Tectonic Processes and Hazards
1.2a Structure of the earth
• Core (inner and outer), mantle (lower and upper), asthenosphere, lithosphere, crust
• Inner core is solid (5000-6000C), outer core is semi-molten
o Both made from iron and nickel
o Heat comes from creation of the planet, collision of rock and entrapment of heat /
radioactive material
• Mantle is widest / thickest later
o Upper mantle = solid, lower = semi-molten (1600-4000C)
▪ Temperature increases with depth
• Crust forms the outer shell
o Oceanic and continental
• Crust + upper mantle = lithosphere
• Continental: 25-65 km, 2.7 grams/cm3, solid, granite
• Oceanic: 5-10 km, 3.3 grams/cm3 (denser), solid, basalt, forms the ocean floor
1.2a The theory of plate tectonics
• Sea floor spreading
o Molten rock oozes up from the Earth’s interior along the mid-oceanic ridges,
creating a new seafloor
o Process happens repeatedly, creating new seafloor, pushing previous ridges away
from the active zone
o Tells us plates are moving
• Palaeomagnetism: study of previous changes in the Earth’s magnetic field
o Every 400,000 years the magnetic field flips, so north and south magnetic poles
switch
o Minerals in the rock line up with the Earth’s magnetic direction
o Mid-Atlantic ridge there is a pattern of magnetic direction either side of the ridge
▪ Could only happen if new rock is being formed at the same time on both
sides
• Mantle convection / convection currents: hot liquid magma currents in the asthenosphere
causes the plates to move
1. Inner core is the hottest
2. Heat from the core is released and transferred to the mantle where there is a steep
temperature gradient
3. With depth, temperature increases, therefore rocks in lower mantle are hot and
plastic, upper mantle = brittle and cool in comparison
4. Rocks in the lower mantle are heated, become less dense and flow upwards through
the mantle to the upper brittle sections
5. As the rock moves up away from the core it losses temperature, cools and sinks back
down through the mantle
6. Resulting heating and cooling creates circulation through the mantle, circulations
called convection currents
7. Convection currents in the upper mantle create friction and drag between the
asthenosphere and lithosphere, the lithosphere is then dragged around the earth’s
surface
8. Movement is very slow
, • Slab pull: now seen as a major driving force for plate movement
o Newly formed crust at the mid ocean ridges becoming denser and thicker as it cools
o Causing it to sink into the mantle under its own weight, pulling the rest of the plate
down with it
• Subduction zone
1.2a Key elements of the plate tectonics theory (including: mantle convection, palaeomagnetism and
sea floor spreading, subduction and slab pull)
1.1a Distribution of tectonic hazards
1.2b Plate boundaries (destructive, constructive, collision and transform)
Destructive plate margins:
• Oceanic / continental (convergent - moving towards each other)
• Denser oceanic forced under lighter continental crust (subduction)
o This forms ocean trenches, deepest parts of the ocean floor
o More the rock descends, the hotter it gets, along with friction this causes part of the
oceanic plate to melt into magma
o This part of subduction called the Benioff Zone
o Magma rises as less dense than the surrounding asthenosphere reaching the surface
and forming volcanoes
o Sediments that have accumulated along continental plate edge are uplifted and
form fold mountains
• Lava is viscous / andesitic which creates composite cones
• As oceanic plate descends it can trigger shallow, intermediate or deep focus earthquakes
o Example: Nazca plate subducting under the South American plate, forming Peru-
Chile trench and the Andes
• Oceanic / oceanic
• When two oceanic plates meet one is forced under the other and so the process of
subduction begins, forming island arcs and ocean trenches
, o Island arcs: volcanoes strung out in a chain, generally curved and follow the line of
the trench
• Magma is formed by the partial melting of the descending plate in the Benioff zone
• Descending plate also provides a source of stress leading to earthquakes
o Example: western side of the Pacific, pacific plate subducting below the Philippine
plate forming the Marianas Trench and the island arc of Guam
• Continental / continental (collision)
