Key terms

• What does Moho stand for in geology?

  Mohorovicic discontinuity
• At what depth does the Moho lie beneath the continents?

  Approximately 35 km
• What is the depth of the Moho beneath the ocean floor?

  Between 10-15 km
• What is the asthenosphere?

  The layer in the Earth's mantle below the lithosphere
• Why do rocks in the asthenosphere become viscous?

  Due to high temperatures
• What is the lithosphere composed of?

  The Earth's crust and the upper mantle
• How is the lithosphere divided?

  Into a series of tectonic plates
• What is the theory of continental drift?

  It states that continents are mobile and have moved across the Earth's surface
• What is sea-floor spreading?

  The lateral movement of new oceanic crust away from a mid-ocean ridge
• What is a tectonic plate?

  A large slab of the Earth's lithosphere and crust
• What are transform faults?

  Large-scale faults in the crust at right angles to a mid-ocean ridge
• What is pillow lava?

  Rounded mounds of lava erupted along mid-oceanic ridges
• What is a black smoker?

  A hydrothermal vent on the ocean floor at plate boundaries
• What is a graben?

  The down-faulted section of a rift valley
• What is subduction?


  The tectonic process where an oceanic plate descends into the Earth's mantle
• What is a trench in geological terms?

  A narrow, deep depression on the ocean floor
• What is the Benioff zone?

  The boundary between a subducting ocean plate and the over-riding continental plate
• What is a pluton?


  A mass of igneous rock injected into overlying rock deep in the crust
• What is an island arc?

  A chain of volcanic islands formed along a subduction zone
• What does viscosity indicate?

  How well a substance flows
• What is an explosive eruption?

  A violent eruption due to the build-up of pressure within a volcano
• What is an effusive eruption?


  A gentle free-flowing basic eruption of lava
• What is a strato-volcano?


  A steep-sided volcano made up of layers of lava and ash
• What is a sill in geology?

  A minor, approximately horizontal intrusion of magma into surrounding rocks
• What is a dyke?


  A vertical or near vertical minor intrusion of magma through surrounding rocks
• What is a caldera?

  A large-scale volcanic crater formed from an explosive eruption
• What is an archipelago?


  A closely grouped cluster of islands
• What is flood basalt?

  A large area of basaltic lava erupted over a long time from multiple vents
• What is denudation?


  The wearing away of the Earth's surface by weathering and erosion
• What is a hot spot?

  An area of intense volcanic activity where a mantle plume reaches the Earth's surface
• What is a super-volcano?

  A volcano that erupts more than 1000 km³ of material in a single eruption event
• What does the Volcanic Explosivity Index (VEI) measure?

  The magnitude and intensity of an eruption on a logarithmic scale
• What is the epicenter of an earthquake?

  The point at the surface directly above an earthquake focus
• What does the Richter scale measure?

  The magnitude of an earthquake using the energy released
• What is the Modified Mercalli scale?

  A subjective measure of earthquake intensity based on human experience and damage
• What is the Moment Magnitude Scale (Mw)?


  A measure of earthquake strength using the amount of physical movement caused by a quake
• What is a rift valley?

  A valley formed by down-faulting between parallel faults
• What is a fault scarp?

  A cliff or escarpment formed by rocks being displaced on either side of a fault
• What is an escarpment?

  A tilted block forming an extensive upland area with a short, steep slope
• What is a phreatic eruption?

  An eruption of steam caused when groundwater meets magma