Plate Tectonics

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. Here you can raise the plate-speed slider and watch plates pull apart at a divergent boundary.

Earth's puzzle pieces

🧩 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

Plate speed5 cm/yr

💡 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

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?

A boundary where two plates move apart is called a divergent boundary.

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?

Separation distance = relative speed × time.

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.

CSAT-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

At a convergent boundary, plates collide or subduct, forming trenches, fold mountains, and active earthquakes and volcanoes.

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).

Summary

Plate tectonics core

mantle convection → plate motion → geological phenomena (quakes, volcanoes, mountains)

Earth's internal heat is the ultimate cause of surface geological activity

🎯 Exam Points

①3 boundary types: divergent / convergent / transform — distinguish the geological phenomena

②Mantle convection drives plate motion

③Divergent = ridges, convergent = trenches/fold mountains, transform = transform faults

④Seafloor spreading evidence: symmetric magnetic stripes, rock-age distribution

⑤Pacific Ring of Fire = quake/volcano belt along plate boundaries

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