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• 1 media/Intro Photo of Meandering river.png 2016-04-05T10:10:11+00:00 Introduction - Shaping The Planet 97 image_header 2016-06-16T13:38:40+00:00
  As we have seen in our tour through the solar system, the surfaces of the planets look very different. Geologists and geomorphologists have played an important role in interpreting the surface features of the planets. Those features can provide information not only about the current conditions on other planets but also provide basis for interpreting what has taken place on those planets in the past.
  
  Geologists have long been interested in unraveling the history of changing environments on Earth based on evidence that is preserved in rocks. To do this we apply the axiom that states:
   
  “The present is the key to the past”.
   
  This means that we can interpret how rocks formed in the past by looking at how they form today. Then, if we see ancient rocks with the same characteristics as the rocks that are forming today we can infer that they formed by the same processes that form them today.
   
  A similar axiom helps us interpret bodies in space for which most of our information is based largely on photographic images of the surface of those bodies.
   
  This axiom is that:
   
  “The Earth is the key to interpreting the planets”
   
  This is based on the assumption that features that what we see on the surfaces the planets and other bodies formed by the same processes that produce the same features on Earth.
   
  For example, river channels form on Earth when water flows over the surface and erodes the substrate form linear channels. These channels form specific patterns of erosion on the Earth surface. When we see similar linear patterns of erosion on the surface of other planets we can infer that they formed by fluids flowing over the surface…..even if there is no water (or other fluids) there today.  Of course, there are processes that act on other solar system bodies that cannot act on Earth.  Flowing glaciers of methane, carbon monoxide and nitrogen are found on Pluto where the average surface temperature is -229ºC but such glaciers could never form on Earth.  In cases like this we can make out best interpretations based on what we know about flowing water ice on Earth.
   
  In this module we will look at processes and the landscapes that they produce on Earth so that we have a basis for interpreting those landscapes when we find them on other solar system bodies.
  
  Plate Tectonics
  
  Many of the large scale features of the Earth, such as mountain belts, ocean basins, volcanoes, are the result of Plate Tectonics which is a collection of processes that affect the entire planet.
  
  You will recall from the section on the differentiation of the planets that the interior of the Earth is divided up in to different regions that differ in terms of their chemical composition, temperature and mechanical properties. The rigid crust makes up the surface of the Earth and it is firmly attached to the upper portion of the mantle and together they make up what geologists call the lithosphere.  Below the lithosphere is the asthenosphere which is the upper part of the mantle which is relatively soft and deformable (it flows in response to an applied stress) and the crust and lithosphere "float" on that mobile portion of the mantle.  There are two types of crust, "continental crust" and "oceanic crust" (see the diagram, below).  Continental crust, as the name suggests, is largely limited to beneath the continents (extending under the oceans only adjacent to the continental margins), it is relatively thick (averaging 35km in thickness) and it has a density that is, on average, similar to that of granite.  Oceanic crust lies beneath most of the worlds oceans and it is thinner (averaging 6km in thickness) and it has a slightly greater density than continental crust because it has an average composition of the rock type called basalt which is rich in relatively heavy elements such as iron and magnesium.
  
  
  The crust and the lithosphere are broken up into a number of fragments that vary greatly in size; these fragments are called plates (specifically, "tectonic plates" or "lithospheric plates").  The next figure (below) is a map showing the Earth's major tectonic plates. All together there are a total of 19 plates that exceed 1 million square kilometres in area and more than 50 "microplates" that are less than 1 million square kilometres in area.
  
  
  These plates move independently of each other and that movement involves forces that also cause the formation of large scale features of the Earth's crust.  The term "tectonics" refers to the major structural features (e.g., mountains, ocean basins, continents, etc.) of the crust so that "plate tectonics" refers to the processes that involve the interaction of crustal plates to produce tectonic features.
  
