EQ1 Key Idea 1.2: Earth's Crust Structure and Tectonic Plates

Pause and Recap

We have completed Key idea 1.1. - the global distribution of tectonic hazards can be explained by plate boundary and other tectonic processes.

Key Idea 1.2: Theoretical Frameworks for Plate Movements

There are theoretical frameworks that attempt to explain plate movements.

Link to the Spec

a. Discuss the theory of plate tectonics (earth's internal structure, mantle convection, palaeomagnetism and sea floor spreading, subduction and slab pull). b. Explain the operation of these processes at different margins (destructive, constructive, collision and transform).

Basic Structure of the Earth

Crust Mantle Outer Core Inner Core Earth's outer layers can be subdivided in terms of Chemical properties (crust, mantle, core) Physical properties (lithosphere, asthenosphere)

Label Your Diagram

Crust 0-100 km thick Lithosphere (crust and upper- most solid mantle) Asthenosphere Mantle Mantle Crust 2900 km Liquid Outer core Core 5100 km Solid Inner core Not to scale 6378 km To scale

OCEAN CRUST 7km CONTINENTAL CRUST } LITHOSPHERE 30km ASTHENOSPHERE UPPER MANTLE 80km Crust + Rigid upper part of the mantle 200km MANTLE 2900km OUTER CORE 5100km INNER CORE 6378km

The Earth's Core

• Dense rock iron and nickel

Mantle Crust Outher core Atmosphere Inner core'

• 5000 degrees Celsius
• Heat is generated by the radioactive decay of elements in the core (e.g. plutonium)
• Primordial - heat left over from when the Earth was formed
• Inner core - solid
• Outer core - molten

Mantle Properties

Molten/semi molten. Rocks containing silicon and oxygen Upper mantle

• Rigid, outermost layer
• Crust plus uppermost mantle Asthenosphere
• Weak, ductile layer
• Moves slowly (convection)
• Lower part of the upper mantle

1 Continent anic crust (mafic) Ocean Moho Continental crust (felsic) Lithospł (rigid Lithospheric mantle (ultramafic) m - Moho Upper Mantle Base of tectonic plate Asthenosphe (weak) Asthenospheric mantle (ultramafic) 660km Lower Mantle Lower Mantle -26 2900km Physical division Chemical division (Not to scale)

The Crust

The crust is solid and where the lightest elements can be found. There are two different types of crust -

1. Continental crust

• Granite
• Less dense
• 30 - 70km
• Older, around 1500 million years old

1. Oceanic crust

• Basaltic
• Denser than continental crust
• Thinner than continental crust, averaging 7km

*Moho = The sharp boundary between the crust and upper mantle

Organise Statements Around Earth Structure Diagram

Temperature: 6,000 degrees C. Made of rocks rich in iron and nickel. Organise the statements around your diagram of the structure of the earth. Composed of silicate rocks rich in iron and magnesium. Thinnest layer, has the coolest, least dense rocks. Semi-molten outer core contains a solid inner core. Rocks are rich in silicon, oxygen, aluminium, potassium and sodium. Approximately the size of Mars and is the densest part of our planet. Semi-molten. Also called the asthenosphere. Temperature: 5,000 degrees C. High temperatures generate convection currents. Together with the rigid upper mantle are known as the lithosphere. Two types: oceanic (6- 10km thick) and continental (up to 70km thick).

Earth Structure Diagram Labels

Temperature: 6,000 degrees C. Inner Core Made of rocks rich in iron and nickel. Core Organise the statements around your diagram of the structure of the earth. Composed of silicate rocks rich in iron and magnesium. Mantle Thinnest layer, has the coolest, least dense rocks. Crust Semi-molten outer core contains a solid inner core. Core Rocks are rich in silicon, oxygen, aluminium, potassium and sodium. Crust Approximately the size of Mars and is the densest part of our planet. Inner Core Semi-molten. Also called the asthenosphere. Mantle Temperature: 5,000 degrees C. High temperatures generate convection currents. Mantle Together with the rigid upper mantle are known as the lithosphere. Crust Two types: oceanic (6- 10km thick) and continental (up to 70km thick). Crust

