Introduction Last week: Physiography of ocean floor- what it looks like. - Earth’s compositional & physical structure. - bathymetric provinces: continental margins, deep-ocean basins, and mid-ocean ridges. - properties of crustal material (basalt vs granite). -isostasy. -Methods of probing sea-floor: seismic reflection, refraction, etc. This week: Sea-floor spreading and global plate tectonics- how the ocean floor (and continents) got this way.
3-1Continental Drift German meteorologist Alfred Wegner 1915 Geologic and paleontological evidence –Continuity of rock and structures –Continents seemed to fit together –Similar fossils on opposite continents
Wegner: Pangaea 200 to 300 Millions of Years Before the Present
3-1Continental Drift Continental drift proposed as hypothesis –Supercontinent of Pangea –Continental crust ‘plowed through’ basalt –Fresh basalt extruded in widening gaps Problems with Wegner’s hypothesis: granite cannot displace basalt! –Granite less dense than basalt –Granite = 2.7 – 2.8 g/cm 3 –Basalt = 2.9 g/cm 3
Geologists in the 1960’s Harry Hess (USA), Fred Vine and Drummond Mathews (Great Britain). Sea floor spreading demonstrates that the sea floor moves apart at the oceanic ridges and new oceanic crust is added to the edges. Let’s look at physical evidence that lead to development of this hypothesis. 3-2Sea-Floor Spreading
Whereas oceanic ridges indicate tension, continental mountains indicate compressional forces are squeezing the land together. –Examples: Appalachians, Rockies, Alps and Himalayas. 3-2Sea-Floor Spreading Sedimentary Rocks Squeezed by Compression
3-2Sea-Floor Spreading The geomagnetic field is the magnetic field of the Earth.
Magnetometers detect and measure Earth’s magnetic field. 3-2Sea-Floor Spreading
Moving across the ocean floor perpendicularly to the oceanic ridges, magnetometers record alternating strong (positive) and weak (negative) magnetic measurements (called magnetic anomalies) in response to the influence of the sea floor rocks. 3-2Sea-Floor Spreading
Magnetic anomalies form parallel bands arranged symmetrically about the axis of the oceanic ridge. 3-2Sea-Floor Spreading
As basaltic rocks crystallize, some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock. Periodically Earth’s magnetic field polarity (direction) reverses poles. 3-2Sea-Floor Spreading
Rocks forming at the ridge crest record the magnetism existing at the time they solidify. 3-2Sea-Floor Spreading
Sea floor increases in age and is more deeply buried by sediment away from the ridge because sediments have had a longer time to collect. 3-2Sea-Floor Spreading
Rates of sea-floor spreading vary from 1 to 10 cm per year for each side of the ridge and can be determined by dating magnetic anomaly stripes of the sea floor and measuring their distance from the ridge crest. 3-2Sea-Floor Spreading
Because Earth’s size has not changed, expansion of the crust in one area requires destruction of the crust elsewhere. - Where and how is crust being destroyed? - let’s look at the evidence. 3-3Global Plate Tectonics
3-3Global Plate Tectonics Seismicity is the frequency, magnitude and distribution of earthquakes.
Subduction is the process at a deep-sea trench whereby one part of the sea floor plunges below another and down into the asthenosphere. 3-3Global Plate Tectonics
Benioff Zone is an area of increasingly deeper seismic activity, inclined from the trench downward in the direction of the island arc. 3-3Global Plate Tectonics South Figi Basin and Cross Section Showing Benioff Zone
3-3Global Plate Tectonics ROCK TYPETEXTUREDENSITY (g/cm3) ASSOCIATION GraniteCoarse2.7 – 2.8Continental Crust BasaltFine/coarse2.9Ocean crust AndesiteFine2.8+Volcanic arcs, Andean type mts. From: Pinet, Table 2.2 (Chapt. 2).
Movement of plates is caused by thermal convection of the “plastic” rocks of the asthenosphere which drag along the overlying lithospheric plates, and gravity which pulls submerged plate downward. 3-3Global Plate Tectonics Driving Mechanisms for Plate Motions
Mantle plumes originate deep within the asthenosphere as molten rock which rises and melts through the lithospheric plate forming a large volcanic mass at a “hot spot”. 3-3Global Plate Tectonics Mantle Plume
The Wilson Cycle Rift valley forms as continent begins to split. Example: Afrcan Rift Lakes. Sea-floor basalts begin forming and continents diverge. Example: Red Sea. Broad ocean basins widen, trenches develop, subduction begins. Example: Atlantic Ocean. Subduction eliminates much of sea-floor and oceanic ridge. Example: Pacific Ocean. Last of sea-floor is eliminated, continents collide forming mountain chain. Example: Mediterranean Sea. Convergence of continental plates and uplifting to form mountain range. Example: Himalayas.
3-4Transform Faults If these plate motions continue, Baja will splinter off California. The San Andreas fault in southern California is a transform fault that connects the sea-floor spreading ridge of the Gulf of California with the spreading ridge off Oregon and Washington.
Because the San Andreas fault has an irregular trace, strike-slip motion can cause local compression or tension. 3-4Transform Faults Fault Geometry
Midoceanic Ridge Province: - continuous submarine mountain range. -covers ~1/3 of the ocean floor & extends ~ 60,000 km around Earth. -Features include: *Rift valley: opposite sides of ridge pulled apart form valley in center. *Transform fault : offset ridge segments- active. *Fracture zone : inactive TF moved out into ocean basins. 2-2 The Physiography of the Ocean Floor
TENSION –Pulling apart, stretching force. –Examples: seafloor spreading at mid- ocean ridges. COMPRESSION –Squeezing together –Examples: collision of crust to form mountains. COMPRESSION 3-2Sea-Floor Spreading
3-3Global Plate Tectonics Two groupings of seismic events. –Along ridges and transform faults –Along margins of N. & S. America, arc around Pacific Ocean S. Asian mainland through Himalayas, etc. These areas are called Subduction zones
3-3Global Plate Tectonics Two groupings of seismic events. –Along ridges and transform faults Shallow, relatively weak earthquakes. Seismicity due to volcanism and faulting. Region of formation of new lithosphere. –Subduction zone
3-3Global Plate Tectonics Two groupings of seismic events. –Along ridges and transform faults Shallow, relatively weak earthquakes. Seismicity due to volcanism. Region of formation of new lithosphere. –Subduction zone Shallow to deep strong earthquakes. Seismicity due to Tectonism (deformation, or buckling, folding, faulting crushing of lithosphere). Region of destruction of lithosphere. Convergent plate boundary.
Wilson Cycle refers to the sequence of events leading to the formation, expansion, contracting and eventual elimination of ocean basins. Stages in basin history are: –Embryonic - rift valley forms as continent begins to split. –Juvenile - sea floor basalts begin forming as continental fragments diverge. –Mature - broad ocean basin widens, trenches eventually develop and subduction begins. –Declining - subduction eliminates much of sea floor and oceanic ridge. –Terminal - last of the sea floor is eliminated and continents collide forming a continental mountain chain. 3-3Global Plate Tectonics
Hot, salty groundwater is dissolving metals from the rocks and depositing them as metal sulfides in dense brine pools like the Atlantis II Deep. 3-4Juvenile Ocean Basin Atlantis II Deep