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A-level plate tectonics background · 15 minutes · no login

Isostasy

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Why this lesson, and why the 3D

Isostasy is a fifteen-minute projector lesson, not a homework packet and not a film with a quiz bolted on. Put this hook on the board before you dim the lights: Scotland is still rising after the ice. Hudson Bay is still rising too. The crust floats. Load it, it sinks. Unload it, it comes back. By the end of the slot a pupil should be able to load a crustal block with ice, watch it sink into the mantle, unload it, and name rebound. Curriculum tags in the header (A-level plate tectonics background; NGSS HS-ESS2-1) are filters, not a flag. The room is worldwide; SI first; International English. Ages on the card (A-level, HS Earth Sci, NGSS MS) are a suggested band, not a gate.

Western Scotland has raised beaches. The ice went; the land is still coming up. Hudson Bay’s shore is doing the same work. A-level and NGSS treat isostasy as why mountains have roots and why ice can bend a continent. If a class thinks crust sits on a rigid floor, rebound is a miracle and gravity anomalies are a rumour. Holderness till is a load too small to see. An ice sheet is not.

The 3D is a named, inspectable object. Click the part, not a caption. If the model will not spin, the Why panel still carries the mechanism, the two photographs, and the film. Do not apologise for the model. Drive it. A silent spin teaches a screensaver. A named mesh that changes under a slider teaches the process.

Fifteen minutes on the projector

A block floating on a denser mantle. Wood in water, school Archimedes.\nAdd ice. The block sinks. Ask why before you name isostasy.\nRemove ice. Rebound.\nScotland’s raised beaches. Hudson Bay’s rising shore.\nThe mantle creeps. It is not lava. Point at the interior lab if they protest.\nMillimetres a year in the real world. This slider is a lie of time.\nL, P. Then Check.

Open the lab full screen (F). Labels on (L). Speak the first step before you touch a slider: A crustal block floats on a denser mantle. Then the second: Add ice. The block sinks. Then the third: Remove ice. Rebound. Scotland and Hudson Bay.

The sliders that earn their keep are Ice load. Space plays; arrows nudge time; R resets. Parameters the URL can remember: ice. If you only have eight minutes, skip the last step and finish in Check. Projector layout (P) is for the back row: one idea, large type, the model still live.

Start with no ice. Note the waterline. Add ice. The block sinks. Remove ice. Watch rebound. Keyboard: Space, arrows on the load, L, R, P. Ask whether the mantle is liquid before you say creep. Caption stays: mantle is solid rock that flows slowly.

The in-site film is “The Deepest Point in the World: Crash Course Geology #12” (Crash Course Geology, about 11 min, Crash Course Geology / Complexly on YouTube. Standard YouTube licence. Educational embed, autoplay off.). Autoplay is off. Play one minute if you need a second voice; do not let the film eat the slot. The 3D still has to move under your hand.

What the third row will get wrong

You will hear the misconception before you have finished the first sentence. Write the false claim on the board and leave it there until the model has contradicted it. Do not skip this. A fluent pupil can recite the hook and still believe the false sentence.

“The ground under our feet is a rigid floor that never moves up or down.” — no. It floats. Ice can sink it. Melt can raise it, slowly.

“Rebound means the mantle is lava.” — no. The mantle is solid rock that creeps. Same story as the interior lab.

“Raised beaches mean the sea fell everywhere.” — no. Local land rose. Global sea level is a different number.

Pitfalls to keep in your pocket: Rigid-floor crust. Mantle as magma ocean. Mixing eustatic sea level with local rebound. Do not let a pupil leave thinking the 3D is a photograph. It is a teaching model. Where vertical exaggeration exists, T collapses it and the caption says so. A labelled lie of scale is honest. An unlabelled one is a cartoon.

Two places, not a slogan

Input is a load: ice, or sediment, or a mountain root. Lithosphere is rigid enough to act as a raft; the mantle beneath is solid but it creeps. Add ice, the raft sinks, mantle flows out. Remove ice, the raft rises, mantle flows back. That is glacial isostatic adjustment. What moves: the block’s height, the mantle under it. What does not: the fact of buoyancy. What is conserved: mass — ice mass leaves, rock mass stays. The lie: a wood-block-in-honey tank, sped up from millennia to a slider. Real rebound is still going, millimetres a year. The mantle is not lava. It is the same solid rock as the interior lab, creeping. Sea level in this scene is a local line so you can see emergence. Global sea level is a different budget.

