How can igneous rock be so black and shiny?
This rock is lava that rapidly cooled on Kilauea volcano in Hawaii Volcanoes National Park on the Big Island of Hawaii. The lava cooled so fast that crystals had little time to form. How does this rock compare with the granite further down this lesson?
The rate at which magma cools determines whether an igneous rock is intrusive or extrusive. The cooling rate is reflected in the rock's texture.
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 below for an example).
Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy).
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 igneous rocks to the surface. Figure below shows a landscape in California’s Sierra Nevada made of granite that has been raised to create mountains.
California’s Sierra Nevada is intrusive igneous rock exposed at Earth’s surface.
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 below).
Extrusive igneous rocks form after lava cools above the surface.
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 below).
Cooled lava forms basalt with no visible crystals. Why are there no visible crystals?
What does the andesite photo in the lesson "Types of Rocks" indicate about how that magma cooled? The rock has large crystals set within a matrix of tiny crystals. In this case, the magma cooled enough to form some crystals before erupting. Once erupted, the rest of the lava cooled rapidly. This is called porphyritic texture.
Cooling rate and gas content create other textures (see Figure below 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.
- Intrusive igneous rocks cool from magma slowly because they are buried beneath the surface, so they have large crystals.
- Extrusive igneous rocks cool from lava rapidly because they form at the surface, so they have small crystals.
- Texture reflects how an igneous rock formed.
Use this resource to answer the questions that follow.
1. How are intrusive rocks formed?
2. What size are the crystals in very coarse rocks?
3. What are the most common coarse rocks?
4. How are extrusive rocks formed?
5. List the three textures for extrusive rocks.
6. Describe rhyolite.
7. Describe pumice.
8. Explain why obsidian appear black.
1. How does a rock develop a vesicular texture?
2. What are the other names for igneous intrusive rock and igneous extrusive rocks and how do they get those names?
3. What sequence of events causes a rock to develop porphyritic texture?