Why Do the Plates Move? Convection Currents

So, why do the plates move? 1 reason is due to Convection currents All tectonic hazards are caused by the Earth's internal heat engine. Radioactive decay of isotopes such as uranium -238 in the Earth's core and mantle generate huge amounts of heat which flow towards the Earth's surface. This heat generates convections currents in the mantle, which in turn move the tectonic plates. These convection currents operate as cells. The interior of the Earth is therefore dynamic rather than static.

Slab Pull Diagram

Ridge Trench "SLAB PULL" I Lithosphere Trench Asthenosphere Mantle 700 km Outer Core Inner Core

Convection Currents in the Mantle

Ridge Lithosphere rench Trenc Asthenosphere Mantle Convection Cells 700 km Convection currents Convection currents are warm currents within the asthenosphere. As they circulate they provide the energy that move the large slabs of crust on which we exist. The crustal plates will then move apart, alongside or against each other depending on where they are in the current.

https://www.bing.com/videos/search?q=convection+vcurrents& &view=detail&mid=5FBE387E88C5C0A72AC65FBE387E88C5COA 72AC6&&FORM=VRDGAR

Question - Why Do Tectonic Plates Move?

Question - why do tectonic plates move? Slab pull Ridge push ridge push slab pull Convection traction convection traction slab pull

Mechanisms of Plate Movement

convection currents plate dragged by a convection current in the mantle convection current hotter mantle rises cooler mantle sinks ridge push plate pushed by the weight of a mid-ocean ridge slab pull plate pulled by the weight of its cold, dense subducting section mid-ocean ridge subduction zone

Ridge Push Explained

Ridge Push This is the term given when magma that rises pushes the plates apart. This is a continuous process that may be linked to sea floor spreading

Slab Pull Explained

Slab Pull

• This is the more accepted theory- that the gravitational pull of subducted lithosphere is a driving force of plate movement.
• The cold, dense lithosphere sinks as an ocean trench pulling the rest of that plate with it.

Ridge Push and Slab Pull Diagram

Oceanic Ridge Sea Level Ridge Push Oceanic Trench Lithosphere Magma Convection Currents in Asthenosphere Slab Pull Cool descending currents Hot rising currents Sea Floor spreading occurs where the convection currents cause plates to be forced apart.

Sea Floor Spreading

1 Sea floor spreading Seafloor Spreading - YouTube Sea Floor spreading occurs where the convection currents cause plates to be forced apart.

Evidence for Sea Floor Spreading

What is the evidence for this idea?

Pleistocene to Recent (0-2 M.Y.A.) Paleocene (58-66 M.Y.A.) Pliocene (2-5 M.Y.A.) Late Cretaceous (66-88 M.Y.A.) Miocene (5-24 M.Y.A.) Middle Cretaceous (88-118 M.Y.A.) Oligocene (24-37 M.Y.A.) Early Cretaceous (118-144 M.Y.A.) Eocene (37-58 M.Y.A.) Late Jurassic (144-161 M.Y.A.) Drilling cores reveal that the age of the oceanic crust increases away from the MOR.

Palaeomagnetism

Palaeomagnetism mid-ocean ridge + + - + - + positive magnetic anomaly 1 magma negative magnetic anomaly Mid-ocean ridge Widening sea floor Widening sea floor Lava from below - -

• Roughly 4 times in every million years the Earth's magnetic field reverses!
• So north becomes south, and south becomes north.
• The sea floor records each of the reversals.

How Palaeomagnetism Works

How ?

• Magma that is erupted at the MOR is basaltic and contains iron
• The iron in the molten magma lines up parallel to the magnetic field at the time of the eruption.

