Download Lesson 6 - Earth Formation

Grade 9 Academic Geography - Strand 3 Lesson 5
Earth Formation
About 4.6 MYA, the Earth formed from cosmic debris
Cut in half, the Earth has four distinct parts:
 Crust
 Mantle
 Outer Core
 Inner Core
Crust
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Crust is the outermost layer
It is between 6 and 100 kilometres thick. The thicker layers are typically mountains, while the
thinner areas are located at the bottom of the ocean.
The crust constantly changes due to erosion, deposition, glacial action and plate tectonics.
There are two types of crust.
 Continental - Granite-like rock rich in silicon, oxygen and aluminum.
 Oceanic - Igneous, basaltic rock containing greater amounts of heavier elements such as
magnesium and iron
Lithosphere
 The lithosphere is composed of the crust and the upper part of the mantle. The lithosphere is
divided into several layers of slow moving plates.
 New crustal rock forms at the “diverging” lithospheric plate boundaries.
 Old crustal rock melts at a subduction zone (...area where one plate plunges under another
plate). A subducted plate moves down to an environment of higher temperature and pressure
where the rocks melt into magma. The magma is released by volcanoes.
 A plate can consist of both crust types.