Plate tectonics
The ground under your feet rides on a plate that moves a few centimetres a year. Start with the inside of the Earth and the earthquake waves that revealed it, then follow plates apart at ridges, together at trenches and past each other on faults, and end with the clues that once joined the continents into Pangaea.
About this course
The ground under your feet rides on a plate that moves a few centimetres a year. The theory of plate tectonics says the Earth's rigid outer layer is broken into a dozen or more large and small plates, and BGS counts about 15 major ones. At 2.5 cm per year, the Atlantic widens by a distance the course has you calculate over a million years. Between 1975 and 1984 the ground around Krafla volcano in Iceland split open again and again, until the cracks had widened by about 7 m.
The eight units start inside the Earth, with its layers and the seismic waves that revealed them. No drill has reached the mantle, yet we know the outer core is liquid because one kind of earthquake wave never arrives on the far side of the planet. The 3D model lets you play S waves only and see every ray end at the outer core, then P waves only and look for the gap between 104° and 140°. Plates and their speeds follows, then divergent, convergent and transform boundaries, and the Ring of Fire. Convergence includes two oceanic plates meeting to build an island arc such as the Mariana and Aleutian Islands. A transform boundary neither makes nor destroys crust, which is why BGS calls it a conservative boundary.
The last unit is Pangaea and the evidence. Wegener found identical fossil species of plants and animals on the matching coasts of South America and Africa, now separated by the Atlantic. He could not say what force could move such large masses of rock. Reid's 1906 elastic rebound, explained after the San Francisco earthquake on the San Andreas fault, appears with the transform boundaries.
Before you start
No earlier course is required. The lessons start from Earth's layers and build up from there. It helps to be comfortable with simple rates, because plate speeds are given in centimetres a year and several questions turn them into kilometres over a million years.
When you finish
You can name Earth's layers and explain how S-waves showed that the outer core is liquid. You can tell divergent, convergent and transform boundaries apart, say why an oceanic plate sinks under a continent, place Iceland on the Mid-Atlantic Ridge and the San Andreas on a transform fault, and list the evidence for Pangaea, from matching fossils to coal in Antarctica.
- 1Earth's layersThe centre of the Earth is 6,371 km below your feet, and the rocky crust you live on is only about 35 km of that under a continent.
- 2How we know: seismic wavesNo drill has reached the mantle, yet we know the outer core is liquid because one kind of earthquake wave never arrives on the far side of the planet.
- 3Plates and their speedsThe floor of the Atlantic grows about 2.5 cm a year, which adds up to 25 km in a million years.
- 4Divergent boundariesBetween 1975 and 1984 the ground around Krafla volcano in Iceland split open again and again, until the cracks had widened by about 7 m.
- 5Convergent boundariesOn 9 June 1994 a magnitude 8.3 earthquake struck 636 km beneath Bolivia, inside a plate that had sunk into the mantle.
- 6Transform boundariesThe San Andreas fault is about 1,300 km long, and for 10 million years the land west of it has been sliding north-west at about 5 cm a year.
- 7The Ring of FireAbout 81 percent of the world's largest earthquakes happen around the rim of the Pacific Ocean.
- 8Pangaea and the evidenceCoal, which forms from swamp plants, has been found in Antarctica.
Sources for this course (13)
- USGS This Dynamic Earth: Inside the Earth, Historical perspective, Understanding plate motions, Ring of Fire, Hotspots (pubs.usgs.gov/gip/dynamic/inside.html, historical.html, understanding.html, fire.html, hotspots.html)
- USGS Earthquake Hazards Program, Shadow Zone (usgs.gov/media/images/shadowzonegif): no direct P waves 104° to 140°
- USGS FAQ Where do earthquakes occur? (usgs.gov/faqs/where-do-earthquakes-occur): circum-Pacific belt about 81% of the largest earthquakes, Alpide belt about 17%
- USGS FAQ How many active volcanoes are there on Earth? (usgs.gov/faqs/how-many-active-volcanoes-are-there-earth): about 1,350 potentially active, about 500 erupted in historical time
- NOAA Ocean Exploration, What is the Ring of Fire? (oceanexplorer.noaa.gov/facts/rof.html): more than 450 volcanoes, nearly 40,250 km
- NOAA Science On a Sphere, Earthquake Magnitude Perspective (sos.noaa.gov/catalog/datasets/earthquake-magnitude-perspective): most great earthquakes are megathrust events at convergent boundaries
- British Geological Survey, Discovering Geology: What causes earthquakes? (bgs.ac.uk/discovering-geology/earth-hazards/earthquakes/what-causes-earthquakes/): crust 25 to 70 km under continents and 5 to 10 km under oceans, mantle to 2,890 km, about 15 major plates, slab pull, ridge push, elastic rebound (Reid, 1906)
- IRIS, Seismic shadow zone animations (iris.edu/hq/inclass/animation/seismic_shadow_zone_basic_introduction)
- Earle, Physical Geology 2e, 9.2 The temperature of Earth's interior (opentextbc.ca/geology/chapter/9-2-the-temperature-of-earths-interior/)
- ESRF 2013, The Earth's centre is 1000 degrees hotter than previously thought (esrf.fr/news/general/Earth-Center-Hotter)
- Lehmann 1936 extracts, Harvard EPS (courses.seas.harvard.edu/climate/eli/Courses/EPS281r/Sources/Inner-Core/Lehmann-1936-extracts+interpretation.pdf)
- layer depths 660 km and 5,150 km from PREM (Dziewonski and Anderson 1981)
- Coastline figure: Natural Earth 1:110m (public domain).
Checked: 2026-10-17