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Plate Tectonics and Earthquakes
Convergent, Divergent, and Transform Boundaries
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Earth's outer shell (lithosphere) is divided into about 15 major tectonic plates that move slowly — typically 2 to 15 centimeters per year — driven by convection currents in the underlying mantle. The theory of plate tectonics explains the distribution of earthquakes, volcanoes, mountain ranges, and ocean trenches, as well as the past movement of continents (continental drift). Alfred Wegener proposed continental drift in 1912, but the mechanism (seafloor spreading and subduction) was not understood until the 1950s–60s.
Plate boundaries come in three types. Divergent boundaries (e.g., Mid-Atlantic Ridge) are where plates move apart, creating new oceanic crust through seafloor spreading. Convergent boundaries are where plates collide: oceanic plates subduct under continental plates (forming ocean trenches and volcanic arcs), or two continental plates collide to form mountain ranges like the Himalayas. Transform boundaries (e.g., San Andreas Fault) are where plates slide horizontally past each other, producing earthquakes but no volcanism.
Key Facts
- Lithosphere = crust + uppermost mantle; broken into ~15 major tectonic plates
- Plates move 2–15 cm/year driven by mantle convection currents
- Divergent boundaries: plates pull apart → new oceanic crust, mid-ocean ridges
- Convergent boundaries: oceanic plate subducts under continental → trench + volcanic arc
- Transform boundaries: plates slide past each other → strike-slip earthquakes (e.g., San Andreas)
- Ring of Fire: ~90% of the world's earthquakes and 75% of volcanoes occur around the Pacific plate edges
Vocabulary
- Tectonic plate
- A large, rigid segment of Earth's lithosphere that moves slowly over the asthenosphere; boundaries between plates are sites of geological activity.
- Subduction
- The process by which a denser oceanic plate descends beneath another plate into the mantle at a convergent boundary.
- Seafloor spreading
- The process at divergent boundaries where magma rises through the ocean floor, creating new oceanic crust as plates move apart.
- Fault
- A fracture or zone of fractures in Earth's crust along which blocks of rock have moved relative to each other.
- Seismic wave
- A wave of energy released by an earthquake or other seismic event; P-waves (compressional) and S-waves (shear) travel through Earth's interior.
Common Mistakes to Avoid
- Thinking continents sit on top of tectonic plates. Continents ARE part of tectonic plates — the plate includes both the continent and adjacent oceanic crust. It is the plate that moves, not just the continent.
- Assuming all earthquakes occur at plate boundaries. Most do, but intraplate earthquakes (like the 1811–12 New Madrid earthquakes in the central US) can occur far from active boundaries due to ancient fault zones.
- Confusing oceanic and continental crust. Oceanic crust is thinner (~7 km) and denser (basaltic). Continental crust is thicker (~35 km) and less dense (granitic). Density difference drives subduction.
- Thinking plate movement is too slow to matter geologically. 5 cm/year over 50 million years = 2500 km — enough to close an ocean or build a mountain range.
Practice Questions
- 1 The Atlantic Ocean is widening at about 2.5 cm per year. How wide will it be in 10 million years (express in km)?
- 2 Explain why there is a volcanic arc (like the Cascades) parallel to and inland of an ocean trench. What is the heat source?
- 3 Compare and contrast the geological features produced at divergent, convergent (oceanic-continental), and transform plate boundaries.