Distribution of Earthquakes and Volcanoes
Earth’s structure:
● Core
○ Inner core
■ 1200 km
■ 6000°C
● Dense, solid ball → due to high pressure
■ Is where radioactive elements decay and release heat
● Produces Earth’s magnetic field → causes movement on the surface
○ Outer core
■ 2266 km
■ 4000°C – 5500°C
■ Liquid
○ Made out of iron and nickel
Mantle
○ Layer surrounding the core
○ Made up of the lower mantle and upper mantle
○ Has solid properties but can flow very slowly
■ Lower mantle
● 670 km and 2900 km
● Hot and dense → due to high pressure
● Made out of semi-solid matter with high viscosity that flows very slowly
■ Upper mantle
● Made up of semi-solid material
● Rock in upper region is stiffer → due to cooler temperatures
● Has 2 sections
○ Lithosphere = crust and solid part of upper mantle → tectonic plates
■ 80 – 100 km
■ Made out of peridotite
■ Is the rigid outer part of Earth
○ Asthenosphere = upper layer of Earth’s mantle
■ 100 – 300 km
■ Weak
■ Denser and more mobile → allows tectonic plates to move over it
○ Temperatures between 1000°C and 3700°C
● Crust
○ 0 – 60 km deep → very thin compared to the thickness of the mantle and core
■ Continental crust = crust that carries land
● 70 km
● Thick and less dense
● Floats higher on the mantle
● Made out of granite (igneous rock; cooled down magma)
● Older crust → up to 3.8 billion years old
■ Oceanic crust = crust that carries water
● 5 – 8 km
● Thin and denser
● Floats lower on the mantle
● Made out of basalt (igneous rock; cooled down magma)
● Recycled at destructive margins
● Younger crust → less than 200 million years old
Plates and plate margins:
● Asthenosphere = upper layer of the Earth’s mantle
● Lithosphere = rigid outer part of the Earth
● Tectonic plates = fragments of crust that floats across heavier, semi-molten
upper mantle
○ Movement can be tracked from space using GPS
● Plate boundary = where 2 plates meet
○ 4 different types of plate boundaries:
■ Conservative
■ Constructive
■ Collision
■ Destructive
○ Interaction leads to the triggering of earthquakes and volcanic activity
Movement of tectonic plates:
● THEORY 1: Convection currents
○ Caused by immense heat from the core
■ Radioactive decay of uranium and thorium takes place in the inner core
■ Releases alpha particles that produce lots of heat
○ Only limited evidence of convection currents
○ Process:
1. Magma is heated in lower mantle by the core → becomes less dense and rises towards mantle
2. Magma begins to cool down as it rises → become more dense
3. Magma reaches asthenosphere but is forced sideways because it’s blocked by lithosphere
○ Convection currents exert a pull on the plates above → causing them to pull
apart/move towards each other
4. Magma cools and sinks
5. Magma reaches outer core but is forced sideways again due to solid iron/nickel layer
6. Currents heat up magma above it in the asthenosphere
7. Currents form and flow beneath lithosphere and carry plates with them
○ Leads to a chain of convection currents going up towards the crust
THEORY 2: Ridge push and slab pull
○ Ridge push = gravity causes the ridge to push on the lithosphere , and move tectonic plates
○ Ridge = underwater mountain range caused by 2 tectonic plates moving away from each other
■ Occurs at constructive margins
● Ocean ridges form and the mantle melts beneath
● Molten magma rises as plates move apart
● Molten magma cools to form new plate material
■ As lithosphere cools it becomes denser
● Starts to slide down away from ridge
● Causes plates to move away from each other
○ Slab pull = weight of a dense tectonic (oceanic) plate is subducted into the mantle
■ Occurs at destructive margins → causes earthquakes/volcanoes
■ Denser plate sinks back into the mantle
■ Pulls the rest of the plate along behind it
● Ridge push may be happening at the other end of the plate
Location of earthquakes and volcanoes:
● Locations match plate boundaries
○ Earthquakes are found at:
■ Collision boundaries
■ Conservative boundaries
■ Destructive boundaries
○ Volcanoes are found at:
■ Constructive boundaries
■ Destructive boundaries
● Found on both land and sea
● Not always along plate boundaries
○ Some occur in the middle of plates
○ Known as “ hot spots ” → particularly where Earth’s crust is thin
■ eg/ Hawaii
● Large band of volcanoes/earthquakes circling the Pacific Ocean = Ring of Fire