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Unit 1: Physical Environments
1.1 Earth Systems – The Earth’s crust is part of a dynamic system.
Students Notes – Lesson 2
Structure of the Earth:
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The inner core is solid and consists of iron & nickel. It has a
density of about 13.6g/cm3, and is fives times denser than the
surface rocks.
The outer core is a liquid, comprising mostly iron. It has a
density of 10-12g/cm3. Movements in the outer core are
thought to be responsible for the Earth’s magnetic field.
The mantle is solid, containing lower density material,
peridotite, formed of silicate minerals. It is about 2900km thick.
The crust is solid and is divided into two main types, oceanic
crust and continental crust. The depth varies between 6km and
70km, and the average density is 3g/cm3.
The mantle and the crust collectively can be divided into three sections:
 Lithosphere – the crust and upper section of the mantle.
 Asthenosphere – the upper part of the mantle that is molten.
 Mesosphere – the lower part of the mantle (solid).
Oceanic & Continental Crust:
The table shows the main characteristics of continental and oceanic crusts.
Thickness
Density
Rock type
Mineral content
Continental Crust
Oceanic Crust
30 – 70 km
5- 9 km
3
2.7g/cm
granite
Silica & aluminium - sial
3.0g/cm3
basalt
Silica & magnesium - sima
Unit 1: Physical Environments
1.2 Earth Systems – The Earth’s crust is part of a dynamic system.
Students Notes – Lesson 2
Tectonic Plates:
The Earth’s crust is divided into a number of plates of different sizes, many of which consist of a combination of oceanic and continental crust. These plates move
slowly and irregularly in relation to each other, typically at rates of a few centimetres a year.
EXAM TIP:
When describing plate movement, which is
not fully understood, you should use terms
like; “it is thought ….” Or “it is likely ….”
The most likely cause of plate movement is the existence
of convection cell currents in the asthenosphere, caused
by heat from the core. This heat comes from a
combination of radioactive decay in the core and residual
primary heat.
This heat causes the convection currents to rise (rising
limb), until they reach the lithosphere where they diverge.
Where the convection current descends (descending
limb), due to cooling, the lithosphere is pulled
downwards (known as dragging).
 Plates are at their hottest nearest the mid-oceanic
ridges and they cool down as they move away. This
means that material will rise near the mid-oceanic
ridges.
 As the material cools its density increases, so it sinks
lower into the molten rock beneath and is relatively
easily dragged downwards into the subduction zone.
 Plate movement may be caused by a combination of both these forces, but the downward drag does seem to have the greater strength.