  The plates move in response to convective motion within the mobile portion of the mantle (asthenosphere).  You have seen that within the earth the temperature increases from a maximum in the core to a minimum at the surface of the crust.  Within the asthenosphere hot material rises from its depths and once at the top of the mantle it flows laterally beneath the crust and lithosphere while it cools which increases its density so that it eventually sinks back to the hotter, deeper regions it heats up again and continues the convective process.  Lithosphere and crust that is in direct contact with horizontally convecting mantle will be dragged in the direction of convection.  Where two convection cells are adjacent to each other convection is either upward where hot material is rising or downward where cool material is sinking into the mantle.  These types of motion create very different boundaries between plates.
  
  
  
  VERY RUDIMENTARY VIDEO OF CONVECTION MOVING PLATES
  
  PLATE BOUNDARIES
  
  There are three important types of boundary that may exist between tectonic plates:
  • Divergent plate boundaries: plates move away from each other on either side of the boundary (e.g., oceanic ridges)
  • Convergent plate boundaries: plates move towards each other on either side of the boundary (e.g., oceanic trenches and/or mountain belts)
  • Transform boundaries: horizontal movement as two plates slide past each other, in opposite directions on either side of the boundary (e.g., transform faults)
  The details of each type of plate boundary and their associated plate movements are described in detail below.
  
  Divergent Plate Boundaries
  
  Most divergent plate boundaries are hidden beneath the oceans where new crusts forms as magma (molten rock) intrudes from the upper mantle into the overlying crust along a global fracture system that is expressed as the largest continuous feature on the planet, the oceanic ridge.  The following map shows the world-wide distribution of the oceanic ridge; it's longest continuous segment is 65,000 km long and it's total length is about 80,000 km.
  
  
  
  The ridge is a submarine mountain chain made up of volcanoes that reach the surface only locally (e.g., Iceland is a volcanic island that formed when volcanoes of the ridge grew to elevations that are above sea level).  Over most of the world's oceans the ridge crest is well below sea level (on average 2.3km below sea level) and lies, on average, about 3 km above the surrounding sea flow.  The oceanic ridge is a massive tectonic feature that has a width of about 1000 km, giving it a total volume of about 80 million cubic kilometres.
  
  Along much the length of the central axis of the oceanic ridge there is a 1 km deep valley or trough.  During the 20th century Cold War this trough had immense value for Soviet and American nuclear submarines because they could move long distances around the world without detection within the cover of the trough.  "Hunt for Red October" is a great movie starring Sean Connery; "Red October" is a Typhoon class Soviet sub and much of the "hunt" takes place along the mid-Atlantic trough.
  
  
  
  Along the ridge magma with the composition of basalt is injected into the crust, forming volcanic mountains that characterize the ridge.  The addition of new crustal rocks along the ridge system pushes older rocks further and further away from the ridge.  This new basaltic crust is relatively thin and dense so it forms “oceanic crust” with its surface below sea level.  This process leads to "sea floor spreading" because the sea floor becomes more and more expansive as new crust forms along the ridge axis.  The animation below illustrates sea floor spreading along an oceanic ridge.  Sea floor spreading rate is defined as the distance that a point on the crust moves away from the ridge axis over a given period of time (distance divided by time) and ranges from about 16cm per year to under 1 cm per year and the average rate of spreading is approximately 2.5 cm per year.
  
  
  
   When a ridge begins to form within a mass of thicker continental crust it first forms a “rift valley” which becomes wider over time, as more crust is added and eventually evolves into a new sea floor and, ultimately, if it continues long enough, a new ocean basin.  The volcanically active East African Rift Valley is in the initial phases of evolution and may well evolve into a new ocean basin separating western Africa from southeastern Africa.  We'll visit the valley in the global video tour of Earth features at the end of the section on Earth processes and features.
  
  Convergent plate boundaries
  
  While new crust forms along the oceanic ridges old crust must be destroyed or else the Earth would just keep on expanding.  Along convergent margins old crust is lost in two different ways: by subduction and/or by compressive deformation.
  
