LESSON 6

• What are divergent boundaries in the Earth's crust?

  Divergent boundaries are zones where tectonic plates move away from each other.
• What is another name for divergent boundaries?

  Divergent boundaries are also known as spreading centers.
• What happens at divergent boundaries?

  Continents split apart and form new continental margins containing resources.
• Why are divergent plate boundaries considered constructive margins?

  Because new ocean floor is generated at these boundaries.
• What occurs in the oceanic lithosphere at divergent boundaries?

  Fractures are created as plates move away from each other.
• What induces decompression melting in the asthenosphere at divergent boundaries?

  The loss of overburden pressure as plates move apart.
• What happens to the magma formed at divergent boundaries?

  The magma rises and fills the fractures in the spreading center.
• What is formed when hot rock cools in the fractures at divergent boundaries?

  A new oceanic lithosphere is formed.
• What is the mid-ocean ridge?


  It is an elevated area in the seafloor that forms a mountain system.
• What are rifts in the context of mid-ocean ridges?

  Rifts are valleys linked to the underwater mountain chains of mid-ocean ridges.
• What is the length of the global mid-oceanic ridge system?

  It extends up to 65,000 km in length.
• What percentage of the Earth's surface does the mid-ocean ridge system represent?

  It represents 20% of the Earth's surface.
• What is sea floor spreading?

  It is a continuous process where both sides of the mid-ocean ridge move apart.
• Who proposed the concept of sea floor spreading?

  Harry Hammond Hess proposed the concept.
• What evidence supported the sea floor spreading theory in the 1960s?

  Paleomagnetic studies and radiometric dating showed the age of oceanic crust increases away from the ridge.
• What is the average rate of sea floor spreading in mid-oceanic ridges?

  The average rate is 5 cm per year.
• What is the rate of sea floor spreading at the mid-Atlantic ridge?

  The rate is 2 cm per year.
• What is the rate of sea floor spreading at the East Pacific rise?

  The rate is 15 cm per year.
• How long has it taken to generate all of Earth's ocean basins?

  It has taken the past 200 million years.
• What mineral in basaltic magmas allows the oceanic crust to record polarities?

  Magnetite minerals allow the oceanic crust to record polarities.
• How does magnetite behave during the crystallization of magma?

  Magnetite aligns with the prevalent magnetic orientation at the time of crystallization.
• What happens to the new crust produced during normal magnetic polarity?

  It splits into two and spreads away from the ridge.
• What forms between older crusts with normal magnetic polarities during reversed magnetic polarity?


  New crust during reversed magnetic polarity forms between older crusts.
• What is the result of the repetition of the splitting process at mid-ocean ridges?

  It forms an oceanic crust with bands of alternating normal and reversed magnetism.
• How does the sea floor move away from the ridge?

  It moves away in a conveyor belt-like manner.
• What can develop within continents as a result of divergent boundaries?

  Continental rifts can develop within continents.
• What happens to continental rifts over time?

  They widen to form new seas.
• What is a modern example of a continental rift?

  The East African Rift is a modern example.
• What are continental rifts also known as?

  Continental rifts are also known as rift valleys.
• What are convergent plate boundaries?


  Convergent plate boundaries are areas where tectonic plates move towards each other.
• What happens to oceanic crust at convergent plate boundaries?

  Portions of oceanic crust descend into the mantle.
• What are subduction zones associated with?

  Subduction zones are associated with convergent plate boundaries.
• What is the primary process operating in oceanic convergent margins?

  Subduction is the primary process in oceanic convergent margins.
• What happens during subduction?

  Oceanic lithosphere is destroyed along trenches and recycled back into the asthenosphere.
• What are trench-arc systems?


  They are deep and long troughs in the ocean floor formed by subduction zones.
• What are arcs in the context of convergent boundaries?

  Arcs are long chains of volcanoes following the orientation of ocean trenches.
• How are arcs formed?

  Arcs are formed due to the introduction of volatile compounds into the asthenosphere.
• What role do volatile compounds play in the formation of arcs?

  They lower the melting point in the asthenosphere, facilitating magma formation.
• What is the relationship between convergent boundaries and tectonic activity?

  Convergent boundaries are associated with significant tectonic activity, including earthquakes and volcanic eruptions.
• How do convergent boundaries affect the Earth's surface?

  They create features such as trenches, arcs, and mountain ranges.