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GCSE AQA 3.1.1 · 15 minutes · no login

Seafloor spreading

All teacher notes · Open the lab · 2118 words · ·

Why this lesson, and why the 3D

Seafloor spreading 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: Magnetic stripes are a tape recorder. New crust at the ridge, older away. By the end of the slot a pupil should be able to watch stripes move apart from the Mid-Atlantic Ridge. Curriculum tags in the header (GCSE AQA 3.1.1; NGSS MS-ESS2-3) are filters, not a flag. The room is worldwide; SI first; International English. Ages on the card (GCSE, A-level, NGSS MS) are a suggested band, not a gate.

Iceland is the ridge with a country on it, which is why every UK specification parks a case study there. New crust at the axis, older as you walk toward Europe or North America, centimetres per year. Magnetic stripes are the tape recorder: geomagnetic reversals frozen in cooling basalt, mirrored on both flanks. The United States pair is not a second Iceland. Cascadia consumes old Pacific crust at a trench; San Andreas is a transform that does not make ocean floor. Together they complete the conveyor: born at Iceland’s ridge, aged across the basin, destroyed or slid at the far edge. If ocean floor were all the same age, those stripes would not exist and Iceland would be a mystery island. A class that treats the Atlantic as primordial water has not met Vine–Matthews. Kola’s borehole never reached this story; this is surface basalt writing time, not a 12 km scratch in continental crust. UK and US maps both freeze today’s ocean as if it had always been there. It has 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

Find the ridge axis. Newest crust sits there. Play spreading. New stripes walk away, mirrored. Point at the mirror. Both sides of the ridge add crust. Magnetic stripes record geomagnetic reversals frozen in cooling basalt. They are not waves. That pitfall is on the board. Ocean floor is not all the same age. Young at ridges, old at trenches. Iceland sits on this ridge with a country on it. Cascadia can swallow old floor. San Andreas does not make stripes. Vine–Matthews is the tape recorder. Reversals are time. Spreading rate is how fast the plates separate, not a weather story. The ridge height is exaggerated ×12. Say the labelled lie out loud. Finish in Check.

Open the lab full screen (F). Labels on (L). Speak the first step before you touch a slider: Ridge. New stripe. Then the second: Play: bands walk away, mirrored. Then the third: Reversals record time. Iceland sits on this.

Time, labels, and T (true scale) are the whole desk. Play walks the process; T removes the vertical lie where one was drawn. Space plays; arrows nudge time; R resets. Parameters the URL can remember: t, labels, true scale. 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.

There is no extra slider on this lab. Find the ridge axis — newest crust sits there. Press Space: new stripes walk away, mirrored. Left and right arrows nudge the same tape. L toggles labels. R resets. P switches projector mode. E explode and X slice do nothing; this is a seafloor, not an onion. Point at the mirror. Pause and say young at the axis, old on the flanks. Name Iceland as the land on this machine, Cascadia as a place old floor can die, San Andreas as a boundary that does not print stripes.

The in-site film is “Geological features of divergent plate boundaries” (Khan Academy, about 12 min, Khan Academy on YouTube. CC BY-NC-SA. 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.

“Ocean floor is all the same age.” — no. Young at ridges, old at trenches.

“Magnetic stripes are waves on the sea.” — no. They are polarity frozen in cooling basalt, a tape recorder, not a swell.

“San Andreas is a spreading centre, like Iceland.” — no. San Andreas is transform. Iceland is the ridge. Cascadia is where old floor can be swallowed.

Pitfalls to keep in your pocket: Stripes as waves. Ocean floor all the same age. Calling San Andreas a spreading centre. Forgetting the vertical exaggeration ×12 on the ridge. 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

At a mid-ocean ridge, plates diverge. Magma fills the gap; new oceanic crust records the field as it cools through the Curie point. Newest crust sits at the ridge axis. Stripes are symmetric because both sides add crust. Vine–Matthews: reversals frozen in basalt, a tape recorder of time. Spreading rate is how fast plates separate, typically centimetres per year, not a weather story. Iceland sits on the Mid-Atlantic Ridge, plume-boosted, which is why there is land. Old crust is not immortal: it ages away and is swallowed at trenches such as Cascadia. San Andreas is a different boundary — transform, no stripe factory. The Himalaya stack continent on continent; they do not print this tape. The model’s labelled lie is printed on the studio: vertical exaggeration ×12. The ridge is a mountain under the sea, but this height field is a classroom cliff. Stripes are colour bands, not a magnetometer record. Spreading that fills a lesson is millions of years of centimetres per year. Transform offsets on the ridge are a sine wobble, not a fracture-zone chart. What remains true: young at the axis, old away, mirrored polarity, Iceland on the ridge. Ocean floor is not all the same age.

The first place to land is Iceland / MAR. The second is A different boundary. 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: Pillow lava: new crust under water. Age increases away from the axis. Credit: NOAA / USGS, public domain.

Mechanism photograph: Field or official figure for teaching Seafloor spreading. A second photograph for the projector script — not the lab cover. Credit: Wikimedia Commons / NASA / USGS.

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.

Spreading rate: How fast plates separate at a ridge. Centimetres per year, not a weather story. Magnetic reversal: The geomagnetic field flips. Cooling basalt keeps a snapshot of polarity. Mid-ocean ridge: Divergent plate boundary. New oceanic crust at the axis. Iceland sits on one. Vine–Matthews: Magnetic stripes as a tape recorder of reversals and spreading. Exam language also accepts “Vine–Matthews–Morley hypothesis”. Oceanic crust: Basalt and gabbro, ~7 km thick, young at ridges, old toward trenches. Magnetic stripe: A band of normal or reversed polarity, mirrored on both flanks of the ridge. Curie point: Temperature below which cooling rock locks in the ambient magnetic field. Fracture zone: A scar of transform offset on the ridge. This lab only hints at it.

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 3D view a ridge axis, magnetic stripes as a mirror, and age increasing away still teach the tape. Newest crust at the axis; both sides add crust. Reversals frozen in cooling basalt are the clock. Iceland sits on this ridge. Cascadia is where old floor can die; San Andreas is not a spreading centre. The fallback caption is the whole mechanism. Draw a centre line, hatch mirrored bands, and arrow them outward. That sketch is Vine–Matthews. If JavaScript is off you still have the glossary and Check. Do not wait for WebGL to say the ocean has an age.

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. USGS seafloor; NOAA.

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 (GCSE AQA 3.1.1; NGSS MS-ESS2-3) are filters, not a flag. A class in Nairobi, London, or Shanghai can use the same 3D. Ages (GCSE, A-level, 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: Magnetic stripes are a tape recorder. New crust at the ridge, older away. Two: the misconception “Ocean floor is all the same age.”. 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 t, labels, true scale. Send them home with the same link; there is no login.

In-site film

Geological features of divergent plate boundaries. Khan Academy. Khan Academy on YouTube. CC BY-NC-SA. Educational embed, autoplay off. Autoplay off.