Plate boundaries

• What type of plate boundary is a convergent/destructive boundary?

  Where one plate moves underneath another
• Why does an oceanic plate subduct under a continental plate?

  Because the oceanic plate is more dense
• What happens when an older oceanic plate subducts under a younger oceanic plate?

  The older, denser plate subducts
• What landforms are associated with destructive plate boundaries?

  • Benioff zones
  • Island arcs
  • Ocean trenches
  • Subduction zones
• What is a Benioff zone?

  An area of seismicity in a subduction zone
• What occurs at the convergent plate boundary in a Benioff zone?

  A slab of cold lithosphere descends into the mantle
• What is the maximum depth of a Benioff zone?

  680 km
• What creates gases that contribute to volcanic eruptions in subduction zones?

  Burnt-off carbon deposits from dead plankton
• What are island arcs?

  Chains of volcanic islands near oceanic trenches
• How are island arcs formed?

  Magma rises from a descending plate
• Where are island arcs typically found in relation to trenches?

  200 km from the trench
• What happens to magma in the formation of island arcs?

  It rises into the overriding plate
• What are ocean trenches?

  Deep sea depressions at subduction zones
• What is the depth range of ocean trenches?

  6000m to 11000m
• How are ocean trenches typically shaped?

  Asymmetric with a steep side towards land
• What happens to sediments at a trench formed by a continental plate?

  They are trapped, forming a nadir plain
• What causes the lithosphere to bend at subduction zones?

  Older, denser plates being pushed beneath lighter plates
• What is subduction?

  When one plate moves under another due to gravity
• What geological features does subduction create?

  New rock, volcanoes, ocean trenches, earthquakes
• What is the typical angle of a subducting plate?

  Between 30 and 70 degrees
• Where do subduction zones commonly occur?

  In a horseshoe shape around the Pacific Ocean
• What is the Ring of Fire?

  The most volcanically active zone in the world
• What percentage of the world's biggest earthquakes occur at the Ring of Fire?

  80%
• What occurs at constructive plate margins?

  • Plates move apart
  • Magma rises to fill the gap
  • Formation of oceanic ridges
  • Example: Mid-Atlantic Ridge
• What occurs at conservative plate margins?

  • Plates move side by side
  • No subduction or volcanic activity
  • Crust is neither created nor destroyed
  • Example: 2010 Haiti earthquake
• What are oceanic ridges?

  Longest uplifted features on Earth
• How high can oceanic ridges be?

  Up to 3000m high
• What type of lava rises at oceanic ridges?

  Basaltic lava, thin and runny
• What is the rate of sea floor spreading?

  1-11 cm per year
• What happens at collision and destructive plate boundaries?

  • Fold mountains occur
  • Two plates move towards each other
  • Example: Himalayas (collision)
  • Example: Andes (destructive)
• What occurs when continental plates collide?

  Neither subduct; they push upwards
• What happens when oceanic and continental plates collide?

  Oceanic plate subducts beneath continental plate
• What occurs when two oceanic plates collide?

  One oceanic plate subducts beneath the other
• What are upfaults and downfaults in geological terms?

  Upfaults are anticlines; downfaults are synclines
• Why are synclines perfect for erosion?

  They are downward folds that collect water