Igneous Rock September 12, 2011
Is That a Fact! Igneous rocks that form deep underground are called plutonic rocks, after Pluto, the god of the underworld in Roman mythology. Volcanic rocks are named after Vulcan, the Roman god of metalworking and fire.
Bellringer • Do you think rock that cooled & solidified from lava on Earth’s surface would look different from those that cooled & solidified from magma inside the earth? • Why? • Write your thoughts in your Science Journal.
Bellringer • Do you think rock that cooled & solidified from lava on Earth’s surface would look different from those that cooled & solidified from magma inside the earth? • When magma cools slowly, under the Earth, large well-developed crystals form; but when magma erupts onto the surface as lava, it cools more quickly where there is not enough time for large crystals to grow.
Objectives • Describe three ways that igneous rock forms. • Explain how the cooling rate of magma affects the texture of igneous rock. • Distinguish between igneous rock that cools within Earth’s crust & igneous rock that cools at Earth’s surface.
Three ways igneous rock forms • Rock is heated • Pressure is released • Rock changes composition
Cooling rate of magma • Felsic rocks-Light colored & less dense than dark colored rocks. Made of Al, K, Si, & Na. Page 794 Periodic Table of the Elements • Mafic rocks-Dark colored & more dense than light colored rocks. Made of Ca, Fe, & Mg. • Longer it takes to cool, the more time the crystals have to grow.
Texture of igneous rock • The larger the crystals are the coarser the texture of the igneous rock. • In contrast, the faster the rock cools, the crystals are fine-grained due to the igneous rock containing smaller crystals.
Using the Figure • Page 99 Figure 2. • Rank the rocks by how fast they cooled. • #1 being the fastest cooled & #4 being the slowest cooled • (Pay attention to the grain size of each rock.)
Intrusive Igneous Rock • Igneous rock that cools & solidifies within Earth’s crust • Usually has a coarse-grained texture • Minerals within are large, visible crystals
Extrusive Igneous Rock • Igneous rock that forms from lava & cools at or near the Earth’s surface. • Fine-grained igneous rock • Contains very small or no crystals at all