Plate tectonics - Year 8 Science

What you will learn

the structure of Earth’s interior and what a tectonic plate is,
the three types of plate boundary (divergent, convergent, transform),
where earthquakes and volcanoes occur and why (the Ring of Fire),
what a hotspot is, with Hawaii as the textbook example,
the main evidence that convinced scientists plate tectonics was real.

Worked example 0 Real-world example: the Himalayas

Explain why Mount Everest is still slowly getting taller.

The Indian Plate is pushing north into the Eurasian Plate.
Both plates are continental (thick, low-density), so neither slides underneath cleanly.
Instead, the crust crumples upward, forming the Himalaya mountains.
The collision is still going on (India moves at roughly $5$ cm/year), so the mountains keep rising by a few millimetres each year — balanced by erosion.

Key idea: plate boundaries are where the slow horizontal drift of the surface builds vertical features we can see.

1. Inside the Earth

Earth is not a uniform ball. From the outside in:

Crust: thin, solid outer skin ( $5$ km under oceans, up to $70$ km under continents).
Mantle: thick hot rock below the crust. The upper part flows very slowly (like warm wax).
Outer core: liquid iron and nickel.
Inner core: solid iron-nickel ball under huge pressure.

The crust plus the rigid top of the mantle make up the lithosphere. The lithosphere is broken into several large and many small plates that slide over the hotter, softer rock beneath.

2. The three kinds of plate boundary

Divergent boundary (plates pull apart, new crust forms); convergent boundary (one plate dives beneath the other, forming a trench and volcanoes); transform boundary (plates slide past each other, earthquakes).

1. Divergent boundary (plates pull apart)

Hot rock rises between the plates and hardens to form new crust.
Example: the Mid-Atlantic Ridge — a giant underwater mountain chain where the Atlantic Ocean is widening by $\sim 2$ cm per year.
In East Africa, a continent is slowly splitting along the Great Rift Valley.

2. Convergent boundary (plates push together)

Ocean + continent: the denser oceanic plate dives beneath (subducts) the continental plate. Trench + volcanoes form. Example: Andes in South America.
Ocean + ocean: one oceanic plate subducts under the other, creating an island arc. Example: Japan.
Continent + continent: both plates crumple upwards forming mountains. Example: Himalayas.

3. Transform boundary (plates slide past each other)

No new crust created, none destroyed — but stresses build and release as earthquakes.
Classic example: San Andreas Fault in California.

3. Hotspots

Not all volcanoes lie on boundaries. A hotspot is a fixed column of extra-hot rock rising from deep in the mantle. As the plate above moves, the hotspot keeps “poking” new holes through it — building a line of volcanoes.

Hawaiian Islands: the Pacific Plate drifts northwest over a stationary hotspot. The Big Island (youngest) sits over the hotspot today; older, eroded islands lie further northwest. The line of islands is a direct record of plate motion over millions of years.

Worked example 1 Reading the Hawaiian chain

The Hawaiian island chain runs roughly northwest from the active Big Island. What can you deduce about the Pacific Plate’s direction of motion?

The hotspot is stationary; the plate above moves.
Older islands sit further northwest.
So the plate has moved northwest past the hotspot, producing each older island in turn.

Key idea: a line of extinct volcanoes is often a fingerprint of past plate motion.

4. Earthquakes, volcanoes and the Ring of Fire

Most of the world’s earthquakes and volcanoes cluster along plate boundaries. The Ring of Fire is a horseshoe-shaped zone around the Pacific Ocean that hosts about 75% of Earth’s active volcanoes and 90% of its earthquakes. It traces the boundaries of the Pacific Plate as it meets many neighbouring plates.

Earthquakes happen when built-up stress in the rock is suddenly released.
Volcanoes erupt where magma from the mantle reaches the surface, typically at convergent or divergent boundaries (and at hotspots).

Worked example 2 Why is Australia tectonically quiet?

Australia has few large earthquakes and no active volcanoes on the mainland. Use plate tectonics to explain.

Most tectonic activity happens at plate boundaries.
Mainland Australia sits near the centre of the Indo-Australian Plate, far from any boundary.
So there is little stress release or magma movement beneath Australia compared with, say, New Zealand (which sits on a boundary).

Key idea: being in the middle of a plate is usually safer than being at the edge.

5. Evidence for plate tectonics

When Alfred Wegener first proposed “continental drift” in 1912, most geologists rejected it. Over the next 50 years, new evidence built an overwhelming case:

Jigsaw fit: the east coast of South America and the west coast of Africa fit together remarkably well, including continental shelves.
Matching fossils: identical fossil species (e.g. Mesosaurus) are found in South America and Africa, where the animal could not have swum across today’s ocean.
Matching rock formations: mountain ranges and ancient glacier scratches line up when continents are reassembled.
Mid-ocean ridges: underwater mountain chains with young rocks at the centre and older rocks further away — showing new crust forming at divergent boundaries.
Palaeomagnetism: rocks lock in the Earth’s magnetic direction when they cool. Rocks on either side of a mid-ocean ridge show symmetrical magnetic stripes, strong evidence that new seafloor is being created and pushed outwards.
Distribution of earthquakes and volcanoes: they trace clear lines around the globe matching proposed plate boundaries.

Worked example 3 Weighing the evidence

Which piece of evidence — the jigsaw fit or the palaeomagnetic stripes — is stronger for plate tectonics? Justify.

