What is Plate Tectonics? ⢠Tectonics is the study of the origin and ...
What is Plate Tectonics?
Tectonics is the study of the origin and arrangement of the broad structural features of the earths surface, including not only folds and faults, but also mountain belts, continents, and earthquake belts Plate tectonics- earths surface is divided into a few large, thick plates that move slowly and change in size Intense geologic activity occurs at plate boundaries toward one another. The eight large plates and few dozen smaller ones make up the outer shell of the earth (crust and upper part of mantle) Plate tectonics can tell us about earthquake distribution, origin of mountain belts, origin of sea floor topography, distribution and composition of volcanoes..plate interactions at plate boundaries Concept of plate tectonics was developed in 1960s by combining two pre-existing ideascontinental drift and sea-floor spreading o Continental drift- continents move freely over the earths surface, changing their positions relative to one another o Sea floor spreading- hypothesis that the sea floor forms at the crest of midoceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench...the two sides are moving in opposite directions like slow conveyor belts
The early case for continental drift
In early 1900s Alfred Wegener, a german meteorologist made a strong case for continental drift. He noted that South America, Africa, india, Antarctica, and Australia had almost identical late paleozoic rocks and fossils The plant Glossopteris is found in Pennsylvanian and Permian-age rock on all five continents, and fossil remains of Mesosauras, a freshwater reptile, is found in Permianage rocks only in Brazil and South Africa. Fossil remains of lang-dwelling reptiles Lystrosaurus and Cynognathus are found in Triassic-age rocks on all five continents Wegener reassembled the continents to form a giant supercontinent, called Pangea (Pangaea). Pangea initially separated into two parts. Laurasia was the northern supercontinent (NA and Eurasia-excluding India). Gondwanaland was the southern supercontinent (southern hemisphere continents and India which has drifted north) The distribution of Late Paleozoic glaciations strongly supports the idea of Pangea The Gondwanaland continents all have glacial deposits of Late Paleozoic age
If continents are arranged according to wegeners Pangea reconstruction, then glaciations in teh southern hemisphere is confined to a smaller area, and the absence of widespread glaciations in the northern hemisphere becomes easier to explain. Also, the present arrangement of the continents would require that late Paleozoic ice sheets flowed from the oceans toward the continents, which is impossible Paleoclimatalogy-study of ancient climates Glacial features:indicate cold climate near north or south pole Coral reefs: indicate warm water near equator Crossbedded sandstones: indicate ancient deserts near 30 degrees N and 30 degrees S latitude Wegener looked at ancient sedimentary rocks and found paleoclimatic reconstructions suggested polar positions different to those at present...he hypothesized that this indicated the continents had moved and gave strong support to continental drift
Scepticism about continental drift
Wegener proposed that continents ploughed through the oceanic crust, crumpling up mountain ranges on the leading edges of the continents where they pushed against the sea floor...most geologists thought that this idea violated what was known about the strength of rocks at the time Wegener said the driving force for continental drift was combo of centrifugal force from earths rotation and gravitational forces that cause tides...but these forces too small to move continents Little support in NA
Study of the Sea Floor
Several ways to get sample of rock and sediments from sea floor o Rock dredge-rocks broken from sea floor by open steel container dragged over ocean bottom at end of a cable o Corer- weighted steel pipe dropped vertically into the mud and sand of ocean floor to sample sediments o Sea floor drilling- both rocks and sediments sampled..offshore oil platforms, ship with drilling derrick...drill cuts long, rodlike rock cores from ocean floor o Submersibles- submarines used to take geologists to sea floor to observe rocks o Single beam echo sounder- basic tool for indirectly studying the sea floor which measures water depth and draws profiles of submarine topography. A sound signal sent down from ship bounces off the sea floor and returns to ship (determines water depth)
