Moving Plates, Boundaries & Earth Evidence — Print · Grade 6 Science

Chapter: Earth Systems, Plates & Earth Materials

Moving Plates, Boundaries & Earth Evidence

Learning goals

I can describe the three kinds of plate boundary and what happens at each.
I can use the global pattern of earthquakes as evidence for plate boundaries.
I can explain how magnetic stripes on the sea floor record plate movement.

Readings for this unit

A Stone and Several Possible Histories

Before this unit

Read the chapter opening and the named observation or diagram. Clarify one word or unit that blocks understanding. Recall the relevant earlier measurement, matter, or energy idea.

Study the unit across three 35–45-minute class sessions. Use the worked examples after an attempt and the supplied records when actual equipment is not available.

The lithosphere, the rigid shell about 100 km thick from the last lesson, is cracked into roughly fifteen major plates and a number of smaller ones. They move over the slowly flowing mantle beneath at speeds between about 2 and 10 cm per year, which satellite measurements now track directly.

That is slow enough to be invisible and fast enough to rebuild the planet's surface. At 2.5 cm a year the Atlantic widens by about 25 m in a thousand years and 25 km in a million, which is why the ocean did not exist at all 200 million years ago.

Plates meet in three ways: pulling apart, pushing together, or sliding past each other. Those three cases account for nearly all of Earth's earthquakes and volcanoes, and the map of where earthquakes happen is effectively a map of the plate edges.

Build your understanding

Where this fits

The last lesson established the layers, including the rigid lithosphere and the slowly flowing rock beneath it. This one is what that structure does. It also completes the story begun in chapter 1, where Wegener's continental drift was rejected for forty years because nobody could supply a mechanism; the mechanism is in this lesson.

Use three flexible 40-45-minute sessions: the plates and what moves them; the three kinds of boundary; the evidence, especially magnetic stripes, then the practice set, revision and the exit check. Workshop, review, quiz and exams have separate time.

Part 1 · A cracked shell on moving rock

The lithosphere behaves rigidly because it is relatively cool, and it rests on hotter mantle rock that is still solid but able to creep over long periods. Two forces do most of the driving. Where a cold, dense plate edge sinks back into the mantle, its weight drags the rest of the plate along behind it, which is called slab pull and is thought to be the larger effect. Alongside it, slow convection currents in the mantle, the same process that moved warm water in chapter 3, push and drag at the base of the plates.

The speeds sound trivial and are not. The Pacific plate moves about 7 to 10 cm per year, the Atlantic sea floor spreads at roughly 2.5 cm per year, and GPS stations bolted to bedrock measure these rates directly to within a few millimetres. Over the 200 million years since the supercontinent Pangaea began breaking up, 2.5 cm per year adds up to about 5000 km, which is close to the present width of the Atlantic. The arithmetic matching the observation is one of the simplest and strongest checks available on the whole theory. It is worth being clear about what the plates are made of, because the name misleads people. A plate is not a continent. The African plate carries the continent of Africa and a large area of ocean floor as one rigid piece, and the Pacific plate is almost entirely ocean floor with almost no land on it. Plates are sections of lithosphere, and continents are passengers riding on them.

Naming the motion first makes the landforms predictable rather than a list to memorise. The clustering of earthquakes along these lines is evidence a learner can check on any world map.

Pause and explain

The Atlantic spreads at about 2.5 cm per year. Show that this is consistent with it opening over 200 million years.

Check your reasoning after answering

At 2.5 cm per year the sea floor spreads 25 km in a million years, so in 200 million years that is about 5000 km, which is close to the Atlantic's present width. A rate measured today and an age estimated from rocks and fossils agree without being fitted to each other, which is what makes the match good evidence.

Part 2 · Three ways plates meet

At a divergent boundary two plates pull apart and hot mantle rock rises into the gap, melts as the pressure on it falls, and solidifies as new sea floor. The Mid-Atlantic Ridge runs the length of that ocean doing exactly this, and Iceland sits on top of it, which is why the island has so many volcanoes and is slowly being pulled in two. The East African Rift is the same process beginning on land, and in perhaps ten million years it will have opened a new ocean.

At a convergent boundary plates push together, and what happens depends on what is colliding. Where dense oceanic plate meets lighter continental plate, the oceanic one bends down and sinks, a process called subduction, producing deep trenches, powerful earthquakes and a line of volcanoes inland: the Andes and Japan are both built this way. Where two continental plates meet, neither sinks easily and the rock crumples upward instead, which is how India colliding with Asia raised the Himalaya, still rising about 5 mm a year. At a transform boundary plates slide past each other, grinding and sticking and then slipping suddenly, which is the San Andreas Fault and why California has earthquakes without volcanoes. The depth of earthquakes tells you which of these you are looking at better than anything visible at the surface. At a spreading ridge or a transform fault, earthquakes occur only in the top few tens of kilometres, because that is as deep as the rigid rock goes. At a subduction zone they occur as deep as 600 km, tracing the descending slab down into the mantle, and no other setting on Earth produces earthquakes at that depth.

Pause and explain

Why does California have major earthquakes but no line of volcanoes, while Japan has both?