The jigsaw fit is striking but on its own could be coincidence or due to erosion shaping similar-looking coasts.
Palaeomagnetic stripes show mirror-image patterns on either side of a ridge — there is no simple alternative explanation. They directly prove new seafloor is forming and spreading apart.
So palaeomagnetism is the stronger, more specific evidence.

Key idea: the strongest science comes from evidence that only fits one explanation.

Practice: Year 8

Fluency

Structure and plates

Name the four main layers of Earth’s interior.
What is the lithosphere made of?
Roughly how fast do tectonic plates move?
Name two of Earth’s major plates.
Is the crust thicker under oceans or continents?

Fluency

Boundaries and features

Name the three types of plate boundary.
Which type forms the Mid-Atlantic Ridge?
Which type is the San Andreas Fault?
What geological feature forms when two continental plates collide?
What is subduction?

Reasoning

Explain

Explain why volcanoes and earthquakes cluster at plate boundaries.
Explain why Australia experiences few earthquakes compared with New Zealand.
How does a hotspot create a chain of islands?
Why do scientists think the Atlantic Ocean is widening?

Problem solving

Applied contexts

The Pacific Plate moves northwest at about $10$ cm per year. Estimate how far Hawaii will have moved in $1$ million years.
Japan sits at the boundary where the Pacific Plate subducts under the Eurasian Plate. Predict and explain the geological hazards Japan faces.
Iceland sits directly on the Mid-Atlantic Ridge. Predict what geological activity you might expect to see there.
A fossil of a land animal from 200 million years ago is found in both South America and Africa. Explain what this tells us about the past positions of the continents.

Challenge

Reasoning

Harder reasoning

Wegener’s idea was rejected for decades, then accepted rapidly in the 1960s. Describe what changed scientifically (think about the mid-ocean ridge and palaeomagnetic data), and what this says about how science progresses.
Explain why earthquakes at transform boundaries are usually shallower than at convergent boundaries.
A volcano sits $500$ km inland from the west coast of South America. Use plate tectonics to explain how magma gets there despite the distance from the coast.
Use the hotspot model to predict what will happen to the Big Island of Hawaii over the next few million years.

Answers

Answer key

Attempt the practice first. When you're ready to check, expand the answers below.

Show the full answer key

Year 8 answers

Fluency

Structure and plates

Crust, mantle, outer core, inner core.
The crust plus the rigid upper part of the mantle.
A few centimetres per year (roughly 2-10 cm/year).
Any two of: Pacific, Eurasian, North American, South American, African, Indo-Australian, Antarctic, Nazca.
Continents (up to $\sim 70$ km); oceanic crust is only $\sim 5$ -10 km thick.

Fluency

Boundaries and features

Divergent, convergent, transform.
Divergent.
Transform.
A mountain range (e.g. the Himalayas).
Subduction is when one plate (usually a denser oceanic plate) dives beneath another plate into the mantle at a convergent boundary.

Reasoning

Explain

At boundaries the plates either push into, pull away from, or slide past each other. All three movements generate stress in the rocks or let magma reach the surface, causing earthquakes and/or volcanic eruptions. Mid-plate regions are relatively stable.
Australia is near the middle of the Indo-Australian Plate, far from the grinding boundaries. New Zealand sits directly on the boundary between the Pacific and Indo-Australian plates, so it gets frequent earthquakes and volcanic activity.
A hotspot is a fixed column of hot mantle that pokes through the moving plate above. As the plate drifts, the hotspot punches new volcanoes through it in a line, leaving older volcanoes behind as a chain.
New seafloor forms at the Mid-Atlantic Ridge (a divergent boundary). As new crust forms and spreads, the ocean floor either side gets pushed apart, widening the Atlantic by $\sim 2$ cm/year.

Problem solving

Applied contexts

$10$ cm/year $\times 10^6$ years $= 10^7$ cm $= 10^5$ m $= 100$ km.
Frequent earthquakes (plate grinding), tsunamis (seafloor earthquakes displace water), and explosive volcanoes (magma rising from the subducting plate).
Active volcanoes, hot springs, geysers, frequent small earthquakes, and new land forming as lava cools.
The species could not cross today’s Atlantic Ocean, so the two continents must have been joined 200 million years ago and have since drifted apart — direct evidence of continental drift.

Reasoning

Challenge

In the 1950s and 60s, mapping of the ocean floor revealed mid-ocean ridges, and palaeomagnetic stripes showed symmetric patterns of reversed/normal magnetism on either side of the ridges. This new evidence only made sense if the seafloor was forming and spreading. Science progresses when decisive new data arrive — theories are revised or accepted when the evidence is strong enough, not just because the idea is old.
Transform boundaries involve plates sliding past each other close to the surface, so fault movement (and the earthquakes it causes) occurs at shallow depth. Convergent boundaries involve one plate diving deep into the mantle, so earthquakes along the subducting slab can be hundreds of km deep.
The oceanic Nazca Plate subducts beneath the South American Plate. As the plate dives, water and minerals in it release, melting the mantle above. Magma rises and breaks through the crust well inland from the trench, forming the volcanoes of the Andes.
The Pacific Plate will continue moving northwest, carrying the Big Island away from the hotspot. The Big Island’s volcanoes will go extinct and erosion will shrink it, while a new volcano rising over the hotspot (Loihi, already building south-east) will eventually breach the surface as a new island.

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