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Chapter 20: The Earth Through Time. Introduction: Tracking Past Plate Motions (1). The evidence in support of plate motions comes from measurements made with the Global Positioning System (GPS). However, evidence of past plate motions is not obtained through GPS.
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Introduction: Tracking Past Plate Motions (1) • The evidence in support of plate motions comes from measurements made with the Global Positioning System (GPS). • However, evidence of past plate motions is not obtained through GPS.
Introduction: Tracking Past Plate Motions (2) • Evidence includes: • Great arc-shaped belts of metamorphic rocks formed by continental collisions. • The eroded remains of island-arc volcanic rocks. • Traces of Earth’s past magnetic field preserved in old lava flows. • Plates have been moving and changing Earth’s surface for at least 2 billion years.
Former Ideas About Continents (1) • In the sixteenth century, it became apparent that the coasts were approximately parallel. • During the nineteenth century, the favored idea was that Earth was originally a molten mass that is cooling and contracting, with the crust being gradually compressed.
Former Ideas About Continents (2) • Scientists discovered at the beginning of the twentieth century that the Earth’s interior is kept hot by radioactive decay. • The Earth might not be cooling but heating up (and therefore expanding). • Heating would cause the Earth that had been covered largely by continental crust to expand and then the continental crust would crack into fragments. • As expansion continued, the cracks would grow into ocean basins and basaltic magma from the mantle would rise through the cracks to form the oceanic crust.
The New Idea: Plate Tectonics (1) • By the middle of the twentieth century, a totally new approach was developed: plate tectonics. • When the lithosphere (岩石圈), called plate, slides sideways across the asthenosphere(軟流圈): • Some parts of the lithosphere can be in compression. • Others can be in tension (that is, being pulled apart). • When a plate split in two, the broken edges of continental crust match perfectly.
The New Idea: Plate Tectonics (2) • The energy needed to move plates comes from the Earth’s internal heat energy, which causes convective flows in the mantle. • Plate tectonics is the only theory ever developed that explains all of the Earth’s major features.
The greatest shortcoming of continental drift was the lack of an adequate mechanism for moving the continents. Wegener proposed that the Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges. Geologists at that time understood enough about the strength of rocks to know that this was highly unlikely. from Cox and Hart, 1986
Pangaea (1) • Alfred Wegener’s theory of continental drift originated when he attempted to explain the match of the shorelines on the two sides of the Atlantic, especially along Africa and South America. • He hypothesized an ancient land mass called Pangaea. • The northern half of Pangaea is called Laurasia, the southern half Gondwanaland.
Pangaea (2) • Laurasia includes: • Eurasia. • North America. • Gondwanaland includes: • India. • Africa. • Antarctica. • Australia. • South America.
300 My BP Figure 20.1 A white area: covered by ice solid lines: contours of 2000-m below sea level, along which the continental crust meets oceanic crust black area: overlapped region; red area: gaps
Present Figure 20.1B white area: old ice sheets arrows: direction of movement of the old ice dashed lines: connecting the glaciated regions if the continents were in the present positions at 300 My BP.
Pangaea (3) • During the late Carboniferous Period, about 300 million years ago, a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia. • This is explained by continental drift: 300 million years ago, the regions covered by ice lay in high, cold latitudes surrounding the south pole, while north America and Eurasia were close to the equator • It remained unconvinced that how the solid rock of a continent could possibly overcome friction and slide across the oceanic crust.
Reunite of Pangaea backward in time Breakup of Pangaea from 200 My BP to the present
Apparent Polar Wandering (1) • From the mid-1950s to the mid-1960s, geophysicists discovered paleomagnetism (古地磁). • Certain igneous and sedimentary rocks can preserve a fossil record of the Earth’s magnetic field at the time and place the rocks formed.
Apparent Polar Wandering (2) • Three essential bits of information are contained in that fossil magnetic record: • The Earth’s polarity (the magnetic field was normal or reversed at the time of rock’s formation). • The location of the magnetic poles at the time the rock formed. • The magnetic inclination (磁傾角, I) (indicating how far from the point of rock formation the magnetic poles lay).
The earth’s magnetic field is close to a dipole field that originated inside the earth.
The earth’s magnetic field is nearly a dipole field, with the dipole axis tilted almost 12° away from the earth’s rotation axis.
Figure 20.2 blue lines: horizontal surface red diamonds: magnetic inclination
Apparent Polar Wandering (3) • The paleomagnetic inclination (I) is a record of the place between the pole and the equator (that is, the magnetic latitude (m) where the rock was formed (use the dipole formula: tanI=2tan m). • In the 1950s, geophysicists determined that the strange plots of paleopole positions indicated apparent polar wandering.
If the continents were not moved in the past, the paleopole of the earth’s magnetic fields would wander along the track denoted by solid lines. Figure 20.3
Seafloor Spreading (1) • In 1962, Harry Hess of Princeton University hypothesized that the topography of the seafloor could be explained if the seafloor moves sideways, away from the oceanic ridges. • His hypothesis came to be called the theory of “seafloor spreading,” and was soon proven to be correct.
Seafloor Spreading (2) • Hess postulated that magma rose from Earth’s interior and formed new oceanic crust along the midocean ridges. • Three geophysicists (Frederick Vine, Drummond Matthews, and Lawrence Morley) proposed that lava extruded at any midocean ridge becomes magnetized and acquires the magnetic polarity that exists at the time the lava cools.