Check your reasoning after answering

Because the boundaries differ. California sits on a transform boundary where two plates slide past each other, which builds up and releases stress as earthquakes but sends no rock down into the mantle. Japan sits above a subduction zone where an oceanic plate descends, and the melting that produces supplies magma for a volcanic chain as well as earthquakes.

Part 3 · The evidence that settled it

The decisive evidence came from the sea floor in the 1950s and 1960s. Earth's magnetic field reverses direction every few hundred thousand years, and new rock forming at a ridge records the field direction as it cools and locks it in permanently. Surveying across the Mid-Atlantic Ridge revealed stripes of alternating magnetic direction running parallel to it, and the pattern on one side is a mirror image of the pattern on the other. There is no way to produce that except by new rock forming at the centre and moving outward in both directions.

Other evidence lines up with it. Plotting every recorded earthquake on a world map draws the plate boundaries almost perfectly, with narrow belts of activity and large quiet interiors. Rock ages on the sea floor increase steadily with distance from the ridges, from nearly zero at the crest to about 180 million years at the oldest margins, and no sea floor anywhere is older than that, because it has all been recycled by subduction. Meanwhile Wegener's original fossil and coastline evidence, dismissed for decades, fits the reconstructed positions exactly. Measurement from space closed the argument completely. GPS stations fixed to bedrock on different plates now record their separation continuously, and the rates they report, a few centimetres a year, match the rates calculated from magnetic stripes laid down over millions of years. A prediction made from rocks and confirmed decades later by satellites is about as convincing as evidence in this subject becomes.

Pause and explain

Why are the magnetic stripes on either side of a mid-ocean ridge such strong evidence for sea-floor spreading?

Check your reasoning after answering

Because the two sides mirror each other. New rock forming at the ridge locks in whichever way Earth's magnetic field is pointing as it cools, and the field reverses periodically. A symmetrical pattern of reversals either side of the crest can only arise if rock is produced at the centre and carried outward in both directions at the same rate.

Part 4 · What the theory lets you predict

A theory earns its place by predicting, and this one does so unusually well. Knowing only the boundary type, you can say what a region should experience: deep trenches and explosive volcanoes at a subduction zone, gentle fissure eruptions and shallow earthquakes at a spreading ridge, high mountains and powerful shallow earthquakes but no volcanoes at a continental collision, and earthquakes without volcanoes at a transform fault. Those predictions can be checked against any region on Earth.

The theory also predicts the future, which is rarer. The Atlantic is widening at about 2.5 cm a year, so it will keep growing; the Pacific is shrinking as subduction consumes its floor faster than its ridges create it; East Africa will eventually separate; and Australia is moving north at about 7 cm a year, enough that the country's official map coordinates had to be shifted by 1.8 m in 2017 so that satellite navigation would match the ground. Predictions at that level of precision are what moved plate tectonics from a proposal to the organising idea of the whole subject. There is one thing the theory still does not deliver, and it is worth knowing honestly. It explains where earthquakes will occur and why, but it does not predict when. Stress builds along a locked fault over decades or centuries and releases unpredictably, so hazard maps give probabilities over long periods rather than dates. Knowing the mechanism and being able to forecast the event are different achievements, and only the first has been managed.

Pause and explain

Australia's official map coordinates were shifted by 1.8 m in 2017. What does this show?

Check your reasoning after answering

That plate motion is large enough to matter at everyday precision. Australia moves north about 7 cm a year, so over roughly twenty-five years the continent had drifted about 1.8 m from the position its coordinates described. Satellite navigation is accurate enough to notice, so the national reference frame had to be corrected to match the ground.

Apply the whole unit · Identify the boundary

Three regions. Region A has a deep ocean trench offshore, earthquakes as deep as 600 km, and a chain of explosive volcanoes inland. Region B has frequent shallow earthquakes along a single line, no volcanoes, and a road offset by 3 m where it crosses that line. Region C has a long valley with volcanoes, hot springs, and the land either side measurably moving apart.

Your task

Name the boundary type in each region and give the evidence that decides it.

What your response should show

Names all three boundary types correctly
Chooses evidence that actually discriminates rather than listing all of it
Uses earthquake depth, not just presence, for the subduction case
Gives a checkable further prediction for each region

Response guidance — after your attempt

Identify the boundary type for each region.
Name the single most decisive piece of evidence in each case.
Predict one further observation you would expect in each region.

Region A is a convergent boundary with subduction. The decisive evidence is the earthquakes as deep as 600 km, because only a plate descending into the mantle can generate earthquakes that far down; a trench alone could be other things, but deep earthquakes cannot. I would also expect the sea floor near the trench to be the oldest in that ocean. Region B is a transform boundary. The decisive evidence is the offset road, which shows the ground on either side has slid horizontally past itself rather than one side rising or sinking. I would expect all earthquakes there to be shallow, since no rock is descending. Region C is a divergent boundary. The decisive evidence is the measurable separation of the land on either side; volcanoes and hot springs alone would not distinguish it from other settings. I would expect the youngest rock to lie along the centre of the valley and ages to increase outward in both directions.

