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Plate tectonics review
A summary of the whole Plate Tectonics chapter, key words, and a mixed test covering the evidence for plate tectonics, what moves plates, earthquakes, volcanoes, plate boundaries, Australia's movement and monitoring hazards.
Use this page to check the whole chapter. Read the summary, make sure you can explain every key word, then do the test without looking back.
Chapter summary
| Topic | What you need to know |
|---|---|
| 5A Evidence | Wegener's continental drift (1912): matching coastlines, fossils (Glossopteris, Mesosaurus, Lystrosaurus, Cynognathus), rock types and glacier scratches. Rejected because he had no mechanism. Sea-floor mapping (Tharp and Heezen), sea-floor spreading (Hess), magnetic stripes symmetrical about ridges, sea floor youngest at ridges and no older than about 180 million years. Pangaea formed about 335 Ma and broke up from about 200 Ma. |
| 5B Forces | Lithosphere (rigid plates) on asthenosphere (hot, solid, slowly flowing). Densities: continental crust 2.7, ocean crust 3.0, mantle 3.3 g/cm³. Heat from radioactive decay drives mantle convection. Ridge push and slab pull (gravity); slab pull is the largest force, so plates with subducting edges move fastest. |
| 5C Earthquakes | Elastic rebound on faults. Focus and epicentre. Shallow, intermediate and deep. P waves (fastest, longitudinal, through liquids), S waves (transverse, not through liquids), surface waves (slowest, most damage). S–P gap × 8 ≈ distance in km; three stations locate the epicentre. Moment magnitude: +1 ≈ 32× energy. Intensity (Mercalli) varies by place. About 100 M3+ earthquakes a year in Australia. |
| 5D Volcanoes | Magma forms by decompression, added water or added heat. Silica raises viscosity; trapped gas makes explosive eruptions. Shield (basalt, gentle) vs composite (andesite/rhyolite, explosive). VEI. Hotspot chains (Hawaiʻi: about 7–10 cm a year). Hazards: pyroclastic flows, lahars, ash, lava, tsunamis. Benefits: fertile soil, geothermal energy, ores. |
| 5E Boundaries | Convergent: ocean–continent (trench, volcanic mountains, e.g. Andes), ocean–ocean (trench, island arc, e.g. Mariana, Challenger Deep about 10 935 m), continent–continent (fold mountains, e.g. Himalayas). Wadati–Benioff zone. Divergent: mid-ocean ridges and rift valleys (East Africa → Red Sea → Atlantic; Iceland). Transform: San Andreas, Alpine Fault. |
| 5F Australia | About 7 cm a year north-north-east, measured by GPS; GDA2020 (1.8 m shift). Separated from Antarctica from about 85 Ma; colliding with South-East Asia. Intraplate earthquakes. Newer Volcanics Province (Mt Gambier about 5000 years ago); Heard and McDonald Islands active. First Nations stories of eruptions (Budj Bim, Craitbul) and of rising seas (Port Phillip Bay, Spencer Gulf). |
| 5G Monitoring | Hazard vs disaster. Seismometers, GPS, InSAR, tiltmeters, gas sensors, DART buoys. Earthquakes forecast (probabilities) not predicted; early warning from P waves. Volcano warning signs. Tsunami speed = √(9.8 × depth); JATWC since 2007; natural warning signs. Base isolation, bracing, building codes. |
Putting it together
The whole chapter runs on one idea: the Earth's outer shell is broken into plates that are slowly moved by the planet's internal heat and by gravity. The evidence for this came from many directions at once: continents that fit, fossils and rocks that match, and a sea floor that is young, striped and spreading. Density decides what happens where plates meet: dense ocean lithosphere sinks at trenches, pulling plates along and feeding explosive volcanoes, while buoyant continents crumple into mountains. Where plates pull apart new crust forms, and where they slide past, faults lock and slip. Earthquakes and volcanoes are the result, mostly along boundaries but also, as in Australia, within plates. Understanding these processes lets scientists monitor them, forecast risks and design communities that can survive them.
Key words
asthenosphere · base isolation · caldera · composite volcano · continental drift · convection current · convergent boundary · density · divergent boundary · early warning · elastic rebound · epicentre · fault · focus · forecast · Gondwana · GPS · hotspot · intensity · intraplate earthquake · island arc · lahar · lithosphere · magma · magnitude · mid-ocean ridge · P wave · Pangaea · pyroclastic flow · rift valley · ridge push · S wave · sea-floor spreading · seismogram · seismometer · shield volcano · silica · slab pull · subduction · surface wave · transform boundary · trench · triangulation · tsunami · viscosity
Test yourself
Which piece of evidence did Wegener NOT have?
Magnetic stripes were found in the 1950s and 1960s, after he died.
Which force is thought to contribute most to plate movement?
Gravity pulling sinking slabs.
The S–P gap at three stations is 10 s, 25 s and 40 s. Which station is closest to the epicentre?
The shortest gap means the shortest distance (about 80 km).
Which pair is matched correctly?
Water from the slab makes sticky, gas-rich magma.
Deep earthquakes (300–700 km) are found
Only sinking slabs are cold and brittle that deep.
A tsunami crosses ocean 4000 m deep. Its speed is about
√(9.8 × 4000) ≈ 198 m/s ≈ 710 km/h.
Which statement about Australia is correct?
It moves north and is squeezed by collisions at its northern edge.
Sort the features by the boundary where they form.
Order these from slowest to fastest.
Plates are pieces of the rigid . Ocean plates sink at zones because they are more dense. Magma with a high content traps gas and erupts explosively. The point on the surface above where an earthquake starts is the . The energy of an earthquake is measured as its .
Explain how the theory of plate tectonics accounts for the distribution of earthquakes and volcanoes on Earth. Use examples.
Plate tectonics says the lithosphere is broken into plates that move a few centimetres a year. Most earthquakes and volcanoes are found in narrow belts along the edges of plates, because that is where plates interact. At convergent boundaries where ocean lithosphere is subducted, such as the Andes and Japan, earthquakes occur from shallow to deep in the sinking slab, and water released from the slab melts the mantle to form explosive composite volcanoes; this is why the Pacific is ringed by the Ring of Fire. Where two continents collide, as in the Himalayas, there are many earthquakes but almost no volcanoes because neither plate sinks deeply. At divergent boundaries, such as the Mid-Atlantic Ridge and Iceland, plates pull apart, mantle rises and melts as the pressure drops, giving gentle basalt eruptions and shallow earthquakes. At transform boundaries such as the San Andreas Fault, plates slide past each other, causing shallow earthquakes but no volcanoes because crust is neither made nor destroyed. The exceptions also fit the theory: Hawaiʻi formed as the Pacific Plate moved over a hotspot, and Australia's earthquakes are caused by stress transmitted through the plate from collisions at its northern edge.