  If a plate of oceanic crust converges on another plate of oceanic crust one slab will dive beneath the other and be "subducted" into the upper mantle where it melts.  The subducting slab bends downward and passes beneath the over-riding slab and the line along which the two slabs intersect forms a linear topographic low called an "oceanic trench".  The subducting crust melts as it descends into the mantle, it rises upwards beneath the over-riding plate, penetrating it locally through vertical fractures and once at the surface produces volcanoes.  These volcanoes commonly built to above sea level where they form linear chains of islands that lie parallel to the oceanic trench; such a chain of volcanoes is called an "island arc complex".  The following figure illustrates this type of convergent plate boundary.
  
  When a plate of oceanic crust converges on a plate of continental crust the oceanic crust is always subduccted (because it is made of basaltic rocks which are lighter than the granitic crust that makes up continents) forming an oceanic trench.  Adjacent continental rocks are compressed by the forces generated by plate convergence and they buckle and break and resulting in a thickening of the continental margin, forming mountain chains that run parallel to the adjacent trench.  In addition, magma that is generated by melting of the subducting slab rises upwards and penetrates the continental margins to produce volcanoes at the surface, within the mountain chain.  The following figure illustrates a convergence of oceanic crust on continental crust.
  
  The map below shows that most convergent plate margins are found surrounding the Pacific Ocean basins (two exceptions are the Sandwich Islands in the south Atlantic Ocean and the region adjacent to Puerto Rica.  The blue lines delineated well-defined trenches whereas the red-shaded areas are regions of plate convergence but without well-defined trenches on the sea floor.  Note that the Aleutian and Japan trench represent convergence of oceanic crust on oceanic crust and the associated island arcs are prominent on the map.  The western edges of North and South America are examples of oceanic crust converging on continental crust and the Rocky Mountains and the Andes Mountains, respectively, lie parallel to the convergent margin, having been constructed by the compressive forces and volcanic activity that characterizes this tectonic setting.
  
  
  
  A third common type of convergent plate boundary exists when continental crust converges on continental crust.  In this case, neither plate subducts because both are too light to sink readily into the upper mantle.  Instead of subduction the plates push against each other and the rocks are folded and broken and pushed upwards to form mountain belts all along the region of subduction.  The modern Himalaya Mountains are still forming in this manner after a plate that was made up of India collided with the southern edge of the Eurasian plate about 20 million years ago.
  
  Transform Plate Boundaries
  
  The tell-tale signs of Plate Tectonics
  
  If we were to evaluate whether or not plate tectonics was operating on Earth on the basis of satellite photos alone then we would looks for the large scale structures that plate tectonics produces.  If we were to find these structures on other planets they may well mean that at plate tectonics is operating there as well.
  Linear mountain belts
  Oceanic ridges
  Linear mountain belts
  Oceanic ridges
  Oceanic trenches
  Island Arc Complexes
  
  ​Oceanic ridges....map
  
  Each plate
  Old notes
  
   Original outline for this module
  Shaping the modern planets
  Plate Tectonics and Volcanism (http://www.imdb.com/title/tt0100814/?ref_=nv_sr_1)
   
  Volcanism
  what volcanoes are
  What they are made up of tephra and lava (mention airfall of tephra including blocks and bombs
  classes of volcano: shield, cinder cone, stratovolcanoes, supervolcanoes
  
  Hawaii and plate tectonics (if they remained stationary would be much larger). 
  