o Multibeam sonar- uses variety of sound sources to produce detailed shaded relief images of the sea floor topography o Sidescan sonar- measures intensity of sound reflected back to the tow vehicle from the ocean floor and provides detailed images of the sea floor and info about sediments and bedforms o Seismic reflection profiler- same as echo sounders but louder noise at lower freq. Sound penetrates the sea floor and reflects from layers within the underlying sediment and rock. The seismic profiler records water depth and reveals the internal structure of the rocks and sediments of the sea floor, such as bedding planes, folds, faults, unconformities o Magnetic, gravity, and seismic refraction surveys also can be made at sea. Deep sea cameras can be lowered to see rock and sediments Detailed maps of ocean floors created in early 1960s..supported dynamic earth surface Geophysical Research
Polar wandering- an apparent movement of the earths poles o New evidence came from rock magnetism..magnetic poles located close to the geographic poles Magnetite in a cooling basaltic lava flow acts like a tiny compass needle, preserving a record of Earth’s magnetic field when the lava cools below the Curie point Paleomagnetism-study of ancient magnetic fields Because magnetic lines of force are inclined more steeply as the north magnetic pole is approached, the inclination (dip) of the magnetic alignment preserved in the magnetite minerals in the lava flows can be used to determine the paleolatitude at which the flow formed. Rocks and their paleomagnetic records have moved as part of migrating tectonic plates. Apparent polar wandering paths are now used to reconstruct continental movement over time Every continent shows a diff position for the Permian pole.. explained in that a single pole stood still while continents split apart and rotated as they moved The paths of European poles and NA poles are of similar shape indicating that the continents were joined together
Recent Evidence for Continental Drift
Most convincing evidence came from greatly refined rock matches btw now separated continents. If continents are fitted together like puzzle then geological features should match from piece to pieces o Esp match btw South America and Africa...match of isotopic ages of rock
o Glacial striations show that during the late Paleozoic Era continental glaciers moved from Africa toward the present Atlantic Ocean, while similar glaciers seemingly moved from the Atlantic Ocean onto South America o Continental glaciers cant move from sea onto land o Many boulders in south American glacial deposits have been traced to source in Africa o Rock in Brazil and Gabon (Africa) Paleomagnetic evidence indicates the direction and rate of drift, allowing maps of old continental positions Pangea was formed by the collision of many small continents long before it split up..movement for past 2 billion years
What is Sea-Floor Spreading (1962)? Harry Hess proposed seafloor might be moving, opposite of Wegener who thought ocean floor was stationary. Seafloor spreading suggests sea floor moves away from mid-oceanic ride as a result of mantle convection. (Away from crest of mid-oceanic ridge, down the flanks of ridge, across deep ocean basin, and finally disappears by plunging beneath continent. Ridge crest also known as spreading axis. Subduction- sliding of sea floor beneath a continent or island arc. Convection- a circulation pattern driven by the rising of hot material and/or the sinking of cold material. (Hot material= lower density so rises, cold= high density) Hot mantle rock rises under these ridges, high heat flow, caused by rise of hot mantle. Basalt eruptions on ridge crests also related to rising rock because here mantle rock is hotter than normal and has partial melting. Horizontal movement of mantle rock away from ridge crests causes tension at crest, forms rift valleys and associated shallow-focus earthquakes. Satellite and global position system (24 satellites) best space geodesy technique to measure earth crust movement. Mantle rock moves horizontally away from ridge crests, piggy backs sea floor (basaltic crest) along with it, cools down, becomes denser, and sinks even further into oceans surface. Downward plunge of cold rock accounts for oceanic trenches, and low heat flow values, also explains large negative gravity anomalies associated with trenches. (sinking of cold rock provides a force that holds trenches out of isostatic equilibrium.)
