F763: Global Issues
Section A - Environmental Issues
Earth Hazards
The world’s main tectonic plates and types of plate boundary
When we talk about tectonic or lithospheric plates, we mean the sections into which the
lithosphere is cracked. The surface of the Earth is divided into 7 major and 8 minor plates. The
largest plates are the Antarctic, Eurasian, and North American plates. Plates are on average
125km thick, reaching maximum thickness below mountain ranges. Oceanic plates (50-100km)
are thinner than the continental plates (up to 200km) and even thinner at the ocean ridges
where the temperatures are higher. Some plates are large enough to consist of both continental
and oceanic crustal portions (e.g. the African or South American plates) whilst the Pacific Plate
is almost entirely oceanic. The Earth is roughly spherical, so these plates are fractured into
curved sections which are in constant motion relative to each other and meet in various ways
along their edges – these are the ‘plate boundaries’, where most volcanoes and earthquakes
occur. The mechanism by which plates move is still a highly controversial subject amongst Earth
scientists.
HOW DO PLATES MOVE?
The mechanism by which tectonic plates move is still a subject of much debate among Earth
scientists. The Earth is dynamic thanks to its internal heat, which comes from deep within the
mantle from the breakdown of radioactive isotopes. This causes convection in the mantle – hot
rocks rise and cold rocks descend. This very slow motion in the solid state transfers stresses to
the lithosphere, just as convection in a boiling pan of thick soup will cause the skin to buckle
where the convection cells meet.
As the theory of plate tectonics developed, mantle convection was long thought to be
responsible for the movement of tectonic plates across the Earth’s surface. This theory is now
largely out of favour, with modern imaging techniques unable to identify convection cells in the
mantle sufficiently large to drive plate movement. Instead, it is thought to be caused by 'slab
pull'. Newly formed oceanic lithosphere at mid ocean ridges is less dense than the
asthenosphere, but becomes denser with age as it cools and thickens. This causes it to sink into
the mantle at subduction zones, pulling slabs of lithosphere apart at divergent boundaries and
resulting in sea floor spreading or rifting. How plate movement operates in detail, however, is
highly controversial.