The Lithosphere and Tectonic System Chapter 12 The Lithosphere and Tectonic System The theory describing the changing configuration of the continents through time is called plate tectonics. Plate tectonic theory maintains that the Earth s outermost solid layer consists of huge, rigid plates that float on a layer of plastic rock. These plates are in slow, constant motion, powered by energy sources deep within the planet. When two plates come together, one may be forced under the other in a process called subduction. The Earth s Structure The Earth s Interior The Earth is an almost spherical body, approximately 6,400 km in radius. The centre of the Earth is occupied by the core, which is about 3,500 km in radius (two zones: the outer core which has the properties of a liquid; the inner core is solid). Even though temperature increases toward the Earth s centre, the inner core remains solid because it is under very high pressure. 1
The Earth s Structure The Earth s Interior Earth s magnetic field is believed to originate from a dynamo effect set up by convection currents within the molten outer core in conjunction with Earth s rotation. Based on various geophysical data, it has long been inferred that the core consists mostly of iron, with some nickel. The core temperature is estimated at 3,000 C to 5,000 C. The Earth s Structure The Earth s Interior Enclosing the metallic core is the mantle, a rock shell about 2,900 km thick, which accounts for about 50 percent of the Earth s mass. Seismic data indicate that mantle rock is composed of magnesium and iron silicates similar to the ultramafic igneous rock peridotite. Temperatures in the mantle range from about 2,800 C near the core to about 1,800 C near the crust, and are maintained by the release of energy through radioactive decay of uranium, thorium, and potassium isotopes. 2
The Earth s Structure The Earth s Interior The outermost and thinnest of the Earth s layers is the crust. It is formed mainly of igneous rock, but also contains substantial proportions of metamorphic rock and a comparatively thin upper layer of sedimentary rock. The Earth s Structure The Earth s Interior Two crustal types are distinguished: Oceanic crust, which accounts for about 0.1 percent of Earth s mass, can be up to 10 km thick under the oceans, but is absent beneath the continents. Continental crust accounts for about 0.4 percent of Earth s mass and is present only in continental areas where it is generally 30 to 40 km in thickness. The Earth s Structure The Lithosphere and Asthenosphere The lithosphere, a zone of rigid, brittle rock, includes not only the crust, but also the cooler, upper part of the mantle (ranges in thickness from 60 to 150 km, and is thickest under the continents consisting of lithospheric plates). Deep within the Earth, the brittle condition of the lithospheric rock gives way gradually to a plastic layer named the asthenosphere, where temperatures reach 1,400 C (density increases from 3.7 to 5.5 g cm -3 with depth). 3
The Earth s Structure The Lithosphere and Asthenosphere At still greater depth in the mantle, the strength of the rock material increases again. Thus, the asthenosphere h forms a soft layer between the hard lithosphere above and a strong mantle rock layer below. Like great slabs of floating ice, lithospheric plates can separate from one another at one location, while elsewhere they may collide and push up mountain ranges. The Geologic Time Scale The solid Earth originated some 4,600 million years ago. This long period of time can be considered in two different ways from a geological perspective. The chronostratic approach to geological time is based on the relative age of rock types and events, and uses fossil assemblages and an idealized set of sedimentary rock strata as its reference. The chronometric approach establishes the absolute age of earth materials from radiometric and other dating methods. 4
The Geologic Time Scale The foundation of chronostratic dating is the geologic column that is made up of all the layers of sedimentary rock that have formed at some time in the geologic past. Differences e in sedimentary e rock types and fossil sequences are used to subdivide the geologic column into the familiar units of the geologic time scale. The major divisions in the geologic time scale are termed eons. All Earth materials and events older than 542 million years (Ma) are assigned to the Precambrian. The Geologic Time Scale The Precambrian is divided into the Proterozoic Eon (542 to 2,500 Ma), the Archean Eon (2,500 to 3,800 Ma), and the Hadean Eon (3,800 to 4,600 Ma). From 542 Ma atot the epese present is scovered eedin the Phanerozoic oc Eon. The Phanerozoic is subdivided into three eras the Paleozoic, Mesozoic, and Cenozoic; these eras are subdivided into periods and epochs. 5