Interaction between moving and stationary rock= Benioff zones of earthquakes. Also produces andesitic volcanism- which is volcanoes forming on continent edges or island arcs. All rocks and sediments of deep sea floor younger than 200 million years old. (Meso/cenozoic eras) Continents contain rocks formed during Paleozoic Era and Precambrian. (3-4 billion years old) deep sea floor is more than half of earth's surface and is very young compared to continents. This is because new young sea floor keeps being formed by basalt eruptions at ridge crest. Through convection and subduction, old sea floor continually destroyed at trenches and new keeps being formed at ridge crest. Overall NEW near ridge crest, OLD near trenches. unifying theory for both sea floor and continent motion was called plate tectonics. Plate- large mobile slab of rock that is part of Earth's surface. Nazca plate= entirely sea floor, North American plate= both oceanic and continental plates. All large plates contain at least some sea floor Plates are part of rigid outer shell of Earth called lithosphere, which includes rocks of crust and uppermost mantle For oceans (young litho=10km thick, old= 100km) For continents (125-250km thickness) Below litho is asthenosphere- zone of low seismic wave velocity that behaves plastically because of increased temperature and pressure. Acts as lubricating layer allowing plates to move. (may extend 70-200km below oceans) below astheno= more rigid mantle rock. plates made of sea floor will subduct and form oceanic trenches, plates made of continental rock will not subduct. Continental rock less dense (specific gravity 2.7) than oceanic rock (specific gravity 3.0) volcanoes etc are caused by plate interactions not within plates themselves. 3 Types of plate boundaries: Divergent plate boundary- boundary btwn plates that are moving apart. Convergent plate boundary- btwn plates moving toward each other. Transform plate boundary- two plates move horizontally past each other. (marked by shallow focus earthquakes) 2 successful tests proved that plates move: 1.Paleomagnetic Evidence- magnetic reversals: when north and south poles change positions. Normal polarity: when magnetic lines flow from south to north (compass points north) Reversed polarity = opposite. Paleomagnetism- study of ancient magnetic fields recorded in rocks.
Lava flow contains magnetic minerals which stack together and can be isotopically dated by making a magnetic polarity time scale. Earth polarity reverses every 500k years, and takes 10k years for reversal to develop. Anomaly- Deviations of magnetic strength from average. Measured by Magnetometer. 2. Marine Magnetic Anomalies- are arranged in band like (stripes) patterns parallel to rift valley of mid oceanic ridge, alternating positive and negative anomalies. The Morley Vine Matthews Hypothesis: (pg34) Pattern of magnetic anomalies was symmetrical on both sides of the ridge crest. Also noticed that these patterns exist over different parts of mid oceanic ridge. Most important finding that magnetic anomalies patterns match magnetic reversals. Suggest that there is continual opening of tensional cracks within the rift valley in mid crest, filled with basaltic magma that cool to form dikes which record Earth's magnetism at the time of magnetic mineral crystallization. When normal polarity, cooling dikes are normally magnetized, when reversed polarity, cooling dikes reversely magnetized. Extension from moving sea floor then cracks dike in two and halves move in opposite directions. SO DIKES CAUSE ANOMALIES How fast do plates move? 2 important points for MVM hypothesis: 1. allows to measure rate of sea floor motion (or plate motion since continents and seafloor move together as plates) since you already know the magnetic reversal ages from lava flows on land, u use distance of sea floor anomaly from ridge divided by age it was formed. Measured rates generally range from 1-10cm per year. 2. allows for predicting the age of sea floor (distinctive patterns of anomalies through time allows age id. Can predict age of igneous rock of sea floor by measuring anomalies at sea surface) Most sea floor sections have measured anomalies, so by matching measured with known patterns, age of sea floor is found. Fractures Zones and Transform Faults Earthquakes occur along fracture zones but only between offset sections of ridge crest called:
Transform Fault- the portion of a fracture zone between two offset portions of ridge crest. It is seismically active, compared to inactive fracture zones. San Andreas California is a ridge-ridge example btwn North American and Pacific Plate. What Happens at Plate Boundaries? Divergent Plate Boundaries (DB) Figure 2.19 pg 38 Divergence is to create or open new ocean basins. DB marked by rifting, basaltic volcanism, and uplift. Produces shallow focus earthquakes on normal faults, and rift valley forms as a central graben (down dropped fault block). Uplift caused by basaltic magma eruptions from mantle beneath. So first rifting (stretching and thinning of crust), then uplift with high heat flow eruptions (eg. African Rift Valleys) Mid Oceanic Ridges- are giant undersea mountain ranges that extend around the world like the seams on a baseball. Made mostly of basalt, more than 80,000 km long and 2500km wide. Rise 3km above adjacent ocean floor. Rift valley- originates by tension and runs down the crest of each ridge, about 2km deep and several km wide. Present in Atlantic and Indian Oceans, not in Pacific. Shallow focus earthquakes, basalt eruptions, and high heat flow all at rift valleys. Crabs, mussels, clams, starfish, and giant tube worms found near black smokers, (metal sulphides) thermophyllic organisms, studied in geomicrobiology. Rift valley hot springs release, iron, copper, zinc, plus smaller amounts of manganese, gold, and silver. Ridge-ridge is two divergent boundaries (most common transform fault), ridge-trench is divergent and convergent, trench-trench is two convergent boundaries. Convergent Plate Boundaries When two ocean crusts converge there is subduction, oceanic plate with continental also subducts, but if two continents they just collide and crumble, neither subducts. OCEAN-OCEAN Convergence Oceanic Trench- is a narrow, deep trough parallel to the edge of a continent or an island arc (a curved line of islands like the Aleutians or Japan) The continental slope on active margins forms the landward wall of the trench. Steepness increases with depth. Slope is typically 4-5 degrees on upper part, 10-15 degrees on lower.
