Earth and space: Earth's history and plate tectonics (grade 12) · lesson

Plates and their boundaries

💡 Mantle convection moves the plates, and what happens at a boundary depends on how the plates move.

Hot mantle rock slowly rises and cooler rock sinks. This convection carries heat upward and helps drive the plates.

At a divergent boundary, plates move apart and new crust forms. At a convergent boundary, plates collide. When ocean crust meets continental crust, the denser ocean plate sinks (subduction), forming a deep trench and a chain of volcanoes. At a transform boundary, plates slide past each other.

Most earthquakes happen at plate boundaries, such as around the edges of the Pacific Ocean and along transform faults.

🧠 How to think about it

Ask how the two plates move: apart, together, or sideways. That one answer predicts the features you will find there.

convection: heat carried by moving material
subduction: one plate sinking beneath another

🔗 Khan Academy: The geosphere · Khan Academy (free) ↗

Seafloor spreading

💡 New ocean crust forms at mid-ocean ridges and is recycled at trenches, so the seafloor is young.

Magma rises at a mid-ocean ridge and cools into new crust, which moves away as more forms. So the rock is youngest at the ridge and older on both sides.

Ocean crust is dense and sinks back into the mantle at trenches. Lighter continental crust stays up. That is why the oldest seafloor is about 200 million years old, while continental rock can be about 4 billion.

A hot spot stays in place while a plate moves over it. Each new volcano is carried away, leaving a chain of islands that get older in the direction the plate moves.

🧠 Let's work it out

How fast a rock moved away from the ridge = distance ÷ age.
100 km = 10,000,000 cm, and 10,000,000 cm ÷ 5,000,000 years = 2 cm per year.
That is one plate's speed away from the ridge.
Both sides move, so the two plates separate twice as fast: 4 cm per year.
Always convert units first, and check which rate is asked.

What do you think comes next?

mid-ocean ridge: an undersea mountain chain where plates pull apart
hot spot: a fixed plume of hot mantle that feeds volcanoes

🔗 USGS: This Dynamic Earth · U.S. Geological Survey ↗

Evidence that continents moved

💡 Things that match across oceans show the continents were once joined.

Fossils of the same land reptile appear on continents now separated by ocean. A land animal could not swim across, so the lands were likely connected.

Rock layers and mountain belts line up across coasts that face each other, as if one piece had split.

🧠 How to think about it

Good evidence is a match that is hard to explain any other way. Salty oceans or similar heights don't connect two specific coasts, so they aren't evidence.

Reading rock layers and radioactive clocks

💡 Lower layers are older, cutting rock is younger, and half-lives give ages in years.

In undisturbed layers, the bottom formed first. A rock that cuts through layers must be younger than all of them.

A radioactive isotope decays at a steady rate. After one half-life, half the parent atoms remain; after two, a quarter. Each decayed parent atom becomes a daughter atom.

🧠 Let's work it out

100% → 50% → 25% is two half-lives.
With a 700-million-year half-life, the rock is 2 × 700 = 1,400 million years old.
If 800 parent atoms drop to 200, then 600 decayed into 600 daughter atoms.

What do you think comes next?

half-life: the time for half of a radioactive sample to decay