■ Earth and Space Science · Quarter 1

Three Ways Two Plates Meet

Apart, toward, or past. The whole of plate tectonics starts with choosing one.

Two sheets of paper and two hardback books on the front desk before the class arrives. 1 min

A geologically dynamic archipelago

The curriculum guide's own words for this country.
Volcanoes along its length. A fault system running nearly end to end.
Earthquakes recorded continuously, most of them too small to feel.

Why here, and not everywhere?

Do not answer it. Week 3 answers it with a map; today builds the tool that reads the map. 4 min

Only three things are possible

That is the complete list.
Every boundary on Earth is one of the three.

Two plates can move apart, move toward, or slide past each other.

Write the three words on the board and leave them up all week: APART · TOWARD · PAST. 3 min

Apart — divergent

The two plates pull away from each other.
Molten rock rises into the gap and cools.
New crust is made.

What forms

A mid-ocean ridge under the sea. A rift valley on land.

What happens

Shallow earthquakes. Frequent, gentle eruptions.

Gentle is the word. Runny lava, low explosivity — the opposite of the volcanoes most learners picture. 5 min

The evidence for it

The sea floor is youngest at the ridge and gets older with distance.
It does this symmetrically — the same pattern on both sides.
Nothing but spreading explains a mirror image.

New crust is made here. The Earth is not getting bigger. So what must be happening somewhere else?

That question is the bridge to slide 6, and the class should reach it before you say it. 4 min

Toward — convergent

Something has to give — but what gives depends entirely on what kind of crust is arriving.
Oceanic crust is thin and dense.
Continental crust is thick and light.
Three combinations. Three different results.

And this is where the topic gets hard.

Slide 6 is the pivot of the week. Everything before it is easy and everything after it is where marks are lost. 4 min

Ocean meets continent

The dense oceanic plate goes under the light continental one. That is subduction.
It melts as it goes down. The molten rock rises through the continent.

What forms

A deep trench offshore. A chain of volcanic mountains inland.

What happens

Powerful, explosive eruptions. Earthquakes at every depth, getting deeper inland.

The Andes. The guide names them in competency 7. 5 min

Ocean meets ocean

Both are dense. The older, colder one goes under.
Same process, but the volcanoes come up through water.

What forms

A deep trench. A curved chain of volcanic islands — an island arc.

What happens

Explosive eruptions. Earthquakes at every depth. Tsunami risk.

This is the boundary type that builds archipelagos, and it is worth saying so out loud here. 4 min

Continent meets continent

Both are thick and light. Neither one will go down.
So both crumple upward instead.

What forms

The highest mountains on Earth. No trench. No volcanoes.

What happens

Enormous shallow earthquakes.

This is convergent. Where is the trench?

The Himalayas — also named in competency 7. This slide breaks *convergent means trench*, which is the plan. 5 min

Past — transform

The two plates slide alongside each other.
No crust is made. No crust is destroyed.
They stick, strain builds, and then they slip all at once.

What forms

A fault line. Streams, fences and roads visibly offset where they cross it.

What happens

Shallow earthquakes, and some of the most destructive ever recorded.

Nothing is built here, which is why learners assume nothing happens here. Say the opposite explicitly. 4 min

The whole thing, once

Apart → ridge or rift · new crust · shallow quakes, gentle eruptions
Toward, ocean + continent → trench + volcanic mountains · deep quakes
Toward, ocean + ocean → trench + island arc · deep quakes
Toward, continent + continent → mountains, no trench, no volcanoes
Past → fault, offset features · shallow quakes only

This is the wall chart. Learners copy it into their notes now, by hand. 4 min

The trap

The asthenosphere is solid rock.
It flows — extremely slowly — the way glacier ice flows while staying solid.
Magma is made in particular places, in small amounts, mostly at boundaries.

The plates do not float on a sea of liquid magma.

If it were liquid, what would happen to earthquake waves passing through it?

They would stop. S-waves cannot cross a liquid, and they cross the asthenosphere. That is Grade 9 Quarter 2 arriving. 4 min

Where this archipelago sits

The guide names the Philippine Plate and asks learners to predict where this country will be in 50 million years from its velocity.
That question only makes sense because the plate is moving now, measurably, and has a direction.

Which of the three is doing this to us?

Convergence, and a major fault system as well. Week 3 puts it on a map — competency 3. 4 min

Four questions for any boundary

1. Which way are the two plates moving?
2. What kind of crust is on each side?
3. What structure does that build?
4. What events happen there?

In that order, always. Question 2 is the one learners skip and it is the one that decides the answer. 3 min

One sentence to leave with

Ask which way, then ask what crust. The structure follows.

Say it together. It is on the worksheet and on the wall chart. 2 min