Plate Tectonics. The Grand Unifying Theory. Tectonics. Plate Tectonics. Geol 104: Plate tectonics

Transcription

1 Plate Tectonics The Grand Unifying Theory Tectonics Study of the origin and arrangement of broad structural features of Earth s surface. Plate Tectonics Idea that the Earth s surface is divided into a few large, thick plates that move slowly and change in size through time. There are eight major plates and twelve smaller plates. 1

2 Plate Tectonics Model Combined two pre-existing ideas: Continental Drift Continents move freely over Earth s surface changing positions through time Seafloor Spreading Seafloor forms at crest of mid-ocean ridges, then moves symmetrically away from ridge toward oceanic trenches. Continental Drift Proposed by Alfred Wegener in early 1900 s. Evidence cited included: Striking fit between east coast of South America and west coast of Africa. Similarities between continents, including: Ancient climate and glaciation 150 million years ago on land areas presently within 30 of the equator. Fossils of reptiles (like Mesosaurus) and plants Rock strata and structures including mountain belts. Wegener s Model 200 million years ago Supercontinent of Pangea consisting of modern-day North America, Eurasia, South America, Africa, India, Antarctic, Australia separated into two parts. Laurasia - consisting of N. A. and Eurasia. Gondwanaland - consisting of S. A., Africa, India, Antarctic and Australia. 2

3 Wegener s Model 135 million years ago Africa and South America began splitting apart 120 million years ago India split from Gondwanaland 65 million years ago North America split from Eurasia 45 million years ago India collided with Eurasia Mechanism of Continental Drift Larger sturdier continents plow though oceanic crust. Driven by combination of: centrifugal forces of Earth s rotation, and gravitational forces like tidal drag of moon and sun. But, oceanic crust is too strong to be broken by proposed forces. Hence, model was not widely accepted. Seafloor Spreading Harry Hess (1962) proposed seafloor was moving as well as continents. Evidence cited included: Topography of ocean floors (mid-ocean ridges, transform faults, ocean trenchs and guyots) Marine magnetic anomalies/reversals Paleomagnetism Location and depth of earthquakes (Benioff zone) and style of volcanism Age of seafloor rocks and sediments Hotspot traces and ocean island chains 3

4 Hess s Model Seafloor is moving like a conveyer belt from mid-ocean ridges toward ocean trenches. Defined: Spreading center - ridge crest, with seafloor moving away from it. Subduction - sliding of seafloor beneath a continent or island arc. Spreading rates range from 1 to 16 cm/year Mechanism of Seafloor Spreading Driven by deep-mantle convection. Convection refers to slow circulation of a substance driven by differences in temperature (heat) and density within that substance. Hot, less dense magma and rock rises at mid-ocean ridges and cool, denser rock sinks at the trenches. Topography of Seafloor Extensive studies of the topography of the seafloor were initiated during WWII. Important findings included existence of: Mid-ocean ridge system Transform faults Deep ocean trenches Guyots 4

5 Mid-ocean Ridge Systems Continuous marine mountain chain that encircles the globe. Total length exceeds 80,000 km. Ridge rises an average of 3 km above surrounding seafloor. Rift valleys, 1-2 km deep, split the ridge crests. Transform Faults Hundreds of fractures cut across rift valley and mid-ocean ridges. Fractures extend through entire thickness of lithosphere. Offset ridge by <1 km to 100 s of km. Location of shallow, low magnitude earthquakes. Deep Ocean Trenches Long, narrow, steep-sided depressions. Depths of 8 to 11 km. Dip of up to 15 5

6 Guyots Flat-topped seamounts. Seamounts are submarine mountains of volcanic origin that rise 1 km or more above surrounding seafloor Hess proposed that guyots formed as ocean islands, eroded flat by wave action, slowly subsided as the oceanic plate cooled and moved away from the ridge. Marine Magnetics Mid-1960 s, magnetic surveys of seafloor indicated magnetic anomalies arranged in bands parallel to the rift valley of mid-ocean ridges. Alternating positive (normal) and negative (reverse) magnetic anomalies form stripelike pattern parallel to ridge crest. Vine-Matthews Hypothesis Pattern of magnetic anomalies is symmetrical about ridge crest. Same pattern of magnetic anomalies exists over different parts of mid-ocean ridges. Pattern of magnetic anomalies at sea match pattern of magnetic reversals established from studies of continental lava flows. 6

7 Vine-Matthews Hypothesis Proposed origin of magnetic anomalies (see fig of textbook): During time of normal magnetism, series of basalt dikes intrude at ridge crest and become normally magnetized. Dike zone is torn in half and moves away from ridge valley as a new group of reverse magnetized dikes form at ridge crest. Process continues through time producing a symmetrical pattern of normal and reverse magnetized rocks about the ridge crest. Paleomagnetism Study of ancient positions of the continents relative to magnetic poles. Iron-rich minerals like magnetite can act as fossil compass. When lava cools through Curie point, magnetite crystals acquire the direction of earth s magnetic field at that time. Paleomagnetism In 1950 s, it was discovered that magnetic alignment of lava flows of different ages varied widely, but in a systematic fashion. Best explanation for apparent polar wandering is that the plates have changed locations through time. 7

