Chapter 3: Plate Tectonics and Earthquakes Plate Tectonics The lithosphere of Earth is broken into pieces called plates. These gigantic pieces pull apart during seafloor spreading at divergence zones, slide past at transform faults, or collide at convergences zones. The study of the movements and interactions of the plates is known as plate tectonics. The tectonic cycle: first, melted asthenosphere flows upward as magma and cools to form new lithosphere on the ocean floor. Second, the new lithosphere slowly moves laterally away from the zones of oceanic crust formation on top of the underlying asthenosphere (seafloor spreading). Third, when the leading edge of a moving slab of oceanic lithosphere collides with another slab, the denser slab turns downward and is pulled by gravity back into the asthenosphere (subduction), while the less-dense, more buoyant slab overrides it. Lastly, the slab pulled into the asthenosphere is reabsorbed. It takes 250 million years to complete this cycle. Development of Plate Tectonic Concept In 1620, Francis Bacon of England noted the parallelism of the Atlantic coastlines of South America and Africa and suggested that these continents had once been joined. During the 1800 s, Eduard Suess presented evidence in support of Gondwanaland; a supercontinent composed of South America, Africa, Antarctica, Australia, India, and New Zealand. Alfred Wegener presented his ideas about continental drift and that all continents had once been united in a supercontinent called Pangaea. Evidence of Plate Tectonics from Seafloor Surveys As the lava erupted from a volcano cools at the surface of Earth, minerals begin to grow as crystals. Some of the earliest form crystals incorporate iron into their structures. After the lava cools below the Curie point, at a out 0 C, ato s in iron-bearing inerals eco e agnetized in the direction of Earth s magnetic field at that time and place. The atoms point towards the magnetic pole of their time: the north magnetic pole (normal polarity) or the south magnetic pole (reverse polarity). Lava flows pile up as sequences of stratified rock, and the magnetic polarity of each rock can be measured. Volcanic rock contains minerals with radioactive elements that allow determination of their age. The oldest rock on the ocean floors are 200 million years old. Evidence of Plate Tectonics from Earthquakes and Volcanoes A popular way of forecasting the locations of future earthquakes uses the seismic-gap method. If segments of one fault have moved recently, then it seems reasonable to expect that the unmoved portions will move next and thus fill the gaps. One segment of a fault can move two or more times before an adjoining segment moves once. A majority of active volcanoes are found on the edges of tectonic plates. The creation of new lithosphere and the destruction of old lithosphere is generally accompanied by volcanic activity. Recycling Earth s Outer Layers
There are 4 tectonic environments: divergent, convergent, and transform plate boundaries, and hot spots o Divergent plate boundaries: Plates are pulled apart under tension at divergent plate boundaries The build-up of magma and heat causes expansion and topographic elevation of the overlying lithosphere, which then fractures and begins to be pulled apart sideways by gravity. It is aided by convection cells moving laterally in the asthenosphere; the main driving mechanism of plate movement. The same pull-apart movement can also split a continent forming a rift zone, a young divergent plate oundary. The Earth s surface ay ulge upward into a dome, causing the elevated rock to fracture into a pattern radiating out from a triple junction, a point where 3 plate edges touch. Gravity can then pull the dome apart, allowing magma to swell up and fill 3 major fracture zones, and the spreading process is initiated. o Convergent plate boundaries: Plates deform under compression at convergent boundaries. If an oceanic plate goes beneath another oceanic plate at a convergent plate boundary, an island arc of violence next to a trench can form. If subducting oceanic plate is pulled beneath a continent-carrying plate, the top of the active volcanoes builds on the continent edge. As the leading edge of the subducting plate turns downward, gravity exerts an even stronger pull on it, which helps tear the trailing edge of the plate away from the spreading centre. o Transform plate boundaries: Oceanic spreading ridges are subjected to shear stress at transform plate boundaries. o Hot spots: Originating deep in the mesosphere, hot spots send up hot, buoyant rock that turns into magma near the surface, building shield volcanoes on the seafloor. Plate Tectonics and Earthquakes The lithosphere is broken into rigid plates that move away from, past, and into other rigid plates. These global-scale processes are seen on the ground as individual faults where Earth ruptures and the two sides move past each other in earthquake-generating events. The varying earthquake hazards that are concentrated at plate edges: o The divergent or pull-apart motion at spreading centres causes rock to fail in tension. o The plates slide past each other in the dominantly horizontal movements of transform faults and are subjected to shear stress. This process creates large earthquakes as the plate boundaries retard movement because of irregularities along the faults. o At subduction zones and in continent-continent collisions, rock deforms mainly
