Plate Tectonics

You’ll be able to

Describe Earth’s internal layers and the driving force behind plate motion
Distinguish the three types of plate boundaries and their landforms
Calculate a plate’s spreading rate from seafloor age and distance data

Earth’s layers and the engine of plate motion

Earth is layered: a thin, rigid crust, a thick mantle of hot, slowly flowing rock, and a dense iron core (liquid outer, solid inner). The crust and the rigid top of the mantle form the lithosphere, broken into moving tectonic plates that ride on the softer, plastic asthenosphere below. The plates move because heat from the core drives convection currents in the mantle: hot rock rises, spreads, cools, and sinks, dragging the plates along like items on a conveyor belt.

Three kinds of plate boundaries

Plates interact at three boundary types. At a divergent boundary, plates move apart and magma rises to form new crust — creating mid-ocean ridges and rift valleys. At a convergent boundary, plates collide: one may slide beneath the other in subduction, building volcanoes and deep ocean trenches, or two continents may crumple upward into mountain ranges like the Himalayas. At a transform boundary, plates slide past each other horizontally, grinding out earthquakes — the San Andreas Fault is the classic example.

Hotspots, earthquakes, and volcanoes

Not all activity sits at plate edges. A hotspot is a plume of rising magma that punches through the middle of a plate; as the plate drifts over it, a chain of volcanoes forms — the Hawaiian Islands are a hotspot track. Boundaries and hotspots concentrate two hazards: earthquakes, sudden releases of energy as rock slips (measured on the Richter scale), and volcanoes, which vent magma, ash, and gases. Subduction zones like the Pacific "Ring of Fire" host the most powerful of both.

Rate of plate movement

rate = distance from ridge / age of that seafloor

Newly formed crust sits at the ridge (age 0). The farther the seafloor and the older it is, the faster the plate has spread. Convert units carefully — 1 km = 100,000 cm.

Worked example

Seafloor at a point 40 km from a mid-ocean ridge is dated to 2 million years old. What is the average spreading rate, in centimeters per year?

1.Convert the distance to centimeters: 40 km × 100,000 cm/km = 4,000,000 cm.
2.Write the rate as distance ÷ time: 4,000,000 cm ÷ 2,000,000 years.
3.Divide: 4,000,000 / 2,000,000 = 2.
4.So the plate has moved that seafloor away from the ridge at about 2 cm per year.

Answer: About 2 cm/yr — roughly the rate your fingernails grow. Plate motion is slow but relentless over geologic time.

Checkpoint

What is the fundamental driving force that moves Earth’s tectonic plates?

Tip

Match each boundary to its landform: divergent → mid-ocean ridge / rift valley (crust created), convergent → mountains, trenches, subduction volcanoes (crust destroyed or crumpled), transform → faults and earthquakes (crust neither made nor destroyed, just sliding).

Checkpoint

The Hawaiian Islands form a chain of progressively older volcanoes in the middle of the Pacific Plate, far from any plate boundary. What best explains their formation?

On the exam

For spreading-rate problems, the unit conversion is where points are lost: 1 km = 1,000 m = 100,000 cm. Set up rate = distance ÷ age, keep the units attached through the whole calculation, and state the final answer in the units the question asks for (usually cm/yr).

Answer the 2 checkpoints as you read.