intrusive and extrusive igneous rocks

intrusive igneous rocks

Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see Figure 1.1 for an example). Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and bi- otite (black, platy). igneous rocks to the surface. Figure 1.2 shows a landscape in Californias Sierra Nevada Mountains made of granite that has been raised to create mountains. Californias Sierra Nevada Mountains are intrusive igneous rock exposed at Earths surface.

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intrusive and extrusive igneous rocks

The rate at which magma cools determines whether an igneous rock is intrusive or extrusive. The cooling rate is reflected in the rocks texture.

extrusive igneous rocks

Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks (Figure 1.3). Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (Figure 1.4). Some volcanic rocks have a different texture. The rock has large crystals set within a matrix of tiny crystals. In this Extrusive igneous rocks form after lava cools above the surface. Cooled lava forms basalt with no visible crystals. Why are there no visible crys- tals? Cooling rate and gas content create other textures (see Figure 1.5 for examples of different textures). Lavas that cool extremely rapidly may have a glassy texture. Those with many holes from gas bubbles have a vesicular texture. Different cooling rate and gas content resulted in these different textures. Click image to the left or use the URL below. URL:

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instructional diagrams

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questions

igneous extrusive rocks exhibit

a) no visible crystals.

-->  b) mostly microscopic crystals with perhaps a few larger crystals.

c) mostly larger crystals with a few microscopic crystals filling in the spaces between them.

d) larger crystals, all visible with the naked eye.

to create a large mountain range of igneous intrusive rocks requires

a) magma to flow over the surface and to pile up in mountains.

b) magma to cool beneath ocean water and then the ocean to move elsewhere.

-->  c) magma to cool deep in the crust and then for the plutons to be uplifted.

d) none of these

igneous intrusive rock has visible crystals because it cools and solidifies slowly.

-->  a) true

b) false

a pluton is a large igneous rock that cooled inside the crust.

-->  a) true

b) false

a rock texture that indicates the presence of gas bubbles.

a) porphyritic

-->  b) vesicular

c) intrusive

d) extrusive

a rock texture in which visible crystals are found in a matrix of tiny crystals.

-->  a) porphyritic

b) vesicular

c) intrusive

d) extrusive

which of these igneous rocks cooled so quickly that crystals did not form, creating a natural glassy look?

a) pumice

-->  b) basalt

c) granite

d) obsidian

which of these igneous rock types have a vesicular texture?

-->  a) pumice

b) basalt

c) granite

d) obsidian

if a rock is cooling in a magma chamber for a while and then suddenly erupts it will have a porphyritic texture.

-->  a) true

b) false

lava that cools extremely rapidly may have a crystalline texture.

a) true

-->  b) false

diagram questions

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