Elongate ocean trenches are 8-10km deep. Deepest spots on earth are in oceanic trenches in south west pacific ocean. (11km+) Island arc- a curved line of volcanoes that form a string of islands parallel to the oceanic trench. The inner wall of a trench (toward the arc) consists of an accretionary wedge (or subduction complex) of thrust faulted and folded marine sediment and pieces of oceanic crust. New slices of sediment are continually added to the bottom of the accretionary wedge, pushing it upward to form a ridge on the sea floor. The forearc basin lies between the accretionary wedge and the volcanic arc. (The trench side of the arc is the forearc, and the other side of the arc is the backarc.) OCEAN-CONTINENT Convergence When an oceanic plate subducts under the continental lithosphere, the accretionary wedge and forearc basin form an active continental margin between the trench and the continent. A Benioff zone of earthquakes dips under the edge of the continent, which is marked by andesitic volcanism and a young mountain belt. (eg. Nazca plate under western South America, or Juan de Fuca plate under North America) The magma that is created by ocean-continent convergence forms a magmatic arc, a broad term used both for island arcs at sea and for belts of igneous activity on the edges of continents. Batholiths- are large plutons in thickened crust seen on land when they are exposed by deep erosion. Continental interior= the craton CONTINENT-CONTINENT Convergence- at first just like O-C convergence, than ocean in between becomes narrower and narrower until the continents eventually collide and destroy or close the ocean basin. (sea floor gets subducted) Continents cannot subduct. RESULT IS Mountain Belt British Columbia pg 45 Two continents are welded together along a dipping suture zone, that marks the old site of subduction. Backarc Spreading- tension causes regional extension behind an arc. Can split the edge of a continent (how Japan formed) Backarc oceanic crust found in most ophiolites (distinctive rock sequences found in many mountain chains on land.)
Oceanic crust has three layers: layer 1= terrigenous sediment (0.5km), layer 2= basalt dikes (1.5km), layer 3= Sill like gabbro bodies (5km) Orogeny- mountain genesis, is an episode of mountain building, often characterized by intense deformation of the rocks in a region. Ocean-Continent Convergence An accretionary wedge develops where newly formed layers of marine sediment are folded and faulted as they are snowploughed off the subducting oceanic plate. Arc Continent Convergence pg 50 Continent-Continent Convergence What causes plate motions? 1. Mid-oceanic ridge crests are hot and elevated, while trenches are cold and deep 2. Ridge crests have tensional cracks 3. The edges of some plates are subducting sea floor, while the edges of other plates are continents (Which cannot subduct). Mantle Plumes- are narrow columns of hot mantle rock that rise through the mantle from thermal boundary layers at the base of the mantle (or upper mantle) Spherical/mushroom shaped haeads. Are hot spots for volcanism. Failed rift= Aulacogen Guyots- are flat topped seamounts found mostly in the western Pacific Ocean. Aseismic ridges- are submarine ridges that are not associated with earthquakes. Earthquakes occur along the rift valley of midoceanic ridges.
