If you are in Year 5 at the Italian liceo delle scienze umane (human sciences high school), it is early October and in natural sciences the block on plate tectonics, atmosphere and climate usually opens. The oral test or written test falls in six to nine weeks, and the topic readily reappears at the maturità (Italy’s school-leaving exam), as an interdisciplinary starting point for the oral. It is one of the most “linkable” themes of the science programme: history of science, geography, seismic risk, civic education.
Here I give you the structure to study it in an orderly way: from the interior of the Earth to the three boundaries, then volcanoes, earthquakes, the two scales and the mistakes I see all the time. For the astronomical side I wrote an article on astronomical geography in the final year; for the motions of the Earth and the seasons this one on the Solar System. I don’t repeat them here: I talk only about what happens beneath our feet.
The interior of the Earth
We can’t dig to the centre, so the structure is worked out from seismic waves, which change speed and direction when they cross different materials. The main layers:
- crust: thin, from a few kilometres under the oceans to tens of kilometres under mountain ranges; it is oceanic (denser) or continental (thicker);
- mantle: the most voluminous layer, rocky, which on very long timescales behaves plastically;
- outer core: liquid, rich in iron and nickel;
- inner core: solid.
The boundaries between them have names of their own: the Mohorovičić discontinuity divides crust and mantle, the Gutenberg one mantle and core.
For tectonics, though, a different division matters, based on mechanical behaviour: the lithosphere, rigid, made of crust plus the upper part of the mantle, which breaks up into plates, and the asthenosphere below, more deformable, on which the plates move.
From Wegener to plates
In 1912 Alfred Wegener proposed continental drift: continents were once joined in a single supercontinent, Pangaea, and later separated. His evidence: the shape of the coasts of South America and Africa that fit together, the same fossils on continents now far apart, rocks and mountain ranges that continue from one shore of the ocean to the other, traces of ancient glaciations in zones that are now tropical.
The hypothesis was long rejected because it lacked a mechanism: how could continents “plough” through the ocean floor? The answer came later, with sea-floor spreading (Harry Hess, 1960s): in the middle of the oceans, along the ridges, magma rises and forms new oceanic crust; the sea floor moves away from the ridge on both sides. The evidence: the age of the sea-floor rocks increases with distance from the ridge, and the magnetic stripes are symmetrical about it, because the Earth’s magnetic field has reversed several times and the rocks “record” it as they cool.
The plates move a few centimetres a year, pushed by convection currents in the mantle and by the weight of plates sinking. It is an extremely slow motion but, over millions of years, an enormous one.
If you are asked for a link of method: Wegener’s story is a classic case of a hypothesis that was solid on the observations but rejected until a physical explanation was found. For a human sciences oral it is a good hook with epistemology.
The three types of boundary
| Boundary | Relative motion | What happens | Examples |
|---|---|---|---|
| Divergent | the plates move apart | new oceanic crust forms, volcanism, shallow earthquakes | Mid-Atlantic Ridge, East African Rift |
| Convergent | the plates move together | subduction or collision, volcanoes and earthquakes, also deep ones | Andes, Japan, Himalayas |
| Transform | they slide sideways | earthquakes, no crust created or destroyed | San Andreas Fault |
Divergent boundary. Magma rises and solidifies: it is the oceanic ridge. If it happens under a continent, it produces a rift, as in East Africa.
Convergent boundary. Three cases, always to be told apart:
- ocean-continent: the oceanic plate, denser, sinks under the continental one (subduction). An oceanic trench forms and, on the continent, a range with volcanoes: the Andes;
- ocean-ocean: one oceanic plate sinks under the other and an island arc with volcanoes forms, as in Japan;
- continent-continent: neither of the two sinks easily, because they are light; the crust deforms and rises, forming a mountain range. It is the case of the Himalayas, born from the collision between the Indian and Eurasian plates. Here earthquakes are frequent, volcanoes are not.
Transform boundary. The plates slide past one another. Tension builds up, then is released suddenly: the San Andreas Fault, in California, is the textbook example.
Volcanoes and earthquakes: the same map
If you overlay the map of active volcanoes and that of epicentres, you see that they coincide with the plate boundaries. It is the simplest proof of the theory, and an oral-test question.