  
  Plate Tectonics
   
   
  3) Surface processes
   
  Water
   
  Air
   
  Gravity (direct, whereas gravity also drives flow of water)....land slides....Frank slide perhaps....in video)
  
  4) External processes (http://www.imdb.com/title/tt0045920/?ref_=fn_al_tt_1)
   
  Meteorite impacts
  
  
  From our meeting
  
  ON Earth - by Rick
  Introduce convection
        Plate Tectonics
        Volcanism
  Surface Water, (liquid/ice - poles) glaciers - advance retreat, water cycle
  Wind
  Impacts
  
  Tour of the Earth on Google Maps
  
  Oceanic ridge, Iceland, Rockies/andes, East African Rift Valley, hawiian islands, island arcs, MAP This out
  
  Other Planets - by others
  Moon
  Mercury - shrink, impacts
  
  Venus - sort of surface renewal, volcanic feature, run, away greenhouse effect
  
  Mars - No Plate tectonics, highlands/lowlands, volcanism (Olypus Mons, Tharsis), Valles Marienris, Gale Crater - impact, central peak, Thin atmosphere, ice caps, - outflow channels, we'll visit water in more details later
  
  
  Ignore gas planets
  
  Discuss selected icy satellites
  Atmoshopere - Titan, no real atmosphere on other satellites
  
  Volcanism on satellites - Fire and Ice
     IO, Enceladous, Pluto - nitrogen,
  
  
  
  
  --------------------- end of meeting
• 1 media/Big PT photo.jpg 2016-06-08T13:03:17+00:00 Plate Tectonics 59 image_header 2019-12-29T15:36:05+00:00
  Many of the large scale features of the Earth, such as mountain belts, ocean basins and volcanoes, are the result of Plate Tectonics which is a collection of processes that affect the entire planet.
  
  You will recall from the section on the differentiation of the planets that the interior of the Earth is divided up into different regions that differ in terms of their chemical composition, temperature and mechanical properties. The rigid crust makes up the surface of the Earth and it is firmly attached to the rigid upper portion of the mantle and together they make up what geologists call the lithosphere.  Below the lithosphere is the asthenosphere, which is the upper part of the mantle that is relatively soft and deformable (it flows in response to an applied stress) and the crust and lithosphere "float" on that mobile portion of the mantle. There are two types of crust, "continental crust" and "oceanic crust" (see the diagram, below).  Continental crust, as the name suggests, makes up the continents (extending under the oceans where seawater laps onto the continental margins), it is relatively thick (averaging 35 km in thickness) and it has a composition that is, on average, similar to that of granite.  Oceanic crust lies beneath most of the worlds oceans and it is thinner (averaging 6 km in thickness) and it has a slightly greater density than continental crust because it has an average composition of the rock type called basalt which is rich in relatively heavy elements such as iron and magnesium.
  
  
  The crust and the lithosphere are broken up into a number of fragments that vary greatly in size; these fragments are called plates (specifically, "tectonic plates" or "lithospheric plates").  The next figure (below) is a map showing the Earth's major tectonic plates. All together there are a total of 19 plates that exceed 1 million square kilometres in area and more than 50 "microplates" that are less than 1 million square kilometres in area.
  
  
  These plates move independently of each other and that movement involves forces that also cause the formation of large scale features of the Earth's crust.  The term "tectonics" refers to the major structural features (e.g., mountains, ocean basins, continents, etc.) of the crust so that "plate tectonics" refers to the processes that involve the interaction of crustal plates to produce tectonic features.
  
  The plates move in response to convective motion within the mobile portion of the mantle (asthenosphere).  You have seen that within the Earth the temperature increases from a maximum in the core to a minimum at the surface of the crust.  Within the asthenosphere hot material rises from its depths and once at the top of the mantle it flows laterally beneath the crust and lithosphere as it cools. Cooling increases its density so that it eventually sinks back to the hotter, deeper regions where it heats up again and continues the convective process.  Lithosphere and crust that is in direct contact with horizontally convecting mantle are dragged in the direction of convection.  Where two convection cells are adjacent to each other convection is either upward where hot material is rising or downward where cool material is sinking into the mantle.  These types of motion create very different boundaries between plates.  The illustration below shows how mantle convection drags the crust to form areas of convergence and divergence within the crust.  The red arrows in the mantle shows the motion of convection and the black arrows above the crust show the direction in which the convective motion will move the crust.
  