Scientific references for this unit

Parent / teacher teaching notes

Begin with the literal observation, quantity, or diagram relationship. Ask the learner to explain why the evidence supports the claim, check a relevant unit or boundary, then discuss an alternative. The supplied numbers are illustrative, not real observations. A parent can read aloud and allow a spoken rehearsal before writing. Judge science and evidence rather than political, faith, or family agreement. Use the complete data option whenever an actual procedure is unavailable or unsuitable.

The key idea

Earth's rigid outer shell is broken into about fifteen pieces that move a few centimetres a year, roughly the rate fingernails grow. Almost every earthquake, volcano and mountain range on the planet sits where two of those pieces meet.

Worked examples

Example 1 · Read relative movement

Calculate the mean separation rate and explain a boundary inference with a limit.

Subtract starting from ending separation.
Divide by the three-year interval.
Use ridge evidence while limiting the conclusion.

Answer: Separation grows by 6.0 cm in three years, a mean relative rate of 2.0 cm/year. Together with the ridge, this supports a divergent-boundary interpretation in the supplied model. It does not establish that each plate moves at exactly 2 cm/year or predict every earthquake or volcano.

Example 2 · Transfer to a different case

Two plates slide horizontally past each other in a supplied model. Must a new mountain range form there?

Identify the distinction and conditions in this new case.
Use the relevant mechanism or quantity; preserve units and the chosen boundary.
Explain why the answer fits and what further evidence would be needed.

Answer: No. That is transform motion; outcomes depend on local conditions, and mountain building is not an automatic result of every boundary interaction.

Example 3 · Connect divergence and convergence with material processes · a focused example

Do all convergent boundaries have identical outcomes?

State the literal condition or observation in the example.
Apply the distinction explained in this teaching part.
Check whether the claim stays within the given evidence.

Answer: No. Plate composition, density, and geometry affect whether subduction, mountain building, and volcanic activity occur.

Avoid these mistakes

Misleading claim: Every coastline is necessarily a plate boundary.
Misleading claim: A boundary map predicts the exact time of the next earthquake.
Omitting units, measurement timing, or a model boundary makes the evidence hard to interpret.
Treating illustrative classroom data as real field measurements or copying a model is not an independent explanation.

Check your understanding

What motion does increasing separation suggest?

Discuss the answer

Divergence between the compared markers, with the method and time interval stated.

Do all convergent boundaries have identical outcomes?

Discuss the answer

No. Plate composition, density, and geometry affect whether subduction, mountain building, and volcanic activity occur.

What can the location pattern support?

Discuss the answer

A relationship between seismic activity and a boundary region, not an exact earthquake prediction.

Why is a coastline not necessarily a tectonic boundary?

Discuss the answer

It marks land meeting water, not necessarily a change between moving lithospheric plates.

Sustained scientific explanation and revision

These activities are scheduled in the parent plan. Save a first response, a correction, and the reason for the change in the science notebook.

Activity 1 · Moving Plates, Boundaries & Earth Evidence · Model and explain

Assigned reading and preparation

Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Moving Plates, Boundaries & Earth Evidence, study Parts 3–4 and attempt the worked case: Calculate the mean separation rate and explain a boundary inference with a limit. Explain the steps before opening the model.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

Separation grows by 6.0 cm in three years, a mean relative rate of 2.0 cm/year. Together with the ridge, this supports a divergent-boundary interpretation in the supplied model. It does not establish that each plate moves at exactly 2 cm/year or predict every earthquake or volcano.

Activity 2 · Moving Plates, Boundaries & Earth Evidence · Practice, revise and exit

Assigned reading and preparation

Read A Stone and Several Possible Histories and the linked unit teaching; use its complete supplied case and diagrams.

Your task

For Moving Plates, Boundaries & Earth Evidence, complete practice questions 1–6 and both choice checks, then answer independently: Two plates slide horizontally past each other in a supplied model. Must a new mountain range form there? Save a correction explaining one changed claim or method.

Success criteria

An accurate scientific claim answering the actual question.
Relevant evidence or a labeled model with quantities and units when applicable.
Reasoning that connects evidence with the mechanism rather than only repeats it.
A specific method, boundary, scale, or alternative-explanation limit.

Hint

Begin with the actual observation, quantity, or diagram relationship. Explain how it bears on the question before making a broad claim.

Response guidance — discuss after your attempt

No. That is transform motion; outcomes depend on local conditions, and mountain building is not an automatic result of every boundary interaction.

Words to know

plate boundary: A region where tectonic plates interact through relative motion.
subduction: One lithospheric plate sinking beneath another at a suitable convergent boundary.
evidence: A relevant observation, measurement, or record used to evaluate a claim.
system boundary: The chosen limit specifying what is included and which exchanges cross it.
uncertainty: A stated limit in measurement or knowledge, distinct from a careless mistake.

In the real world

Exit check

Two plates slide horizontally past each other in a supplied model. Must a new mountain range form there?

Check your explanation after answering

No. That is transform motion; outcomes depend on local conditions, and mountain building is not an automatic result of every boundary interaction.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.