Volcanoes are found mostly where crust is created or consumed: ridges, subduction zones, the Pacific “Ring of Fire”. There are also volcanoes far from the boundaries, the hot spots, but they are the exception, and usually not needed for the test.
Earthquakes arise from the sudden release of energy accumulated along a fault. Three terms not to be confused:
- hypocentre (or focus): the underground point where the rupture starts;
- epicentre: the point on the surface directly above the hypocentre;
- seismic waves: primary (P, faster, longitudinal), secondary (S, transverse) and surface waves, the most destructive.
In subduction zones the hypocentres lie on an inclined plane that descends beneath the continent: this too is a way of recognising the boundary.
Richter and Mercalli
The two scales measure different things, and it is the most frequent mistake.
The Richter scale (today the moment magnitude is mostly used) measures magnitude: the energy released at the source, a single number for each earthquake. It is logarithmic: each extra unit means a wave amplitude 10 times greater and an energy about 32 times greater.
The Mercalli scale measures intensity: the effects on people, buildings and ground in a given place, in degrees from I to XII. It changes from one place to another: the same earthquake has high intensity near the epicentre and low far away. It also depends on the ground and the quality of the buildings.
Why there are earthquakes in Italy
In cautious terms: Italy lies in a zone of slow convergence between the African and Eurasian plates. This deforms the crust along the peninsula and produces active faults, in particular along the Apennines; the same context is linked to the volcanism of Etna, Vesuvius and Stromboli. The geological history is complex and books present it with different nuances, so use the model you find in your textbook, without oversimplifying it.
The mistakes I see every year
Confusing magnitude and intensity. Richter tells you how strong the earthquake is in itself; Mercalli tells you what it caused in a place.
Thinking the Richter scale is linear. A magnitude 7 earthquake is not “a bit stronger” than a 6: the energy is about 32 times greater.
Saying that the continents “float on the sea”. What moves are the lithospheric plates, which include crust and part of the upper mantle, on the asthenosphere.
Putting volcanoes in the Himalayas. A continent-continent boundary: range, earthquakes, no subduction volcanism.
Swapping epicentre and hypocentre. The first is at the surface, the second at depth.
Saying that drift is “a wrong theory of Wegener’s”. He was right about the fact, the mechanism was missing; plate tectonics has integrated it.
Three worked exercises
1. How long has the Atlantic been opening?
Two coasts are today 3000 km apart and are moving apart at 3 cm a year. If the speed had always been constant, how long have they been separating?
I convert to centimetres: . Time is distance divided by speed:
It is a plausible order of magnitude for the scale of geological time; real values depend on a motion that is not exactly constant.
2. Magnitude and energy
A magnitude 7 earthquake and a magnitude 5 one: how much do they differ in wave amplitude and in energy?
The difference is . The amplitude grows by a factor of 10 per unit:
The energy follows :
The magnitude 7 quake has waves 100 times larger and releases about 1000 times the energy. Check: for a single unit the energy rises by about 32; .
3. Age of the sea floor and recognising the boundary
At 600 km from a ridge, the sea floor is moving away at 2 cm a year. What age do we expect? Then: an earthquake with a hypocentre at 300 km depth under Japan, what kind of boundary does it suggest?
Distance in centimetres: .
For the earthquake: a hypocentre this deep is not found at a divergent or transform boundary, where the crust is thin and the quakes are shallow. It suggests a subduction zone, where the plate descends into the mantle: so an ocean-ocean or ocean-continent convergent boundary, and it is that of Japan.
Test yourself
For the human sciences maturità you can train on the dedicated simulator, but there is not yet a practice test on tectonics alone: it is useful for seeing where you stand with the rest of the exam. For other timed tests see the practice tests category.
How to prepare over the next weeks
- Draw a cross-section of the Earth and a map with the main plates, marking the three types of boundary with their examples.
- Learn the table of boundaries by heart, and the two scales with the difference between magnitude and intensity.
- Redo the three exercises changing the numbers.
- Prepare a link to bring to the oral: Wegener and the scientific method, or seismic risk and prevention.
Other resources are on the science page.
If you want a hand
I give one-to-one online tutoring, and on this topic I work a lot with diagrams drawn together: if you want to understand where you stand, you can book a free introductory call of 30 minutes.
