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Place 04 · The Ridge and the Trench · Chapter 3 of 7
Where the Seafloor Goes Back In
At the far edge of a plate, the seafloor bends, dives and slides back into the planet, and on the way down it digs the deepest holes on Earth.
About 15 minutes · 1 video · 1 live earthquake map · 2 quick activities
Start here
Three ways to meet a neighbor
Earth’s rocky shell is cracked into plates that move a few centimeters a year. Where two plates meet, they pull apart, push together or grind past each other, and each kind of plate boundary builds something different. Remember the Nazca Plate carrying the Galápagos east? At its far edge it dives under South America into the Peru–Chile Trench, and it is not coming back.
Watch this
See it move
MinuteEarth — Plate Tectonics Explained. Under three minutes of hand-drawn plates doing what plates do, with a lot less jargon than this chapter.
While you watch, look for
- What a tectonic plate actually is
- What happens where two plates push into each other
- Something you did not expect. Hold onto it.
The short version
Three things going on here
Divergent
Pulling apart
At a divergent boundary two plates move apart and magma rises to fill the gap, building new crust. Under the ocean that makes a mid-ocean ridge. On land it tears open a rift valley, like the ones running through Iceland and East Africa.
Convergent
Diving under
Where two plates push together, the older, colder, denser ocean plate usually sinks beneath the other: that is subduction. The seafloor bends down into an ocean trench, and water squeezed out of the sinking plate helps melt the rock above it, feeding a chain of volcanoes. The deepest trench of all is the Mariana, where the Pacific Plate dives under the Philippine Sea Plate; its Challenger Deep is about 10,935 m down, deep enough to swallow Mount Everest with about 2 km of water to spare.
Transform
Grinding past
At a transform boundary two plates slide sideways past each other, making no new crust and destroying none, just a lot of earthquakes. Most transform faults zigzag across mid-ocean ridges. A famous one runs on land: the San Andreas Fault, where the Pacific Plate has been grinding northwest past North America at about 5 cm a year.
Photos: Laura Paredis, Jess Loiterton, Celeo Sun / Pexels
Play with it
Let the earthquakes draw the plates
This live map plots recent earthquakes around the world. Nobody drew plate boundaries on it: the earthquakes do that by themselves. Find the lines of dots tracing the rim of the Pacific, then zoom in on the west coast of South America and on the Mariana Islands.
Live map: Interactive Earthquake Browser by the EarthScope Consortium (formerly IRIS), from global earthquake catalogs.
Two questions to answer while you are in there
- Are the earthquakes scattered evenly, or do they fall in long lines? Where are the lines?
- Along a mid-ocean ridge the quakes make a thin line; next to the Peru–Chile or Mariana Trench they make a wide band. Why might a plate diving down at an angle spread its earthquakes out?
Try it yourself
Check yourself
Try it
Which Boundary Is This?
Eight quick calls: read the clue, decide whether the plates are pulling apart, pushing together or sliding past.
Try it · interactive
Plate Boundary Builder
Pick two plates and push them apart, together or past each other, and watch a cross-section build a mid-ocean ridge, a trench with a chain of volcanoes, a pile of collision mountains or a transform fault, with a real-world example of each.
Open the Plate Boundary Builder in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Plate boundary
Where two tectonic plates meet
Subduction
One plate sinking beneath another into the mantle
Ocean trench
Deep, narrow seafloor trough where a plate sinks
Transform fault
Boundary where plates slide past each other
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: Where the Seafloor Goes Back In
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 4.6 — Convergent Plate Boundaries — LibreTexts · 8 minutes
- Webb 4.7 — Transform Plate Boundaries — LibreTexts · 4 minutes
- USGS This Dynamic Earth — Understanding Plate Motions — U.S. Geological Survey · 12 minutes
Photo credits (Pexels): Lucas Valentim, Jose Luis Vanasco, Laura Paredis, Jess Loiterton, Celeo Sun.