8 Earthquakes and Volcanism Pattern of earthquake and volcanic activity is strongly correlated with boundaries of plates. Earthquake depths and magnitudes, and the style of volcanism also correlated with type of plate boundary. Boundary Earthquakes Volcanism Divergent shallow, low magnitude passive basaltic volcanism Transform shallow, low-high magnitude no volcanism Convergent shallow-deep, low-high magnitude violent andesitic volcanism (Benioff zone) Age of Seafloor & Sediments Seafloor Drilling (ODP) Dating of microfosssils in sediments indicates youngest oceanic crust is at ridge crest and oldest is at ocean trenches. No sediment >160 million years old has been found in ocean basins. Dates of sediments and rocks match estimates from magnetic reversal patterns. Hotspot Traces and Ocean Island Chains Hotspots are narrow columns of hot mantle magma/rocks associated with deep plumes. As seafloor moves over a hotspot, a chain of ocean islands and/or seamounts forms. Dating of Hawaiian islands - Emperor Seamount chain showed systematic increase in ages going away from current volcanism. Show direction and rate of plate motions. 8

9 Plate Tectonics Definitions Plate Tectonics - theory that the Earth s surface is divided into a few large, thick plates that are slowly moving and changing in size. Plates are segments of the lithosphere made of rigid, strong rock that move as a unit over the ductile asthenosphere. Plate Boundaries Tectonic activity is concentrated at plate boundaries where plates interact with each other. There are three types of boundaries based upon relative motion of the plates. Divergent - plates are moving apart. Convergent- plates are moving toward each other. Transform - plates are moving horizontally past each other. Divergent Plate Boundaries Marked by: Rift valleys Shallow focus earthquakes Normal faulting High heat flow Passive basaltic volcanism 9

10 Transform Plate Boundaries Marked by: Shallow focus earthquakes (some high magnitude) Strike-slip faulting Absence of volcanism Convergent Plate Boundaries Cause subduction or continental collision. Marked by: Deep trenches Shallow to deep focus earthquakes (Benioff zone) Reverse faulting low heat flow Violent andesitic volcanism Young mountain ranges or island arcs Ocean-Ocean Convergent Boundary Deep oceanic trench Accretionary wedge Volcanic island arc Violent andesitic volcanism Benioff zone of earthquakes Reverse faulting (compression) along slab edges Normal faulting (tension) within slab Example: West Aleutian Islands 10

11 Ocean-Continent Convergent Boundary Oceanic trench Accretionary wedge Magmatic arc Benioff zone Young mountain belt on edge of continent Uplift by crustal thickening Backarc thrust (reverse) faults Example: Andes mountain chain Continent-Continent Convergent Boundary Suture zone Shallow, large magnitude earthquakes Thrust belts and subsiding basins Crustal thickening by: Shallow underthrusting of one continent Accretion of original island arc Stacking of thrust sheets Mountain belt in interior of continent Example: Himalaya Mountains Characteristics and Examples of Plate Boundaries Type of Type of Geologic Geologic Modern Boundary Plate Features Events Examples Divergent Ocean-Ocean Mid-ocean ridge Sea-floor spreading Mid-Atlantic ridge shallow earthquakes basaltic volcanism normal faulting Continent-Continent Rift Valley Continent torn apart East African Rift shallow earthquakes basaltic volcanism normal faulting Convergent Ocean-Ocean Island arcs and Subduction West Aleutians ocean trenches deep earthquakes andesitic volcanism reverse faullting Ocean-Continent Mountains and Subduction Andes ocean trenches deep earthquakes andesitic volcanism reverse faulting Continent-Continent Mountains deep earthquakes Himalayas reverse faulting Transform Ocean-Ocean Offset of mid-ocean Shallow earthquake East Pacific Rise ridge axis strike-slip faulting Continent-Continent Small mountain Shallow earthquakes San Andreas fault ranges Strike-slip faults 11

12 Plate Motion Caused by: Ridge push Plates cool, thicken, and subside while moving away from ridge axis (plate slides downhill). Slab pull Dense cool slab sinking at steep angle through hot mantle pulls the slab along. Trench suction When plates fall into mantle at angles steeper than their dip, trench literally sucks-down slab. Summary Plate Tectonics explains: Distribution and composition of volcanoes. Distribution and relative magnitude of earthquakes. Occurrence of young mountain belts. Seafloor features Mid-ocean ridges, ocean trenches, fracture zones, seamounts, etc. 12