Tectonics is the study of the origin and arrangement of the broad structural features of the earths surface, including not only folds and faults, but also mountain belts, continents, and earthquake belts Plate tectonics- earths surface is divided into a few large, thick plates that move slowly and change in size Intense geologic activity occurs at plate boundaries toward one another. The eight large plates and few dozen smaller ones make up the outer shell of the earth (crust and upper part of mantle) Plate tectonics can tell us about earthquake distribution, origin of mountain belts, origin of sea floor topography, distribution and composition of volcanoes..plate interactions at plate boundaries Concept of plate tectonics was developed in 1960s by combining two pre-existing ideascontinental drift and sea-floor spreading o Continental drift- continents move freely over the earths surface, changing their positions relative to one another o Sea floor spreading- hypothesis that the sea floor forms at the crest of midoceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench...the two sides are moving in opposite directions like slow conveyor belts
The early case for continental drift
In early 1900s Alfred Wegener, a german meteorologist made a strong case for continental drift. He noted that South America, Africa, india, Antarctica, and Australia had almost identical late paleozoic rocks and fossils The plant Glossopteris is found in Pennsylvanian and Permian-age rock on all five continents, and fossil remains of Mesosauras, a freshwater reptile, is found in Permianage rocks only in Brazil and South Africa. Fossil remains of lang-dwelling reptiles Lystrosaurus and Cynognathus are found in Triassic-age rocks on all five continents Wegener reassembled the continents to form a giant supercontinent, called Pangea (Pangaea). Pangea initially separated into two parts. Laurasia was the northern supercontinent (NA and Eurasia-excluding India). Gondwanaland was the southern supercontinent (southern hemisphere continents and India which has drifted north) The distribution of Late Paleozoic glaciations strongly supports the idea of Pangea The Gondwanaland continents all have glacial deposits of Late Paleozoic age
If continents are arranged according to wegeners Pangea reconstruction, then glaciations in teh southern hemisphere is confined to a smaller area, and the absence of widespread glaciations in the northern hemisphere becomes easier to explain. Also, the present arrangement of the continents would require that late Paleozoic ice sheets flowed from the oceans toward the continents, which is impossible Paleoclimatalogy-study of ancient climates Glacial features:indicate cold climate near north or south pole Coral reefs: indicate warm water near equator Crossbedded sandstones: indicate ancient deserts near 30 degrees N and 30 degrees S latitude Wegener looked at ancient sedimentary rocks and found paleoclimatic reconstructions suggested polar positions different to those at present...he hypothesized that this indicated the continents had moved and gave strong support to continental drift
Scepticism about continental drift
Wegener proposed that continents ploughed through the oceanic crust, crumpling up mountain ranges on the leading edges of the continents where they pushed against the sea floor...most geologists thought that this idea violated what was known about the strength of rocks at the time Wegener said the driving force for continental drift was combo of centrifugal force from earths rotation and gravitational forces that cause tides...but these forces too small to move continents Little support in NA
Study of the Sea Floor
Several ways to get sample of rock and sediments from sea floor o Rock dredge-rocks broken from sea floor by open steel container dragged over ocean bottom at end of a cable o Corer- weighted steel pipe dropped vertically into the mud and sand of ocean floor to sample sediments o Sea floor drilling- both rocks and sediments sampled..offshore oil platforms, ship with drilling derrick...drill cuts long, rodlike rock cores from ocean floor o Submersibles- submarines used to take geologists to sea floor to observe rocks o Single beam echo sounder- basic tool for indirectly studying the sea floor which measures water depth and draws profiles of submarine topography. A sound signal sent down from ship bounces off the sea floor and returns to ship (determines water depth)
