Features and Processes at Plate Boundaries

Question 1

The global pattern of plates and plate boundaries

Answer 1

• The lithosphere is divided into seven large and three smaller tectonic plates
• The plates are moved by convection currents operating as convection cells within the asthenosphere
• The can move towards each other (destructive), move away from each other (constructive) or slip alongside each other (conservative)
• Most plate movement is slow and continuous but sudden movements produce earthquakes
• It is at the plate margins that most landforms (e.g. fold mountains, volcanoes) are found

Question 2

Features and processes associated with divergent/constructive plate boundaries

Answer 2

New crust forms at constructive plate margins where rising plumes of magma from the upper mantle stretch the crust and the lithosphere. The resulting intense volcanic activity builds submarine mountain ranges

• Ocean ridges: formed when plates move apart in oceanic areas. The space between the plates is filled with basaltic lava from below to form a ridge. Volcanoes also exist along this ridge and may rise above sea level (e.g. Surtsey, south of Iceland)
• Rift valleys: formed when plates move apart in continental areas. Sometimes the brittle crust fractures as sections of it move and areas of crust drop down between parallel faults to form a valley (e.g. East African Rift Valley)

Question 3

Features associated with convergent/destructive plate boundaries (Oceanic-Continental plate margins)

Answer 3

• Different densities of plates (oceanic is more dense)
• Denser oceanic plate subducts under the continental plate
• A deep ocean trench is formed at the plate boundary
• Sediments and rocks fold and are uplifted along the leading edge of the continental plate
• Continental crust buckles and mountain chains form (e.g. the Andes)
• The angle of subduction on the oceanic plate is between 30-70 degrees; faulting occurs in the Benioff zone, releasing energy in the form of earthquakes

Question 4

Features associated with convergent/destructive plate boundaries (Oceanic-Oceanic plate margins)

Answer 4

• The slightly denser plate will subduct under the other, creating a trench
• Descending plate melts, magma rises and chains of volcanoes - island arcs - form (e.g. the Antilles)

Question 5

Features associated with convergent/destructive plate boundaries (Continental-Continental plate margins)

Answer 5

• Little if any subduction because of similar densities

- Impact and pressure tends to form fold mountains (e.g. the Himalayas)

Question 6

Features associated with conservative plate boundaries

Answer 6

At a conservative plate margin, two plates slide past each other. The movement can be violent and an additional build-up of pressure which eventually gives way results in powerful earthquakes. There is no volcanic activity (e.g. San Andreas fault between the Pacific and North American plates)