Rock layers and Earth's history
Interpret cross-section diagrams of the Earth's interior, geological strata, and rock cycle; read and label layers (crust, mantle, outer core, inner core); understand that deeper layers in sedimentary sequences are older
What a learner can do afterwards
- Label the four layers of the Earth on a cross-section diagram using the correct terms
- Interpret a diagram of sedimentary rock layers and identify which layer was deposited first
- Read a rock cycle diagram and trace the pathway of a rock from igneous to sedimentary to metamorphic
The lesson
Earth is built like a hard-boiled egg. The thin shell is the crust, the ground under your feet. Under that is the mantle, a thick layer of hot rock that flows very slowly, like thick honey. Deeper still is the outer core, made of melted metal. At the very center sits the inner core, a solid ball of metal, the hottest part of the whole planet.
A scientist cuts open a cliff and sees flat bands of rock stacked like a sandwich. The bottom band formed first, long before the top band. Each new layer of sand or mud settles on top of the layer before it, so in an undisturbed stack, the deepest layer is always the oldest.
Reading rock layers is like reading a diary from the bottom up. The oldest entry sits at the bottom, and every new page gets added on top.
Rocks change from one kind to another in a cycle. Melted rock cools and hardens into igneous rock. Wind and water break igneous rock into tiny pieces that pile up and press together into sedimentary rock. Heat and pressure deep underground squeeze sedimentary rock into metamorphic rock, which can melt again and start the cycle over.
Earth has four layers, crust to inner core, and in a stack of rock layers the bottom layer is always the oldest.
Watch it
Where it sits
Learn first
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.