The first place to land is Scottish / Lake District rebound. The second is North American ice load. Same process, different rock or climate. If a pupil can only name one place, they have a postcard, not a mechanism. Atlas entries for those names are a click from the case list; send them back to the lab with one sentence they can say in Check.

Do not flatten the comparison into “this country versus that country.” Name the landform. Holderness is till, not chalk. Hawaii is a hotspot, not an island arc. Yosemite is a glacial trough, not a river canyon. The 3D is allowed to be schematic; the place names are not allowed to be sloppy.

Figures and the film

Field photograph: A raised shoreline after ice left. A raised shoreline after ice left. Buoyancy of the crust (and lithosphere) in a denser mantle. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC. Credit: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.

Mechanism photograph: Field photograph or official scientific figure for teaching isostasy. Field photograph or official scientific figure for teaching isostasy. A second photograph for the projector script — not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC. Credit: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.

Two photographs, two jobs. The field picture is the place. The mechanism picture is the process. Do not run the lesson on a single Apollo Earth disc. A carbon-cycle class is a forest and a seam, not a blue marble. An ENSO class is a Pacific temperature map, not Africa from Apollo 17.

Words they must be able to say

Say these out loud once while you click the matching mesh. If a word has no mesh, it still has a photograph in Terms.

Isostasy: Buoyancy of the crust (and lithosphere) in a denser mantle. Load down, unload up. Glacial rebound: Land rising after ice has gone. Scotland, Hudson Bay, Fennoscandia. Exam language also accepts “Glacial isostatic adjustment”. Raised beach: An old shoreline now above the sea. A rebound fossil. Lithosphere: The rigid raft: crust plus the stiff top of the mantle. Asthenosphere: Weaker mantle below that can creep. Solid, not a magma ocean. Root: Thick crust under a mountain, the submerged part of the raft. Eustatic: Global sea-level change. Not the same as local rebound.

Exam-language toggle (the Teacher tab) swaps UK/US pairs where they exist — levée/levee, arête/arete — so a mixed room is not punished for spelling.

Close the slot

Without the canvas: crust floats on a denser mantle. Ice is a load. Load down, unload up. Scotland’s raised beaches and Hudson Bay’s rising shore are rebound still in progress. The mantle creeps; it is not a magma ocean. Mountains have roots for the same reason. That is enough for Check.

Finish in Check — ten questions, not a mood. The bank mixes definition, process, the misconception you just killed, a named place, and a read-the-model item (click the part; T). If a pupil can only recite the hook, they have not used the geometry. Wrong answers still show the one-line explain. Do not skip the explain.

Sources on the Teacher tab are the trail, not a bibliography to copy. BGS glacial rebound; USGS / NRC Hudson Bay uplift.

Link back to the live lab and keep the same slug. Do not invent a parallel worksheet that contradicts T, SI, or the named parts.

This note is worldwide. Curriculum tags (A-level plate tectonics background; NGSS HS-ESS2-1) are filters, not a flag. A class in Nairobi, London, or Shanghai can use the same 3D. Ages (A-level, HS Earth Sci, NGSS MS) are a suggested band. If a sentence only works in one exam board, rewrite it before you project it.

Write these three lines on the board before you dim the lights. One: Scotland is still rising after the ice. Hudson Bay is still rising too. The crust floats. Load it, it sinks. Unload it, it comes back. Two: the misconception “The ground under our feet is a rigid floor that never moves up or down.”. Three: the named parts they must click. Then drive the model. Check is ten questions covering definition, process, the misconception, a place, and a read-the-model item. Do not skip the explain on a wrong answer.

Board, units, and what not to say

SI first. Metres, kilometres, degrees Celsius, metres per second. A conversion in brackets is courtesy, not a second system. Do not let a pupil leave thinking the 3D is a photograph. It is a teaching model. Where vertical exaggeration exists, T collapses it and the caption says so.

Do not say “teaching mesh”, “sprinkle”, or “thin free surface not a fat pipe” out loud. Say the landform. Seasons are ~23.44° of axial tilt, not Earth–Sun distance. The outer core is liquid; S-waves do not pass. Yosemite is a glacial U-trough. San Andreas is a transform. Hawaiʻi and Yellowstone are hotspots. Florida is not a glacial trough.

If the room is noisy, projector layout (P) is the back-row setting: one idea, large type, the model still live. Full screen (F). Labels (L). Space plays. Arrows nudge time. R resets. The URL can remember ice. Send them home with the same link; there is no login.

In-site film

The Deepest Point in the World: Crash Course Geology #12. Crash Course Geology. Crash Course Geology / Complexly on YouTube. Standard YouTube licence. Educational embed, autoplay off. Autoplay off.