Major Relief Features of the Earth s Surface About 29 percent of the Earth s surface is land, with the remaining 71 percent comprising the oceans. However, if the seas were to drain away, broad sloping areas lying close to the continental shores would be exposed. These continental shelves are covered by shallow water, less than 150 m deep. If these continental shelves are considered part of the continents, land area would increase to 35 percent, with a reduction in the area of the ocean basins to 65 percent. 6
Major Relief Features of the Earth s Surface Relief Features of the Continents Continents can be subdivided into two basic regions: active belts where mountain-building processes are still occurring and shield areas composed of old, stable rock. The mountain ranges in the active belts grow either through volcanism or by tectonic activity. Major Relief Features of the Earth s Surface Relief Features of the Continents Volcanism can result in massive accumulations of volcanic rock by extrusion of magma, as seen in the Cascade Mountains of western North America. Tectonic activity involves the breaking, bending, and upthrusting of the Earth s crust due to internal forces associated with the collision of the lithospheric plates. 7
Major Relief Features of the Earth s Surface Ancient Mountain Roots The majority of the crust consists of comparatively inactive regions of much older rock: continental shields and mountain roots. Continental shields are low-lying continental surfaces, beneath which are complex arrangements of ancient igneous and metamorphic rocks. Some core areas of the shields are composed of rocks dating back to the Archean Eon, 2,500 to 3,800 million years ago. Major Relief Features of the Earth s Surface Ancient Mountain Roots Remains of older mountain belts lie within the continental shields in many places. These mountain roots are mostly formed of Paleozoic and early Mesozoic sedimentary rocks that have been intensely bent and folded, and in some locations changed into metamorphic rocks for example, slate, schist, and quartzite. 8
Major Relief Features of the Earth s Surface Relief Features of Ocean Basins The crustal rock of the ocean floors consists almost entirely of basalt, which is generally covered by a comparatively thin accumulation of sediments. Age determinations of the basalt and its overlying sediments show that the oceanic crust is, geologically, quite young. Much of the oceanic crust is less than 60 million years old, although in some cases it has been dated to 135 million years. Mid-Ocean Ridge and Ocean Basin Features The major relief feature of ocean basins is a central ridge, which in the case of the Atlantic Ocean divides the basin approximately in half. This mid-oceanic ridge consists of submarine hills that rise gradually to a rugged central zone. In the centre of the ridge is a narrow, elongated depression known as the axial rift. The location and form of the axial rift suggest that this is a region where the crust is being pulled apart (the age of the rocks on both sides of the ridge increases symmetrically with distance from the rift). 9
Mid-Ocean Ridge and Ocean Basin Features The deepest parts of ocean basins are the ocean trenches that mark the positions of subduction arcs where oceanic crust is being forced down into the mantle. Of the 22 ocean trenches that have been located, 18 are in the Pacific Ocean, three are in the Atlantic Ocean, and one is in the Indian Ocean. The deepest is the Mariana Trench in the Pacific Ocean, about midway between Indonesia and Japan; it is 2,540 km in length, descends to 11,033 m, and is 69 km in width. Mid-Ocean Ridge and Ocean Basin Features Continental Margins The continental margin is the narrow zone where oceanic lithosphere is in contact with continental lithosphere. As the continental margin is approached from the deep ocean, the ocean floor begins to slope gradually upward, forming the continental rise, then becomes much steeper on the continental slope. The top of this slope marks the edge of the continental shelf, a gently sloping platform with vertical relief of less than 20 m. The average width of the continental shelves is about 80 km, but this is quite variable. 10
The Global System of Lithospheric Plates The Earth s crust consists of distinctive lithospheric plates of various sizes; seven of these are considered to be major or primary plates. The Pacific Plate is the largest primary plate and covers about 103.3 million km 2 ; the smallest is the South American Plate, covering 43.6 million km 2. Plate Tectonics The place where two plates meet is a plate boundary. Different types of boundaries are recognized depending on how the adjacent plates are moving in relation to each other. The motion of lithospheric plates and how they interact at their boundaries is collectively called plate tectonics. Many of Earth s large-scale structural and topographic features, or primary landforms, are recognized by the deformation that occurs when moving lithospheric plates collide. Plate Tectonics Plate Motions and Interactions Lithospheric plates move about the Earth s surface at a rate of about 5 to 10 cm a year. It is generally postulated that plate movement is caused primarily by immense convection currents generated by heat energy, derived from radioactive decay of unstable isotopes in the crust and mantle. Plate movement is also linked to density differences within the mantle due to rock chemistry and mineral composition. 11