• Plates not subducted, tend to buckle and be pushed upwards, with the edges of the plates
and sediment is forced upward
o Movement can trigger shallow focus earthquakes
• Results in fold mountains
o Example: Indo-Australian plate forced in the Eurasian plate
o Used to be a sea there, now sediments forced upward to create the Himalayas
Constructive plate margins:
• Plates are moving apart and the space between is filled with basaltic lava upwelling from
below causing new crust to be continuously formed
o Where plates move apart in oceanic areas = ocean ridges
o Where plates move apart in continental areas = rift valley
• Constructive plate margins in oceanic areas
• New crust is formed here, magma rises up from the mantle and creates a new crust which
forces plates either side of the boundary apart (seafloor spreading)
• Most constructive plate margins are along the central zone of the world’ major ocean basins
(e.g., Atlantic or Pacific Ocean)
o Boundary normally marked by a ridge
o Ridge sometimes offset by a transform fault (not all in a straight line together)
• Oceanic ridges:
• Form of the ridge influenced by the rate of plates separating
o Slow rate = produces wide ridge axis and deep central rift valley with inward-facing
fault scarps
o Intermediate rate = less well marked rift and smoother outline
o Fast rate = produces smooth crest and no rift
• Volcanic activity occurs along the ridges (forms submarine volcanoes) - Iceland formed this
way
o Gently sloping sided volcanoes due to low viscosity lava, eruption are frequent but
relatively timid
• New crust forming creates transform faults at right angles to the plate boundary
• Spreading at different speeds on either sides of the fault can cause friction and shallow-
focus earthquakes
• Constructive plate margins in continental areas
• Continental crust differs from oceanic = thicker, less dense, different chemical composition
o Continental rifting, creates rift valleys
• Molten rock rises to the surface, forcing land to break and separate, when crust stretches
beyond its limit, tension cracks appear on earth’s surface
o Magma squeezes through cracks, sometimes erupting and forming volcanoes
1.2a Structure of the earth
• Core (inner and outer), mantle (lower and upper), asthenosphere, lithosphere, crust
• Inner core is solid (5000-6000C), outer core is semi-molten
o Both made from iron and nickel
o Heat comes from creation of the planet, collision of rock and entrapment of heat /
radioactive material
• Mantle is widest / thickest later
o Upper mantle = solid, lower = semi-molten (1600-4000C)
▪ Temperature increases with depth
• Crust forms the outer shell
o Oceanic and continental
• Crust + upper mantle = lithosphere
• Continental: 25-65 km, 2.7 grams/cm3, solid, granite
• Oceanic: 5-10 km, 3.3 grams/cm3 (denser), solid, basalt, forms the ocean floor
1.2a The theory of plate tectonics
• Sea floor spreading
o Molten rock oozes up from the Earth’s interior along the mid-oceanic ridges,
creating a new seafloor
o Process happens repeatedly, creating new seafloor, pushing previous ridges away
from the active zone
o Tells us plates are moving
• Palaeomagnetism: study of previous changes in the Earth’s magnetic field
o Every 400,000 years the magnetic field flips, so north and south magnetic poles
switch
o Minerals in the rock line up with the Earth’s magnetic direction
o Mid-Atlantic ridge there is a pattern of magnetic direction either side of the ridge
▪ Could only happen if new rock is being formed at the same time on both
sides
• Mantle convection / convection currents: hot liquid magma currents in the asthenosphere
causes the plates to move
1. Inner core is the hottest
2. Heat from the core is released and transferred to the mantle where there is a steep
temperature gradient
3. With depth, temperature increases, therefore rocks in lower mantle are hot and
plastic, upper mantle = brittle and cool in comparison
4. Rocks in the lower mantle are heated, become less dense and flow upwards through
the mantle to the upper brittle sections
5. As the rock moves up away from the core it losses temperature, cools and sinks back
down through the mantle
6. Resulting heating and cooling creates circulation through the mantle, circulations
called convection currents
7. Convection currents in the upper mantle create friction and drag between the