o Multibeam sonar- uses variety of sound sources to produce detailed shaded relief images of the sea floor topography o Sidescan sonar- measures intensity of sound reflected back to the tow vehicle from the ocean floor and provides detailed images of the sea floor and info about sediments and bedforms o Seismic reflection profiler- same as echo sounders but louder noise at lower freq. Sound penetrates the sea floor and reflects from layers within the underlying sediment and rock. The seismic profiler records water depth and reveals the internal structure of the rocks and sediments of the sea floor, such as bedding planes, folds, faults, unconformities o Magnetic, gravity, and seismic refraction surveys also can be made at sea. Deep sea cameras can be lowered to see rock and sediments Detailed maps of ocean floors created in early 1960s..supported dynamic earth surface Geophysical Research
Polar wandering- an apparent movement of the earths poles o New evidence came from rock magnetism..magnetic poles located close to the geographic poles Magnetite in a cooling basaltic lava flow acts like a tiny compass needle, preserving a record of Earth’s magnetic field when the lava cools below the Curie point Paleomagnetism-study of ancient magnetic fields Because magnetic lines of force are inclined more steeply as the north magnetic pole is approached, the inclination (dip) of the magnetic alignment preserved in the magnetite minerals in the lava flows can be used to determine the paleolatitude at which the flow formed. Rocks and their paleomagnetic records have moved as part of migrating tectonic plates. Apparent polar wandering paths are now used to reconstruct continental movement over time Every continent shows a diff position for the Permian pole.. explained in that a single pole stood still while continents split apart and rotated as they moved The paths of European poles and NA poles are of similar shape indicating that the continents were joined together
Recent Evidence for Continental Drift
Most convincing evidence came from greatly refined rock matches btw now separated continents. If continents are fitted together like puzzle then geological features should match from piece to pieces o Esp match btw South America and Africa...match of isotopic ages of rock
o Glacial striations show that during the late Paleozoic Era continental glaciers moved from Africa toward the present Atlantic Ocean, while similar glaciers seemingly moved from the Atlantic Ocean onto South America o Continental glaciers cant move from sea onto land o Many boulders in south American glacial deposits have been traced to source in Africa o Rock in Brazil and Gabon (Africa) Paleomagnetic evidence indicates the direction and rate of drift, allowing maps of old continental positions Pangea was formed by the collision of many small continents long before it split up..movement for past 2 billion years
What is Sea-Floor Spreading (1962)? Harry Hess proposed seafloor might be moving, opposite of Wegener who thought ocean floor was stationary. Seafloor spreading suggests sea floor moves away from mid-oceanic ride as a result of mantle convection. (Away from crest of mid-oceanic ridge, down the flanks of ridge, across deep ocean basin, and finally disappears by plunging beneath continent. Ridge crest also known as spreading axis. Subduction- sliding of sea floor beneath a continent or island arc. Convection- a circulation pattern driven by the rising of hot material and/or the sinking of cold material. (Hot material= lower density so rises, cold= high density) Hot mantle rock rises under these ridges, high heat flow, caused by rise of hot mantle. Basalt eruptions on ridge crests also related to rising rock because here mantle rock is hotter than normal and has partial melting. Horizontal movement of mantle rock away from ridge crests causes tension at crest, forms rift valleys and associated shallow-focus earthquakes. Satellite and global position system (24 satellites) best space geodesy technique to measure earth crust movement. Mantle rock moves horizontally away from ridge crests, piggy backs sea floor (basaltic crest) along with it, cools down, becomes denser, and sinks even further into oceans surface. Downward plunge of cold rock accounts for oceanic trenches, and low heat flow values, also explains large negative gravity anomalies associated with trenches. (sinking of cold rock provides a force that holds trenches out of isostatic equilibrium.)
