Earth's surface is not one shell but many plates floating on the mantle.
Plates move a few centimeters per year, driven by mantle convection, and earthquakes, volcanoes, and mountains form at their boundaries.
Divergent, convergent, and transform boundaries each produce different geological features.
Raise the plate speed and watch plates pull apart at a divergent boundary.
Earth's puzzle pieces
If you memorize coastlines, seismic belts, and volcanic belts as separate lists, they never join on one map. Split the surface into plates and name the relative motion at the edges as divergent, convergent, or transform, and only then do ridges, trenches, and transform faults fit inside one account. Mantle convection is the driver of that relative motion, and the real speeds sit between about 1 cm and 15 cm per year, about as fast as a nail grows. Treat only continents as floating and you drop the oceanic lithosphere pieces, so this chapter has to fix the plate as crust plus part of the upper mantle before it names the boundaries.
🧩 What is plate tectonics?
①Earth's surface isn't one piece — it's divided into giant plates like a puzzle
②These plates float on the mantle and move slowly (a few cm/year)
③Where plates meet: earthquakes, volcanoes, and mountains form
Plate motion and mantle convection
The picture of a whole continent sliding as one block is easy to confuse with a plate. A plate is a lithosphere piece that includes crust and part of the upper mantle, and oceanic plates belong on that list too. Rock age increasing away from a ridge and magnetic stripes that stay symmetric left and right mean new crust forms in the middle; they do not mean a continent is shoved as one lump. Bundle every mountain range at only one kind of boundary and you mix trenches at subduction zones with fold mountains, and a transform boundary mostly scrapes with little creation or destruction, so the three types must not share one sentence. A belt where earthquakes and volcanoes gather, such as the circum-Pacific orogenic belt, is read by picking the boundary name first and then overlapping the hazards.
5 cm/yr
seegongsik.com
Cross-section: divergent boundary plates spreading apart driven by mantle convection
This panel is a cross-section, not a map. The green band on top is the continental plate, the gray-blue band the oceanic plate, and under the gold dashed line in the middle the label reads divergent boundary. The width of the gap between the two plates and the length of the green arrows are drawn wider and longer in proportion to the speed slider (1 cm/year to 12 cm/year). In the brown field below, two arcs sit still with the label mantle convection, and on the right a single red line slants downward. At the bottom the travel speed is written in cm/year, and the green arrows point away from each other across the gap. Raising the speed value does not spin the arcs or shift the plates; only the gap and the arrow lengths are restamped.
💡 Key observations
①Higher speed → wider gap between plates (divergent boundary)
②Mantle convection is the driving force
③Real plate speed: ~1–15 cm/year (about as fast as fingernails grow!)
Three plate boundary types
Boundary classification
Types of plate boundaries
🌋 Features by type
①Divergent → new crust forms (constructive)
②Convergent → old crust destroyed (destructive)
③Transform → crust neither created nor destroyed (conservative)
Why plates move — mantle convection
Driving force of mantle convection
core's heat → mantle convection → plate motion
Earth's internal heat circulates the mantle, which drags plates along
Plate motion speed
v ≈ 1 ~ 15 cm/year
measurable by GPS
🔬 Evidence
①Seafloor spreading: rock age increases away from ridges
②Paleomagnetic stripes: symmetric on both sides of ridges
③Coastline fit + matching fossil distribution
④The Pacific Ring of Fire follows plate boundaries with quakes and volcanoes
Worked Examples and Exam Practice
Example 1
Like the Mid-Atlantic Ridge, what is the boundary called where two plates move apart and new oceanic crust forms?
1
A boundary where two plates move apart is called a divergent boundary.
2
Magma rises through the gap and solidifies, creating new oceanic crust.
▸ Divergent boundary (mid-ocean ridge)
A divergent boundary is constructive (it makes new crust); a convergent boundary is destructive (crust is consumed).
Example 2
If two plates move apart at 5 cm/year, about how far do they separate in 2 million years?
1
Separation distance = relative speed × time.
2
5 cm/year × 2 million years = 10 million cm = 100 km.
▸ About 100 km
Plates creep a few cm/year, but over millions of years that adds up to hundreds of km.
exam-style
Which statement about plate boundaries is correct?
①Old crust is consumed at a divergent boundary
②Trenches and fold mountains form at a convergent boundary
③New crust is actively created at a transform boundary
④Mantle convection is unrelated to plate motion
⑤Plates move a few cm every second
▸ ② Trenches and fold mountains form at a convergent boundary
1
At a convergent boundary, plates collide or subduct, forming trenches, fold mountains, and active earthquakes and volcanoes.
2
A divergent boundary makes new crust (①), a transform boundary neither creates nor destroys crust (③), mantle convection drives plates (④), and plates move a few cm per year (⑤).