Plate Tectonics.
A planet in pieces. A surface in motion.
Where plates meet.
Choose a process. Watch the transformation unfold, or jump between its stages.
The plates approach
Oceanic and continental crust move toward one another.
Interactive schematic · distances and time are not to scale.
Convergent boundary
A tectonic plate boundary where two plates collide, resulting in subduction (if one plate is oceanic) or folding/mountain-building (if both are continental).
What is plate tectonics?
Everything else on this site — mountains, volcanoes, earthquakes, even where erosion happens fastest — traces back to one idea: the Earth's outer shell isn't one solid piece. It's broken into roughly a dozen major pieces called tectonic plates, each one a slab of crust and the uppermost rigid mantle beneath it, floating on the hotter, slowly-flowing mantle underneath. These plates are moving, constantly, at roughly the speed your fingernails grow — a few centimeters a year. That sounds trivial, but multiply it by hundreds of millions of years and you get continents that have crossed oceans, mountain ranges pushed miles into the sky, and ocean floor that's constantly being made and destroyed.
What actually moves them
The mantle beneath the plates is solid rock, but it's hot enough to flow extremely slowly over long timescales — hotter material rises, cooler material sinks, in huge convection currents, similar to water heating in a pot (just millions of times slower). The plates ride on top of this movement, dragged and pushed along with it.
Where plates meet is where everything interesting happens
The middle of a plate tends to be geologically quiet; it's the boundaries — where one plate meets another — that produce almost all of Earth's mountain-building, volcanism, and earthquakes. There are three kinds of boundaries, and each produces a completely different result:
Divergent boundaries
plates pulling apart. As they separate, magma rises to fill the gap, creating new crust. On the ocean floor, this forms a mid-ocean ridge — an underwater mountain chain where new oceanic crust (basalt) is constantly being generated. On land, this same pulling-apart process creates rift valleys, like East Africa's Great Rift Valley, which is slowly tearing the continent apart.
Convergent boundaries
plates colliding. What happens here depends on what kind of crust is involved. If dense oceanic crust collides with lighter continental crust, the oceanic plate gets forced underneath in a process called subduction, melting as it descends and feeding volcanoes above (this is how the Andes and Cascades formed). If two continental plates collide, neither is dense enough to subduct, so the crust crumples and folds upward instead — this is how the Himalayas formed, and still are forming, as India continues pushing into Asia.
Transform boundaries
plates sliding past each other horizontally, neither colliding nor separating. No new crust is created or destroyed, but the friction of two plates grinding past one another builds up stress that releases suddenly as earthquakes. California's San Andreas Fault is the best-known example.
The big takeaway
mountain-building, volcanism, and earthquakes aren't three separate, unrelated phenomena — they're different consequences of the same underlying process, plate tectonics, depending entirely on what kind of boundary you're standing on.