Interaction between moving and stationary rock= Benioff zones of earthquakes. Also produces andesitic volcanism- which is volcanoes forming on continent edges or island arcs. All rocks and sediments of deep sea floor younger than 200 million years old. (Meso/cenozoic eras) Continents contain rocks formed during Paleozoic Era and Precambrian. (3-4 billion years old) deep sea floor is more than half of earth's surface and is very young compared to continents. This is because new young sea floor keeps being formed by basalt eruptions at ridge crest. Through convection and subduction, old sea floor continually destroyed at trenches and new keeps being formed at ridge crest. Overall NEW near ridge crest, OLD near trenches. unifying theory for both sea floor and continent motion was called plate tectonics. Plate- large mobile slab of rock that is part of Earth's surface. Nazca plate= entirely sea floor, North American plate= both oceanic and continental plates. All large plates contain at least some sea floor Plates are part of rigid outer shell of Earth called lithosphere, which includes rocks of crust and uppermost mantle For oceans (young litho=10km thick, old= 100km) For continents (125-250km thickness) Below litho is asthenosphere- zone of low seismic wave velocity that behaves plastically because of increased temperature and pressure. Acts as lubricating layer allowing plates to move. (may extend 70-200km below oceans) below astheno= more rigid mantle rock. plates made of sea floor will subduct and form oceanic trenches, plates made of continental rock will not subduct. Continental rock less dense (specific gravity 2.7) than oceanic rock (specific gravity 3.0) volcanoes etc are caused by plate interactions not within plates themselves. 3 Types of plate boundaries: Divergent plate boundary- boundary btwn plates that are moving apart. Convergent plate boundary- btwn plates moving toward each other. Transform plate boundary- two plates move horizontally past each other. (marked by shallow focus earthquakes) 2 successful tests proved that plates move: 1.Paleomagnetic Evidence- magnetic reversals: when north and south poles change positions. Normal polarity: when magnetic lines flow from south to north (compass points north) Reversed polarity = opposite. Paleomagnetism- study of ancient magnetic fields recorded in rocks.
Lava flow contains magnetic minerals which stack together and can be isotopically dated by making a magnetic polarity time scale. Earth polarity reverses every 500k years, and takes 10k years for reversal to develop. Anomaly- Deviations of magnetic strength from average. Measured by Magnetometer. 2. Marine Magnetic Anomalies- are arranged in band like (stripes) patterns parallel to rift valley of mid oceanic ridge, alternating positive and negative anomalies. The Morley Vine Matthews Hypothesis: (pg34) Pattern of magnetic anomalies was symmetrical on both sides of the ridge crest. Also noticed that these patterns exist over different parts of mid oceanic ridge. Most important finding that magnetic anomalies patterns match magnetic reversals. Suggest that there is continual opening of tensional cracks within the rift valley in mid crest, filled with basaltic magma that cool to form dikes which record Earth's magnetism at the time of magnetic mineral crystallization. When normal polarity, cooling dikes are normally magnetized, when reversed polarity, cooling dikes reversely magnetized. Extension from moving sea floor then cracks dike in two and halves move in opposite directions. SO DIKES CAUSE ANOMALIES How fast do plates move? 2 important points for MVM hypothesis: 1. allows to measure rate of sea floor motion (or plate motion since continents and seafloor move together as plates) since you already know the magnetic reversal ages from lava flows on land, u use distance of sea floor anomaly from ridge divided by age it was formed. Measured rates generally range from 1-10cm per year. 2. allows for predicting the age of sea floor (distinctive patterns of anomalies through time allows age id. Can predict age of igneous rock of sea floor by measuring anomalies at sea surface) Most sea floor sections have measured anomalies, so by matching measured with known patterns, age of sea floor is found. Fractures Zones and Transform Faults Earthquakes occur along fracture zones but only between offset sections of ridge crest called:
Transform Fault- the portion of a fracture zone between two offset portions of ridge crest. It is seismically active, compared to inactive fracture zones. San Andreas California is a ridge-ridge example btwn North American and Pacific Plate. What Happens at Plate Boundaries? Divergent Plate Boundaries (DB) Figure 2.19 pg 38 Divergence is to create or open new ocean basins. DB marked by rifting, basaltic volcanism, and uplift. Produces shallow focus earthquakes on normal faults, and rift valley forms as a central graben (down dropped fault block). Uplift caused by basaltic magma eruptions from mantle beneath. So first rifting (stretching and thinning of crust), then uplift with high heat flow eruptions (eg. African Rift Valleys) Mid Oceanic Ridges- are giant undersea mountain ranges that extend around the world like the seams on a baseball. Made mostly of basalt, more than 80,000 km long and 2500km wide. Rise 3km above adjacent ocean floor. Rift valley- originates by tension and runs down the crest of each ridge, about 2km deep and several km wide. Present in Atlantic and Indian Oceans, not in Pacific. Shallow focus earthquakes, basalt eruptions, and high heat flow all at rift valleys. Crabs, mussels, clams, starfish, and giant tube worms found near black smokers, (metal sulphides) thermophyllic organisms, studied in geomicrobiology. Rift valley hot springs release, iron, copper, zinc, plus smaller amounts of manganese, gold, and silver. Ridge-ridge is two divergent boundaries (most common transform fault), ridge-trench is divergent and convergent, trench-trench is two convergent boundaries. Convergent Plate Boundaries When two ocean crusts converge there is subduction, oceanic plate with continental also subducts, but if two continents they just collide and crumble, neither subducts. OCEAN-OCEAN Convergence Oceanic Trench- is a narrow, deep trough parallel to the edge of a continent or an island arc (a curved line of islands like the Aleutians or Japan) The continental slope on active margins forms the landward wall of the trench. Steepness increases with depth. Slope is typically 4-5 degrees on upper part, 10-15 degrees on lower.
