawater is discharged.
(d) New ocean floor moves laterally away from the spreading center at rates ranging from 1-18 cm/yr.
(e) Figure 20.11: The age of oceanic crust increases away from the ridge axis.
(f) Figure 20.7: Ridges are divided into segments separated by transform faults that compensate for variable spreading rates along the ridge axis.
(g) The overlying thickness of deep-sea sediments increase away from the ridge axis.
(h) Figure 20.11: The oldest crust within an ocean basin is generally adjacent to a subduction zone or continental margin while the youngest occurs along the ridge axis.
Seafloor Magnetic Anomalies and Polarity Reversals
1. F.J. Vine and D.H. Matthews (1963) did the following:
(a) Figure 20.9: Studied ocean floor magnetic anomaly patterns, noting symmetry with respect to mid-ocean ridges.
(b) Established relationships among magnetic anomalies, polarity reversals and seafloor spreading.
(c) Figure 19.16: noted the symmetry of magnetic anomaly profiles with respect to the ridge axis.
(d) Figure 20.10: Vine and Matthews also compared the magnetic anomaly patterns on the seafloor with the magnetic polarity time scale that had been established on land and were able to assign ages to the various magnetic stripes on the seafloor. In addition, isotopic and fossil ages obtained through deep sea drilling of sediments directly above oceanic basalt helped bracket the ages of individual magnetic stripes.
2. Figure 20.11: Age dating of seafloor magnetic anomaly patterns eventually led to construction of maps depicting the ages of various segments of oceanic crust.
1. Figure 18.14 & 20.12: The distribution of earthquake epicenters and most active volcanoes define the plate boundaries.
2. Figure 1.14: There are three types of plate boundaries:
(a) A divergent (constructive) plate boundary is where two plates move away from one another along a mid-ocean ridge. New lithosphere is created along a divergent plate boundary.
(b) A convergent (destructive) plate boundary is where one plate subducts beneath or collides with another plate. Lithosphere is generally destroyed along a convergent plate boundary.
(c) A transform boundary is where one plate slides against another. Lithosphere is conserved (neither destroyed nor created) along a transform boundary.
3. Figure 20.25: There are several proposed Driving Forces for Plate Tectonics
(a) Convection: Large convection cells in the mantle carry lithosphere in a conveyor-belt manner. Cells may reach all the way down to the core boundary.
(b) Slab Pull: Dense subducting oceanic lithosphere pulls the rest of the slab behind it. Convection cells are restricted to the upper mantle.
(c) Sliding: Gravitational sliding of the slab down the slope of mid-ocean ridges.
(d) Slab Push: A spreading ocean ridge pushes the plates away from the ridge.
(e) Hot Plume Model: All upward convection is confined to a few narrow plumes originating at the core/mantle boundary. The downward limbs of convection cells are cold and dense, carrying subducted oceanic plates with them.
Convergent Plate Boundaries
1. Figure 20.18: There are several parts to a typical subduction zone:
(a) Trench: A depression marking the site where one plate descends beneath another.
(b) Accretionary Wedge or Prism: Generally located on the overriding plate close to the trench. Sediments scraped off the subducting plate accumulate on the edge of the overriding plate as a chaotic mixture called a mélange.
(c) Volcanic Arc: The main line of volcanism associated with subduction zones. Volcanic arcs within oceans are called island arcs. Andesitic lavas are common.
(d) Forearc Basin: A depression located between the accretionary wedge and the main volcanic arc. Sediments eroded from adjacent highlands are deposited and accumulate within the forearc basin.
2. Figure 19.22: There are three types of convergent plate boundaries:
(a) Figure 20.18: Ocean-ocean convergence with volcanic island arcs
(b) Figure 20.19: Ocean-continent (Andean-type) convergence with a continental volcanic arc, thick accretionary wedge and large continental volcanic arc
(c) Figure 20.20: Continent-continent convergence with extensive deformation and thickening of continental crust into huge mountain belts.
1. Figures 20.21 & 20.22: A collage is an agglomeration or assemblage of micro-continents (terranes) and arcs separated by suture zones.
2. Collage tectonics results in accreted terrances consisting of old volcanic arcs, ancient seafloor and fragments of continents.
3. The boundaries between adjacent micro-continents and arcs may be marked by suture zones, faults and zones of metamorphism.
4. The process of assembling arcs and micro-continents is referred to as collage tectonics.
Parts Of A Continent
1. Figures 21.2 & 21.3: Cratons: Part of a continent that has not been subjected to major deformation for a prolonged period of time, typically since Precambrian or early Paleozoic time (since ~500 m.y. ago).
2. Basement: The oldest rocks recognized in a given area and composed of igneous and metamorphic rocks over ~500 m.y. old. Consists of one or more old cratons.
3. Shield: A large region of stable, ancient basement rocks within a continent.
4. Orogenic Belt (Orogen): A linear region that has been subjected to folding and other deformation in a mountain-building episode.
5. Platform: A tectonically stable, almost level region of a continent typically containing flat-lying sedimentary rocks. Platforms are usually much younger than basements and shields.