  
  PLATE BOUNDARIES
  
  There are three important types of boundary that may exist between tectonic plates:
  • Divergent plate boundaries: plates move away from each other on either side of the boundary (e.g., oceanic ridges)
  • Convergent plate boundaries: plates move towards each other on either side of the boundary (e.g., oceanic trenches and/or mountain belts)
  • Transform boundaries: two plates slide horizontally past each other, in opposite directions on either side of the boundary (the boundary is called "transform fault"; a vertical fracture in the crust on either side of which movement is in opposite directions)
  The details of each type of plate boundary and their associated plate movements are described in detail below.
  
  Divergent Plate Boundaries
  
  Most divergent plate boundaries are hidden beneath the oceans where new crusts forms as magma (molten rock) intrudes from the upper mantle into the overlying crust along a global fracture system that is expressed as the largest continuous feature on the planet, the oceanic ridge.  The following map shows the world-wide distribution of the oceanic ridge; its longest continuous segment is 65,000 km long and its total length is about 80,000 km.
  
  
  
  The ridge is a submarine mountain chain made up of volcanoes that reach the surface only locally (e.g., Iceland is a volcanic island that formed when volcanoes of the ridge grew to elevations that are above sea level).  Over most of the world's oceans the ridge crest is well below sea level (on average 2.3 km below sea level) and lies, on average, about 3 km above the surrounding sea floor.  The oceanic ridge is a massive tectonic feature that has a width of about 1000 km, giving it a total volume of about 80 million cubic kilometers.
  
  Along much the length of the central axis of the oceanic ridge, there is a 1 km deep valley or trough.  During the 20th century Cold War this trough had immense value for Soviet and American nuclear submarines because they could move long distances around the world without detection within the cover of the trough.  "Hunt for Red October" is a great movie starring Sean Connery; "Red October" is a Typhoon class Soviet sub and much of the "hunt" takes place along the mid-Atlantic trough.
  
  
  
  Along the ridge, magma with the composition of basalt is injected into the crust and erupts at the the surface, forming the volcanic mountains that characterize the ridge.  The addition of new crustal rocks along the ridge system pushes older rocks further and further away from the ridge.  This new basaltic crust is relatively thin and dense so it forms “oceanic crust” with its surface below sea level.  This process leads to "sea floor spreading" because the sea floor becomes more and more expansive as new crust forms along the ridge axis.  The animation below illustrates sea floor spreading along an oceanic ridge.  Sea floor spreading rate is defined as the distance that a point on the crust moves away from the ridge axis over a given period of time (distance divided by time) and ranges from about 16 cm per year to under 1 cm per year and the average rate of spreading is approximately 2.5 cm per year.
  
  
  
   When a ridge begins to form within a mass of thicker continental crust, it first forms a “rift valley” that becomes wider over time.  As more crust is added, the rift valley eventually evolves into a new sea floor and, ultimately, if it continues long enough, a new ocean basin.  The volcanically active East African Rift Valley is in the initial phases of this evolution and may well evolve into a new ocean basin separating western Africa from southeastern Africa.
  
  Convergent plate boundaries
  
  While new crust forms along the oceanic ridges, old crust must be destroyed or else the Earth would just keep on expanding.  Along convergent margins old crust is lost in two different ways: by subduction and/or by compressive deformation.  The map below shows the distribution of convergent and divergent plate boundaries and the direction of plate movement around the world.
  
  
  
  If an oceanic plate converges on another oceanic plate, one slab will dive beneath the other and be "subducted" into the upper mantle.  The subducting slab bends downward and passes beneath the over-riding slab and the line along which the two slabs intersect forms a linear topographic low called an "oceanic trench".  The subducting crust heats up as it descends into the mantle and releases water into the overlying materials, which then melt and rise into the over-riding plate, penetrating it locally through vertical fractures and once at the surface produces volcanoes.  These volcanoes commonly build to above sea level where they form linear chains of islands that lie parallel to the oceanic trench; such a chain of volcanoes is called a "island arc".  The following figure illustrates this type of convergent plate boundary.
  