Plate Tectonics Plate Motions and Interactions The process in which one plate is carried beneath another is called subduction; the descending slab is heated and softened, and is eventually incorporated into the mantle. The zone of separation, or rift, between the lithospheric plates along the axis of a mid-oceanic ridge is called a divergent or spreading boundary. 12
Plate Tectonics Plate Motions and Interactions A third type of lithospheric plate boundary occurs where two lithospheric plates are in contact, but one plate merely slides past the other (transform boundary where movement of the plates causes neither separation nor convergence San Andreas Fault). 13
Plate Tectonics Oceanic Processes Prominent mountain masses and mountain chains can also be elevated by one of two basic tectonic processes: extension and compression. Extensional tectonic activity occurs where oceanic plates pull apart or where a continental plate undergoes breakup. Plate Tectonics Oceanic Processes Compressional tectonic activity occurs at converging plate boundaries. The result is often an alpine mountain chain consisting of intensely deformed rock strata (folds). The entire deformed rock mass produced by compression is called an orogen, and the event that produced it is an orogeny. Fold Belts Plate Tectonics The wavelike shapes imposed on the strata consist of alternating arch-like upfolds, called anticlines, and troughlike downfolds, called synclines. 14
Plate Tectonics Fault Landforms One common type of fault associated with crustal rifting is the normal fault in which the plane of slippage, or fault plane, is steeply inclined, and the rocks on one side are displaced relative to the other. This may arise either through the land on one side being lowered, or downthrown, or because movement has occurred on both sides of the fault. If the land is raised, or upthrown, the process is referred to as a reverse fault (thrust sheet or nappe). 15
Plate Tectonics Fault Landforms Normal faults commonly occur in multiple arrangements, often as intersecting sets of parallel faults, where land may drop down to form a graben or rise as a horst. The resulting topography is a series of trenches and plateaus (block mountains, as in the Basin and Range district of the western US). The East African Rift Valley system, which extends some 3,000 km from the Red Sea southward through the Afar region of Ethiopia to the Zambezi River, illustrates the process on a continental scale. 16
Plate Tectonics Fault Landforms Where two continental lithospheric plates converge along a subduction boundary and collide the result is an orogeny that causes complex folding and faulting of the crustal rocks. A mass of metamorphic rock is formed between the joined continental plates, welding them together and terminating further tectonic activity along that collision zone (continental suture). 17
Plate Tectonics Continental Ruptures and New Ocean Basins Passive continental margins are formed when a single plate of continental lithosphere breaks apart. Continental rupture - starts when the crust is lifted and Continental rupture starts when the crust is lifted and fractured, creating a region of upthrown block mountains and intervening basins (long narrow rift valleys). 18
Plate Tectonics Continental Ruptures and New Ocean Basins As separation continues, the rift valley widens and opens to the ocean to form a narrow sea with a spreading plate boundary running down its centre. Continued widening of the basin results in a large ocean with the continental lithosphere moved far apart. 19
Continents of the Past Geologic evidence suggests a supercontinent, known as Rodinia, was fully formed about 700 million years ago. Rodinia broke apart and its fragments were carried away in different directions. Later these ancient continents converged toward a common o centre, where they collided and joined to form a more recent supercontinent, called Pangaea, about 200 million years ago. Assuming that similar supercontinent cycles began 3 billion years ago (Middle Archean time), it is feasible that six to ten such events could have occurred (Wilson Cycle). 20
Continents of the Past Credit for the first full-scale scientific hypothesis of the breakup of a single large continent belongs to Alfred Wegener, who offered geologic evidence as early as 1915 that the continents had once been united and had drifted apart. He postulated that the supercontinent, Pangaea, existed about 300 million years ago in the Carboniferous Period. A Look Ahead The tectonic activity processes discussed in Chapter 12 can also lower crustal masses to form depressions that may be occupied by ocean embayments or inland seas. Chapter 13 discusses the distribution, morphology, and p p gy forces that create these major structural features. 21