The Earth's crust and the top of the mantle are broken into huge pieces called tectonic plates. They float on the slowly flowing mantle and move a few centimetres a year, about as fast as your fingernails grow. Heat from the core makes convection currents in the mantle, which drag the plates along.
Plates meet at boundaries. At a divergent boundary plates move apart and magma rises to make new crust, as along the Mid-Atlantic Ridge. At a convergent boundary plates move together: an oceanic plate may sink under another (subduction), making volcanoes and deep trenches, or two continents may crumple into mountains like the Himalayas. At a transform boundary plates slide past each other, causing earthquakes, as along the San Andreas Fault.
Worked example
Why is the Atlantic Ocean getting wider?
- The plates under it are moving apart.
- Magma rises at the Mid-Atlantic Ridge and makes new sea floor.
Answer:So the ocean widens by a few centimetres a year.
Key words
- tectonic plate
- a huge slab of crust and upper mantle
- convection current
- circulating flow in the mantle
- subduction
- one plate sinking under another
- boundary
- where two plates meet
New questions every time.
NGSS-referenced: MS-ESS2-3