Earth and space (grade 7) · lesson

The rock cycle

💡 Rocks form in three ways, and any rock can slowly change into another type.

Igneous rock forms when melted rock cools and hardens. Melted rock underground is magma. At the surface, it is lava. Granite is igneous.

Sedimentary rock forms in layers. Weathering breaks rock into bits. Water carries them away and drops them in layers. The layers are pressed and cemented into rock.

Metamorphic rock forms when heat and pressure change a rock without melting it. Limestone becomes marble. Granite can become gneiss.

🧠 Let's work it out

Find the clue in how it formed.
Melted, then cooled? Igneous.
Pieces laid down in layers? Sedimentary.
Heated and squeezed but never melted? Metamorphic.

What do you think comes next?

igneous: rock made from cooled melted rock
sedimentary: rock made from layers of pressed pieces
metamorphic: rock changed by heat and pressure
magma: melted rock under Earth's surface

🔗 Khan Academy: The geosphere · Khan Academy ↗

Fast and slow changes

💡 Some changes to Earth's surface take minutes, but most take millions of years of tiny steps.

A landslide or an eruption can change land in minutes.

Most changes are slow. Rivers carve canyons and mountains rise over millions of years.

Tiny changes add up. A few centimeters a year becomes many kilometers over a million years.

🧠 Let's work it out

Use units carefully.
The Atlantic widens about 2.5 cm a year.
25 km is 2,500,000 cm.
Divide: 2,500,000 ÷ 2.5 = 1,000,000 years.

What do you think comes next?

weathering: breaking rock into smaller pieces
erosion: carrying broken rock away

Reading rock layers

💡 In flat, undisturbed rock layers, lower layers are older than the layers above them.

Sedimentary layers stack up over time. Each new layer settles on top of the last one.

So, if the layers have not been flipped or tilted, the bottom layer formed first. The top layer is youngest.

Fossils in a layer are about as old as that layer.

🧠 Let's work it out

Picture a stack of mail.
The first letter you got is at the bottom.
A fossil in layer D, below layer B, is older than a fossil in layer B.

What do you think comes next?

fossil: the remains or traces of a living thing kept in rock

Moving plates

💡 Earth's surface is broken into moving plates, and matching rocks, fossils, and seafloor ages are the evidence.

Earth's outer layer is broken into huge plates. They move a few centimeters a year.

Most earthquakes and volcanoes happen where plates meet. At a mid-ocean ridge, plates pull apart. Magma rises and hardens into new seafloor, so the youngest rock is at the ridge.

Continents now an ocean apart have matching coastlines, rare rock layers, and fossils of land animals and plants. These clues suggest they were once joined.

🧠 Let's work it out

Judge each clue: could it match by chance?
Sandy beaches form everywhere, so they prove nothing.
A freshwater reptile could not swim across a salty ocean.
Finding it on both sides means the land itself moved.

What do you think comes next?

tectonic plate: a huge moving piece of Earth's outer layer
mid-ocean ridge: an underwater mountain chain where plates pull apart

🔗 USGS: This Dynamic Earth · USGS ↗