under compression. The convergent motions pulling a 7 to 100 km thick oceanic plate back into the asthenosphere at a subduction zone or pushing continents together. Spreading Centres and Earthquakes Iceland is a nation that exists solely on a hot spot-fed volcanic island portion of the mid- Atlantic ridge. Small to moderate-size earthquakes tend to occur in swarms, as is typical of volcanic areas where magma is on the move. A model explaining how spreading began: o The northeastern portion of Africa sits above an extra-hot area in the upper mantle. The heat contained within this mantle hot zone is partially trapped by the blanketing effect of the overlying African plate and its embedded continent. The hot rock expands in volume and some liquefies to magma. o This volume expansion causes doming of the overlying rock, with resultant uplift of the surface to form topography. The doming uplift sets the stage for gravity to pull the raised landmasses downward and apart, thus creating pull-apart faults with centrally located, down-dropped rift valleys (pull-apart basins). o As the faulting progresses, magma rises up through the cracks to build volcanoes. As rifting and volcanism continue, seafloor spreading processes take over, the down-dropped linear rift valley becomes filled by the ocean, and a new sea is born. The first valley holds the oldest humanoid fossils found to date and is the probable homeland of the first human beings. Convergent Zones and Earthquakes The 3 basic classes of collisions are (1) oceanic plate vs. oceanic plate, (2) oceanic plate vs. continent, (3) continent vs. continent. o These collisions result in either subduction or continental upheaval. If oceanic plates are involved, subduction will occur. o The younger, warmer, less-dense plate will override the older, colder, denser plate, which will then bend downward and be pulled back into the mantle. o If two continents are involved, they will subduct because their huge volume of low-density, high-buoyancy rock simply cannot sink to great depth and cannot be pulled into the denser asthenosphere rock below. Earthquakes result from compressive movements where the overriding plate moves upward and the subducting plate moves downward. Shallow earthquakes occur (1) in the upper portion of the down-going plate, (2) at the bend in the subducting plate, and (3) in the overriding plate. At depths below 100 km, the upper and lower surfaces of the subducting slabs are too warm to generate large earthquakes. Thus, earthquakes occur in the cooler interior area of rigid rock, where stress stored as gravity pulls against the asthenosphere to slab penetration. Continent-Continent Collision o When Gondwanaland began its breakup, India moved northward toward Asia. o With no seafloor left to separate them, India punched them into the exposed underbelly of Asia. India has moved another 2,000 km farther north, causing
complex accommodations within the two plates as they shove into, under, and through each other accompanied by folding, overriding, and stacking of the two continents into the huge mass of the Himalaya and the Tibetan Plateau. o Each year, India continues to move about 5 cm into Asia along a 2,000 km front. This ongoing collision jars a gigantic area with great earthquakes. The affected area includes: India, Pakistan, Afghanistan, the Tibetan Plateau, much of eastern Russia, Mongolia, and most of China. The deadliest earthquake in history occurred in 1556 when about 830,000 Chinese were killed in and near Xi`an on the banks of the mighty Huang River. The sever shaking caused much of the soft silt and sand sediments of the region to vibrate apart and literally behave like fluids. The deadliest earthquake in recent times occurred directly beneath the city of Tangshan in 1976. A fault ruptured at a depth of 11 km in a local response to the regional stress created by the ongoing collision of India with Asia- killed 240,000 people. Hot Spots and Earthquakes When rock liquefies, its volume expands, and neighbouring brittle rock must fracture and move out of the way. The sudden breaks and slips of brittle rock are fault movements that produce earthquakes. When magma is on the move at shallow depths, it commonly generates a nearly continuous swarm of relatively small earthquakes referred to as harmonic tremors. The land surface is commonly uplifted due to the injection of magma below the ground surface. But the land surface is also commonly down-dropped due to withdrawal of magma. Terms to Remember: Plate: a piece of lithosphere that moves atop the asthenosphere. There are a dozen large plates and many smaller ones. Divergence zones: A linear zone formed where plates pull apart as at a spreading centre. Transform faults: A strike-slip fault that connects the end of two offset segments of plate edges such as spreading centres or subduction zones. Convergence zones: A linear area where plates collide and move closer together. This is a zone of earthquakes, volcanoes, mountain ranges, and deep-ocean trenches. Plate tectonics: the description of the movements of plates and the effects caused by plate formation, collision, subduction, and slide past. Continental drift: the movement of continents across the face of the Earth, including their splitting apart and recombination into new continents. Pangaea: a supercontinent that existed during late Paleozoic time when all the continents were unified into a single landmass. Curie point: the temperature above which a mineral will not be magnetic. Magnetic field: a region where magnetic forces affect any magnetized bodies or electric currents. Magnetic pole: either of two regions- the north or the south poles- where the times of force of the magnetic field are perpendicular to the Earth s surface. Hot spots: a place on Earth where a plume of magma has risen upward from deep in the
mantle and through a plate to reach the surface. Plumes: an arm of magma rising upward from deep in the mantle. Bathymetry: the mapping of depths of water in oceans, rivers, and lakes; the underwater equivalent of topography. Spreading centres: the site where plates pull apart and magma flows upward to fill the gap and then solidifies as new ocean floor. Trench: the elongate and narrow troughs where ocean water can be more than twice as deep as usual. Trenches mark the downgoing edges of subducting plates. Epicentre: the point on the surface of the Earth directly above the fault that moved to generate an earthquake (ex. Point directly above the hypocentre) Hypocentre: the initial portion of a fault that moved to generate an earthquake. Hypocentres are below the ground surface; epicentres are projected above them on the surface. Triple junction: a place where three plate edges meet. Island arc: a curved linear belt of volcanoes above an oceanic-oceanic subduction zone (Japan) Panthalassa: a assive, single ocean that occupied 60% of Earth s surface in Late Paleozoic time. Laurasia: a northern supercontinent that included most of North America, Greenland, Europe, and Asia from about 180 to 75 million years ago. Gondwanaland: a southern supercontinent that included South America, Africa, Antarctica, Australia, New Zealand and India from about 180 to 75 million years ago. Slips: the actual displacement along a fault surface or formally continuous points.