Explain how the cooling rate affects the crystal size and texture of igneous rocks. Provide examples of rocks with different crystal sizes.

A slower cooling rate allows for the formation of larger crystals, resulting in a coarse texture, as seen in intrusive rocks like granite. Faster cooling rates result in smaller crystals or a glassy texture, as seen in extrusive rocks like obsidian.

Describe the difference between mafic and felsic rocks in terms of their composition, color, and silica content.

Mafic rocks are dark in color, contain a lower percentage of silica (0-45%), and are rich in magnesium and iron. Felsic rocks are light in color, contain a higher percentage of silica (65% or greater), and are rich in feldspar and silica.

Classifying Rocks By Formation Lesson Plan for Grade 6th Grade

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Learning Objective

I can classify rocks based on their formation.

1
Igneous rocks are formed from hot molten rock that cools and solidifies from either magma or lava.
2
Extrusive igneous rocks form on the surface, cool quickly, and have small or no crystals, while intrusive rocks form inside the Earth, cool slowly, and have larger crystals.
3
Igneous rocks can be classified as either mafic, which are dark in color and contain magnesium and iron, or felsic, which are lighter and contain feldspar and silica.

Classifying Rocks by Formation

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