Seafloor Spreading (3) • The oceanic crust contains a continuous record of the Earth’s changing magnetic polarity. • Successive strips of oceanic crust are magnetized with normal and reversed polarity. • The magnetic striping allows the age of any place on the seafloor to be determined. • Magnetic striping also provides a means of estimating the speed with which the seafloor had moved.
Relative velocity of plate motion v can be determined by distance to the ridge d divided by age of the seafloor t, i.e., v = d / t. Figure 20.4
Magnetic Record And Plate Velocities (1) • The most recent magnetic reversal occurred 730,000 years ago. • The oldest reversals so far found date back to the middle Jurassic, about 175 million years ago. • From the symmetrical spacing of magnetic time lines it appears that both plates move away from a spreading center at equal rates.
Magnetic Record And Plate Velocities (2) • All that can be deduced from magnetic time lines is the relative velocity of two plates. • Absolute velocities requires information from GPS measurements (for present-day plate motions), or hot spot tracks, or past plate motions; • Plates with only oceanic lithosphere tend to have high relative velocities (Pacific and Nazca plates).
Magnetic Record And Plate Velocities (3) • Plates with a great deal of thick continental lithosphere, such as the African, North American, and Eurasian plates, have low relative velocities. • Plate velocities vary with the geometry of motion of a sphere. • Plates of lithosphere are pieces of a shell on a spherical Earth. • The motion of a rigid plate on the surface of a sphere can be described by a rotation pole (Euler pole) and a constant angular velocity rotating around the pole.
Magnetic Record And Plate Velocities (4) • One consequence of different plates velocities is that the width of new oceanic crust bordering a spreading center increases with the distance from the rotation pole. • Each transform fault lies on a line analogous to a line of latitude around the rotation pole.
Relict Plate Boundaries In The Geologic Record (1) • Seafloor magnetic strips help us reconstruct plate motion only as far back in time as the Jurassic, some 175 million years ago. • All large expanses of older oceanic crust have been subducted back into the mantle at convergent plate boundaries.
Relict Plate Boundaries In The Geologic Record (2) • The paleomagnetism of continental rock can be used to follow plate motion further back in time, but without the breadth and continuity of seafloor data. • Today’s continents were assembled from many distinct plates or plate fragments. • Small fragments of continental crust that have drifted as a single unit in the Earth history are called terranes.
Ophiolites (1) (蛇綠岩系) • The igneous rock formed at a spreading center, known as midocean ridge basalt (MORB), has a distinctive chemistry; • When MORB is found on land, it usually lies within a body of rock that appears to be a fragment of oceanic crust caught up in a continental collision.
Ophiolites (2) • The minerals that characterize basalt, if buried deep within the collision zone, transform into a assemblage dominated by a distinctive green fibrous mineral called serpentine (蛇紋石). • These serpentine-dominated fragments of oceanic crust found on continents are called ophiolites, from the Greek word for serpent, ophis.
Ophiolites (3) • Their structure matches well the crustal structure expected at a midocean ridge: • At the top is a thin layer of sediment that was deposited on the ocean floor. • Beneath the sediment is a layer of pillowed basalt. • Still deeper are sills of gabbro (輝長岩), the plutonic equivalent of basalt. • Beneath the gabbro sills there is often a layer of peridotite. • The contact between gabbro and peridotite (橄欖岩) is interpreted to be the former Moho at the base of what was formerly oceanic crust.
Subduction Mélange (混雜(成)岩) And Blueschists (1) • Along convergent margins, a distinctive feature is the development of a mélange: a chaotic mixture of broken, jumbled, and thrust-faulted rock. • A sinking plate drags the sedimentary rock formed from accumulated sediment downward beneath the overriding plate.
Subduction Mélange And Blueschists (2) • Caught between the overriding plate and the sinking plate, the sediment becomes shattered, crushed, sheared, and thrust-faulted, forming a mélange. • As the mélange thickens, it undergoes metamorphism, common in many mélange zones, to form low-temperature metamorphism blueschists (藍片岩)and eclogites (榴輝岩).
Back-arc Basins And Plate Extension (1) • When the sinking of a subducting plate is faster than the forward motion of the overriding plate, the margin of the overriding plate can be subjected to tensional (pulling) stress. • If the overriding plate is oceanic or if the extension of a continental margin has progressed to an extreme state, an arc-shaped basin forms behind and parallel to the magmatic arc of the subduction zone.
Back-arc Basins And Plate Extension (2) • Basaltic magma may rise into such a back-arc basin at a newly formed spreading center, and new oceanic crust may form.
Supercontinents And Vanished Oceans (1) • The average composition of continental lithosphere is quartz-rich, compared to olivine and pyroxene-rich oceanic lithosphere. • Continental lithosphere is always less dense than oceanic lithosphere and is not subducted. • Seismic evidence suggests that a root of mantle rock is attached to the base of old, cool continental crust.
Supercontinents And Vanished Oceans (2) • Continental buoyancy can be enhanced by the detachment and loss of this dense mantle root during the final stages of a continental collision. • When plate movement bunches continental fragments together, the heat of the mantle beneath still must escape Earth’s interior. • A supercontinent impedes heat flow from the deep mantle to the surface, effectively forming a layer of insulation.
Supercontinents And Vanished Oceans (3) • Convective motion within Earth’s mantle: • Will accumulate heat and cause thermal buoyancy at the base of the continental lithosphere. • Warms and softens the lithosphere, which begins to rift. • The opening of the Atlantic ocean was heralded by the eruption of large basalt flows; • In some cases, these eruptions persisted and became hot spots (example: the Parana flood basalt in Brazil).