asthenosphere and lithosphere, the lithosphere is then dragged around the earth’s
surface
8. Movement is very slow
, • Slab pull: now seen as a major driving force for plate movement
o Newly formed crust at the mid ocean ridges becoming denser and thicker as it cools
o Causing it to sink into the mantle under its own weight, pulling the rest of the plate
down with it
• Subduction zone
1.2a Key elements of the plate tectonics theory (including: mantle convection, palaeomagnetism and
sea floor spreading, subduction and slab pull)
1.1a Distribution of tectonic hazards
1.2b Plate boundaries (destructive, constructive, collision and transform)
Destructive plate margins:
• Oceanic / continental (convergent - moving towards each other)
• Denser oceanic forced under lighter continental crust (subduction)
o This forms ocean trenches, deepest parts of the ocean floor
o More the rock descends, the hotter it gets, along with friction this causes part of the
oceanic plate to melt into magma
o This part of subduction called the Benioff Zone
o Magma rises as less dense than the surrounding asthenosphere reaching the surface
and forming volcanoes
o Sediments that have accumulated along continental plate edge are uplifted and
form fold mountains
• Lava is viscous / andesitic which creates composite cones
• As oceanic plate descends it can trigger shallow, intermediate or deep focus earthquakes
o Example: Nazca plate subducting under the South American plate, forming Peru-
Chile trench and the Andes
• Oceanic / oceanic
• When two oceanic plates meet one is forced under the other and so the process of
subduction begins, forming island arcs and ocean trenches
, o Island arcs: volcanoes strung out in a chain, generally curved and follow the line of
the trench
• Magma is formed by the partial melting of the descending plate in the Benioff zone
• Descending plate also provides a source of stress leading to earthquakes
o Example: western side of the Pacific, pacific plate subducting below the Philippine
plate forming the Marianas Trench and the island arc of Guam
• Continental / continental (collision)
• Plates not subducted, tend to buckle and be pushed upwards, with the edges of the plates
and sediment is forced upward
o Movement can trigger shallow focus earthquakes
• Results in fold mountains
o Example: Indo-Australian plate forced in the Eurasian plate
o Used to be a sea there, now sediments forced upward to create the Himalayas
Constructive plate margins:
• Plates are moving apart and the space between is filled with basaltic lava upwelling from
below causing new crust to be continuously formed
o Where plates move apart in oceanic areas = ocean ridges
o Where plates move apart in continental areas = rift valley
• Constructive plate margins in oceanic areas
• New crust is formed here, magma rises up from the mantle and creates a new crust which
forces plates either side of the boundary apart (seafloor spreading)
• Most constructive plate margins are along the central zone of the world’ major ocean basins
(e.g., Atlantic or Pacific Ocean)
o Boundary normally marked by a ridge
o Ridge sometimes offset by a transform fault (not all in a straight line together)
• Oceanic ridges:
• Form of the ridge influenced by the rate of plates separating
o Slow rate = produces wide ridge axis and deep central rift valley with inward-facing
fault scarps
o Intermediate rate = less well marked rift and smoother outline
o Fast rate = produces smooth crest and no rift
• Volcanic activity occurs along the ridges (forms submarine volcanoes) - Iceland formed this
way
o Gently sloping sided volcanoes due to low viscosity lava, eruption are frequent but
relatively timid
• New crust forming creates transform faults at right angles to the plate boundary
• Spreading at different speeds on either sides of the fault can cause friction and shallow-
focus earthquakes
• Constructive plate margins in continental areas
• Continental crust differs from oceanic = thicker, less dense, different chemical composition
o Continental rifting, creates rift valleys
• Molten rock rises to the surface, forcing land to break and separate, when crust stretches
beyond its limit, tension cracks appear on earth’s surface
o Magma squeezes through cracks, sometimes erupting and forming volcanoes