  
  
  When a plate of oceanic crust converges on a plate of continental crust, the oceanic crust is always subducted (because it is made of basaltic rocks, which are denser than the granitic crust that makes up continents) forming an oceanic trench.  Adjacent continental rocks are compressed by the forces generated by plate convergence and they buckle and break, resulting in a thickening of the continental margin, and forming mountain chains that run parallel to the adjacent trench.  In addition, magma that is generated by melting of rocks above the subducting slab rises upwards and penetrates the continental margins to produce volcanoes at the surface, within the mountain chain.  The following figure illustrates a convergence of oceanic crust on continental crust.
  
  
  
  The map below shows that most convergent plate margins are found surrounding the Pacific Ocean basins (two exceptions are the Sandwich Islands in the south Atlantic Ocean and the region adjacent to Puerto Rico).  The blue lines delineate well-defined trenches whereas the red-shaded areas are regions of plate convergence but without well-defined trenches on the sea floor.  Note that the Aleutian and Japan trench represent convergence of oceanic crust on oceanic crust and the associated island arcs are prominent on the map.  The western edges of North and South America are examples of oceanic crust converging on continental crust and the Rocky Mountains and the Andes Mountains, respectively, lie parallel to the convergent margin, having been constructed by the compressive forces and volcanic activity that characterizes this tectonic setting.
  
  
  
  A third common type of convergent plate boundary exists when continental crust converges on continental crust.  In this case, neither plate subducts because both are too buoyant to sink readily into the upper mantle.  Instead of subduction, the plates push against each other and the rocks are folded and broken and pushed upwards to form mountain belts all along the region of subduction.  The modern Himalaya Mountains are still forming in this manner after a plate that was made up of India collided with the southern edge of the Eurasian plate about 20 million years ago.  The following animation shows the collision between converging continental plates.  Note that it begins with ocean crust subducting beneath continental crust but then a continent that is attached to the subducting plate converges to form the mountains within the larger continental mass that results from the collision.
  
  
  
  Transform Plate Boundaries
  
  Transform plate boundaries develop between offset segments of oceanic ridge as shown in the figure below.  Transform boundaries are also called "transform faults".  In geology a "fault" is a fracture or system of fractures in the crust and on either side of the fractures the crust is moving in opposite directions.  In the figure, below, rocks on either side of the transform fault move away from the associated oceanic ridge axis but due to the offset in the ridge the movement on either side of the fault is in opposite directions.
  
  
  Along the oceanic ridge there are thousands of transform faults as shown in the following realistic illustration of the sea floor along the mid-Atlantic oceanic ridge.  In this image the ridge trends more-or-less north south (up and down) whereas the transform faults trend approximately east-west, offsetting the ridge axis.
  
  
  
  The tell-tale signs of Plate Tectonics
  
  The following figure summarizes the various types of plate boundary and how plate tectonics operates.  Note that it also identifies specific types of volcanoes that are associated with various plate boundaries.  We will see more about volcanoes in the next section.
  
  
  
  If we were to evaluate whether or not plate tectonics was operating on Earth on the basis of satellite photos alone, then we would look for the large scale structures that plate tectonics produces.  If we were to find these structures on other planets, they may well mean that plate tectonics is (or was at some time in the past) operating there as well.
  
  These large scale structures include linear mountain belts that are produced in association with continental-oceanic crust convergence and along zones of continental collision.  Oceanic trenches and volcanic island arcs also indicate that plate tectonics has been at work.  Oceanic ridges in otherwise flat basins are a particularly prominent outcome of plate tectonic processes but these are largely hidden on Earth by a couple of kilometers of seawater.  On a dry planet, such ridges would appear as long chains of volcanic mountains with a central trough and transform faults that appear as valleys extending at right angles to the oceanic ridge.