The rocks will cool and 'set' this permanently marks the magnetic field at that time- it cannot be changed. This then leaves a striped pattern on the sea floor showing the polarity of the earth. mid-ocean ridge + - ++ + + positive magnetic anomaly magma negative magnetic anomaly

Sea Floor Spreading and Magnetic Field

Ocean surface Mid-oceanic ridge N N N S 1 N S S S Oceanic crust Mantle Sea floor spreading at the mid-oceanic ridge - direction of Earth's magnetic field (recorded in solidified lava)

Sea Floor Spreading Activity

Sea floor spreading activity

• Complete the sea floor spreading: the evidence worksheet

The Benioff Zone and Subduction Processes

The Benioff Zone and subduction processes Link to the spec c. Understand the physical processes impact on the magnitude and type of volcanic eruption, and earthquake magnitude and focal depth (Benioff zone).

Benioff Zone and Subduction

The Benioff Zone and subduction

• The Benioff Zone is an area of seismic activity with the slab being thrust downwards in a subduction zone.
• The different speeds and movements of rock at this point produce numerous earthquakes that vary in depth. Shallow: 0-70 Intermediate: 70-300 Deep: 300-700km

Wadati-Benioff Zone Characteristics

Wadati-Benioff Zone Was named after two seismologists Hugo Benioff and Kiyoo Wadati. Known for deep-focus earthquake. The zone of Seismicity corresponds to the down-going slab in a subduction zone. The dip can be between 30° to 60° The Benioff zone may extends from near surface to depths of up to 650-700 km. Most of the earthquakes occur within 1000℃ isotherm due to internal deformation and dehydration embrittlement of the subducting slab.

Earthquakes, Volcanism and Subduction

Earthquakes, Volcanism and Subduction Oceanic Crust Continental Crust Lithosphere Lithosphere Asthenosphere Benioff Asthenosphere zone Kay: (Earthquakes) Shallow Inbarmediato Deep continental volcanoes trench sea-level 1 oceanic lithosphere XX continental lithosphere Xx Benioff XX _ Znne Diagram of Wadati-Benioff zone, from the United States Geological Survey

Focal Depth of Earthquakes

Focal depth Earthquakes always occur below the earth surface but their depth varies from: 0-700km deep. Volcanic island arc Trench Shallow: 0-70 Intermediate: 70-300 Deep: 300-700km

Wadati-Benioff Zone in Japan

Marginal sea China Oceanic crust 250 km Japan Wadati-Benioff zone 500 km Asthenosphere Japan trench 700 km Key

• Shallow
• Intermediate
• Deep Partial melting Subducting oceanic lithosphere

Benioff Zone and Subduction at Convergent Boundaries

Benioff Zone and Subduction 10 Convergent boundary Andes Mountains Sea level South American Plate Nazca Plate -+ Continental lithosphere -100 Elevation (km) -200 -300 -400 -500 -600 500 1000 Distance (km) Convergent boundary 10 - Oceanic lithosphere Kermadec arc Sea level -5 Australian Plate -+ ..... .... .. ... Pacific Plate -100 Elevation (km) -200 -300 -400 -500 -600 500 1000 Distance (km) Convergent boundary Himalayas 10 - Tibetan Plateau Sea level + Eurasian Plate Elevation (km) -5 Indian Plate -+ . ... ... Continental lithosphere Continental lithosphere -100 -200 Oceanic lithosphere -300 500 1000 Key Distance (km) Depth of hypocentre · 0-33 . 34-100 . 101-400 . 401-700 Figure 1.9 Variations in earthquake hypocentres at convergent boundaries

Key Term: Hypocentre

Key term: The hypocentre (focus) is the focus point within the ground where the strain energy of the earthquake in the rock is first released. The distance between this and the epicentre (on the surface) is called the focal length. The hypocentre can occur at any depth between the earth's surface and about 700km below This shows the variations in hypocentres at earthquakes along the subduction zones at destructive and collision plate boundaries. The most damaging earthquakes have a shallow focus of less than 40km .""Oceanic lithosphere Oceanic lithosphere