--- title: "What moves the plates?" subtitle: "Year 8 Earth and space · Plate tectonics · Lesson 3" --- # What moves the plates? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Australia is moving north about 7 centimetres a year, nearly twice as fast as your fingernails grow. A whole continent. What could possibly push it? :::teacher-note **In one sentence:** Students explain that the lithosphere is divided into moving plates and describe mantle convection, driven by Earth's inner heat (from the core and from radioactive decay in the mantle), as the mechanism (AC9S8U03, node 2, a hinge node). **Students should be able to say:** I can explain that the lithosphere is divided into plates that move. I can describe mantle convection as the mechanism that moves the plates. I can state that plates move a few centimetres a year. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **Earth's surface is broken into huge plates that move a few centimetres a year. What do you predict moves them?** ☐ The pull of the Moon ☐ Earthquakes pushing them along ☐ Slow currents in the hot rock below them ☐ Ocean currents and waves My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Slow currents in the hot mantle below: heated rock rises, spreads under the plates, cools and sinks, carrying the plates with it. The heat comes from the core below and from radioactive elements in the mantle rock. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: plates and convection :::keep-together ### The layers **Put each description with its part of the Earth.** Groups: Lithosphere (the plates), Mantle, Core - Rigid pieces that move a few centimetres a year: {{field-15}} - Hot solid rock that flows very slowly: {{field-15}} - Heats the mantle from below: {{field-15}} - Also heated from within, by radioactive elements in its rock: {{field-15}} - Where the convection currents are: {{field-15}} - The crust and the top of the mantle: {{field-15}} ::: :::teacher-note **Answer:** Rigid pieces that move a few centimetres a year → Lithosphere (the plates); Hot solid rock that flows very slowly → Mantle; Heats the mantle from below → Core; Also heated from within, by radioactive elements in its rock → Mantle; Where the convection currents are → Mantle; The crust and the top of the mantle → Lithosphere (the plates) ::: :::keep-together ### Why rock rises **Why does hot rock above the core rise?** ☐ Hot things always go up because heat rises ☐ It is less dense than the cooler rock around it ☐ The core pushes it out ☐ It has melted into liquid magma, which floats up ::: :::teacher-note **Answer:** It is less dense than the cooler rock around it Heated rock expands a little, so it is less dense and rises. Cooled rock is denser and sinks. ::: :::keep-together ### Plates move apart **On the board, where did the two plates move apart?** ☐ Above the sinking currents ☐ Above the rising current ☐ Where the board was coolest ::: :::teacher-note **Answer:** Above the rising current Rising rock spreads out sideways under the plates, dragging them apart. This is a divergent boundary, like the Mid-Atlantic Ridge. ::: :::keep-together ### How fast **Australia moves about 7 cm north each year. How many kilometres is that in a million years? (100,000 cm = 1 km)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 70 7 cm × 1,000,000 = 7,000,000 cm = 70 km. Slow, but over 50 million years that is 3,500 km. ::: :::typical ## Elaborate: the same idea in water :::keep-together ### Water and rock **What is the biggest difference between the water and the mantle?** ☐ Mantle rock is solid and flows millions of times more slowly ☐ Mantle rock rises when it cools ☐ There is no heat source under the mantle ::: :::teacher-note **Answer:** Mantle rock is solid and flows millions of times more slowly The mechanism is the same; the timescale is not. A loop in the beaker takes seconds; in the mantle, tens of millions of years. ::: ::: :::wider ## Elaborate: pushes and pulls :::keep-together ### Three forces **Match each force to how it moves a plate.** A. A plate slides away, downhill, from a high mid-ocean ridge B. Flowing mantle carries the plate above it along C. Old, cold, dense plate sinks into the mantle and drags the rest of the plate behind it {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Slab pull` | | | `Ridge push` | | | `Mantle drag` | | ::: :::teacher-note **Answer:** Slab pull → C. Old, cold, dense plate sinks into the mantle and drags the rest of the plate behind it; Ridge push → A. A plate slides away, downhill, from a high mid-ocean ridge; Mantle drag → B. Flowing mantle carries the plate above it along ::: :::keep-together ### Which matters most? **The fastest plates (like the Pacific plate) are the ones with long edges sinking into the mantle. Which force does that suggest matters most?** ☐ Neither: plate speed is random ☐ Slab pull ☐ Ridge push ::: :::teacher-note **Answer:** Slab pull Most geologists now think slab pull is the biggest force. All three are parts of the same convection system. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - The **lithosphere** (crust and top of the mantle) is broken into rigid **tectonic plates**, moving a few centimetres a year. - Earth's inner heat (from the **core** below, and from radioactive elements in the mantle rock) drives **convection currents** in the **mantle**: hot rock is less dense and rises, spreads, cools and sinks. - The currents carry the plates: apart above rising rock, together above sinking rock. - The mantle is solid rock that flows extremely slowly, not liquid magma. ## Exit ticket :::keep-together **What drives the movement of tectonic plates?** {{lines-3}} ::: :::teacher-note **Answer:** Convection currents in the mantle: heated rock rises, spreads, cools and sinks, carrying the plates. The heat comes from the core and from radioactive elements in the mantle. :::