Elongate ocean trenches are 8-10km deep. Deepest spots on earth are in oceanic trenches in south west pacific ocean. (11km+) Island arc- a curved line of volcanoes that form a string of islands parallel to the oceanic trench. The inner wall of a trench (toward the arc) consists of an accretionary wedge (or subduction complex) of thrust faulted and folded marine sediment and pieces of oceanic crust. New slices of sediment are continually added to the bottom of the accretionary wedge, pushing it upward to form a ridge on the sea floor. The forearc basin lies between the accretionary wedge and the volcanic arc. (The trench side of the arc is the forearc, and the other side of the arc is the backarc.) OCEAN-CONTINENT Convergence When an oceanic plate subducts under the continental lithosphere, the accretionary wedge and forearc basin form an active continental margin between the trench and the continent. A Benioff zone of earthquakes dips under the edge of the continent, which is marked by andesitic volcanism and a young mountain belt. (eg. Nazca plate under western South America, or Juan de Fuca plate under North America) The magma that is created by ocean-continent convergence forms a magmatic arc, a broad term used both for island arcs at sea and for belts of igneous activity on the edges of continents. Batholiths- are large plutons in thickened crust seen on land when they are exposed by deep erosion. Continental interior= the craton CONTINENT-CONTINENT Convergence- at first just like O-C convergence, than ocean in between becomes narrower and narrower until the continents eventually collide and destroy or close the ocean basin. (sea floor gets subducted) Continents cannot subduct. RESULT IS Mountain Belt British Columbia pg 45 Two continents are welded together along a dipping suture zone, that marks the old site of subduction. Backarc Spreading- tension causes regional extension behind an arc. Can split the edge of a continent (how Japan formed) Backarc oceanic crust found in most ophiolites (distinctive rock sequences found in many mountain chains on land.)
Oceanic crust has three layers: layer 1= terrigenous sediment (0.5km), layer 2= basalt dikes (1.5km), layer 3= Sill like gabbro bodies (5km) Orogeny- mountain genesis, is an episode of mountain building, often characterized by intense deformation of the rocks in a region. Ocean-Continent Convergence An accretionary wedge develops where newly formed layers of marine sediment are folded and faulted as they are snowploughed off the subducting oceanic plate. Arc Continent Convergence pg 50 Continent-Continent Convergence What causes plate motions? 1. Mid-oceanic ridge crests are hot and elevated, while trenches are cold and deep 2. Ridge crests have tensional cracks 3. The edges of some plates are subducting sea floor, while the edges of other plates are continents (Which cannot subduct). Mantle Plumes- are narrow columns of hot mantle rock that rise through the mantle from thermal boundary layers at the base of the mantle (or upper mantle) Spherical/mushroom shaped haeads. Are hot spots for volcanism. Failed rift= Aulacogen Guyots- are flat topped seamounts found mostly in the western Pacific Ocean. Aseismic ridges- are submarine ridges that are not associated with earthquakes. Earthquakes occur along the rift valley of midoceanic ridges.
