Unit 2: Plate Boundaries

• Divergent boundaries are a zone in the Earth’s crust where the plates move away from each other. It is also known as the spreading centers.
• Divergent Boundaries


  It is where the continents split apart and form new continental margins containing precious resources of salt, natural gas, and petroleum.
• Divergent plate boundaries are considered as constructive margins because it's where the new ocean floor is generated.
• Divergent Plate Boundaries

  Fractures are created in the oceanic lithosphere as plates move away from each other, and the loss of overburden pressure induces decompression melting in the hot asthenosphere.
• Divergent Boundaries


  There is an upward movement of the mantle to an area of low pressure enabling the rocks to melt that form magma. This magma rises and fills the fractures in the spreading center. As this hot rock cools in these fractures, it forms a new oceanic lithosphere.
• Elevated areas in the seafloor forms mountain system called the mid-ocean ridge
• Mid-ocean ridge


  These underwater mountains are linked in chains with valleys known as a rift. Example of which is mid-Atlantic ridge located along the floor of the Atlantic ocean and East Pacific rise located along the Pacific Ocean.
• Mid-ocean ridges on Earth are all connected forming the global mid-oceanic ridge system
• Mid-oceanic ridge system


  It is the most extended topographic feature on Earth which extends up to 65,000 km in length. It is located in major ocean basins of the world and represents 20% of the Earth’s surface.
• Seafloor spreading is the main mechanism operating along divergent margins
• Seafloor Spreading


  It is a continuous process occurring in mid-oceanic ridges in which both sides of the ridge move apart, causing decompression melting and widening (i.e., spreading) of the seafloor.
• Seafloor Spreading


  This concept was first proposed in the 1960s by a professor of geology at Princeton University named Harry Hammond Hess.
• Harry Hammond Hess


  In his paper, “History of Ocean Basins”, he outlined the concept of molten rocks oozing up from the Earth’s interior along mid-ocean ridges. Paleomagnetic studies and radiometric dating performed during the 1960s convincingly supported the seafloor spreading theory by showing that the age of oceanic crust increases systematically away from the ridge in opposite directions.
• On average, the rate of seafloor spreading in mid-oceanic ridges is 5 cm per year. In the mid-Atlantic ridge, however, rates are found to be slow at 2 cm per year.
• Spreading along the East Pacific rise is much faster at 15 cm per year. Although these rates are small, they are enough to generate all the Earth’s ocean basins within the past 200 million years.
• The oceanic crust can record polarities due to the presence of magnetite minerals in basaltic magmas.
• Divergent Boundaries


  Magnetite aligns with the prevalent magnetic orientation at the time of crystallization of the magma. As new crust is produced during a period of normal magnetic polarity, they split into two and spread away from the ridge.
• Divergent Boundaries


  Subsequent production of the new crust during a period of reversed magnetic polarity will then form between older crusts with normal magnetic polarities. Repetition of this splitting process forms an oceanic crust with bands of alternating normal and reversed magnetism with age increasing away from the ridge.
• Divergent Boundaries


  As new seafloor moves away from both sides of the ridge, more melts arise from the asthenosphere. This process is repeated in a conveyor belt-like manner.
• Divergent boundaries may also develop within continents. Spreading of landmasses into two segments forms continental rifts.
• Continental Rifts


  In time, these rifts widen to form new seas. A modern example of a continental rift is the East African Rift. Continental rifts are also known as rift valleys
• Convergent plate boundaries are areas where tectonic plates move towards each other.
• Along convergent plate boundaries within the oceanic lithosphere, portions of oceanic crust descend into the mantle at a rate equal to the production of new seafloor along divergent plate boundaries.
• Convergent plate boundaries are often associated with subduction zones.
• Subduction Zones


  It is where oceanic crust descends towards the mantle due to differences in density of the subducting plate, the asthenosphere, and the overriding plate.
• Subduction is the primary process operating in oceanic convergent margins
• During subduction, oceanic lithosphere is destroyed along trenches and is recycled back into the asthenosphere.
• Subduction

  It is for this reason that convergent plate boundaries are called destructive margins.
• The manifestations of these subduction zones at the surface are the trench-arc systems, which are deep and remarkably long troughs in the ocean floor.
• Trench-arc systems


  These troughs form along the boundary between the two plates as the subducting plates force the overlying plate to bend downward.
• Arcs are long, sublinear chains of volcanoes following the orientation of ocean trenches.
• Arcs


  These were formed due to the introduction of volatile compounds (mainly water derived from subducted pelagic minerals) into the hot asthenosphere wedge between the two plates.
• Volatiles


  lower the melting temperature of the mantle rocks, inducing flux melting; the magma rises into the overriding plate and produces volcanism.
• The angle at which a plate subducts underneath another plate depends largely on its age and density. Young and buoyant lithosphere tend to have low angles of descent.
• As the asthenosphere resists the downward motion of the subducting plate, stress is produced in the cool interior of the subducting plate. This generates earthquakes along an inclined seismic zone called Wadati-Benioff zone.
• Wadati-Benioff Zone


  Earthquakes along an inclined seismic zone
• Sunda Arc


  is a place of the world's most dangerous and explosive volcanoes. The most massive is the Mount Tabora eruption in 1815.
• Krakatoa eruption in 1883 recorded the loudest noise that can be heard even at 5,000 km away.
• There are three main types of convergent plate boundaries classified according to the type of plates involved and their associated landforms. These are oceanic–continental (O-C), oceanic-oceanic (O-O), and continental-continental (C-C).
• In oceanic-continental convergent margins, the denser oceanic crust subducts under the lighter continental crust. This process forms continental volcanic arcs.