In this article, we will discuss the field identification of igneous rocks in detail.
First, we should know what are igneous rocks.
What are Igneous Rocks?
Igneous rock is one of the three main rock types and is formed when hot molten rock (magma) solidifies. Igneous rock is found all over the world and makes up the majority of the Earth’s crust.
There are two main types of igneous rock: intrusive and extrusive.
The most common igneous rocks are granite, basalt, and obsidian.
Classification of Igneous Rocks
In terms of modes of occurrence the igneous rocks can be:
Intrusive igneous rock forms when magma cools slowly beneath the Earth’s surface. This type of rock is typically very hard and has large crystals.
Granite is an intrusive igneous rock that is very strong and durable. It is often used in construction and is the most common type of rock in the continental crust.
Extrusive igneous rock forms when magma cools quickly on the Earth’s surface. This type of rock is typically very fine-grained and has small crystals.
Basalt is an extrusive igneous rock that is commonly found in lava flows.
Now, we will discuss how igneous rock is identified in the field.
Identification of igneous rocks
Igneous rock can be identified in the field by keeping in mind the following features:
There are 7 different texture classes for igneous rocks, each with its own unique features.
- Pegmatite igneous rocks have very large crystals, more than 1 cm in size. These are the slowest cooling type of igneous rocks. Remember, the slower a rock cools, the larger the crystals will be.
- Phaneritic igneous rocks are composed of interlocking crystals that are smaller than crystals in pegmatite but still visible to the naked eye.
- Porphyritic igneous rocks have crystals of two different sizes, often with large crystals set in areas of smaller crystals.
- Aphanitic igneous rocks have a fine-grained texture and most of their crystals are too small to see with the naked eye. You will need to use a magnifying glass to observe the crystals in aphanitic rocks.
- Igneous rocks that form too quickly for crystals to form have what is called a glassy texture. Obsidian is the only glassy igneous rock and can be identified by its dark color. This looks like dark black glass.
- Vesicular igneous rocks, such as pumice, look bubbly and form before gases are able to escape as lava forms the rock. This also formed with very rapid cooling.
- Pyroclastic igneous rock is a texture composed of volcanic fragments ranging from very fine (ash) to very coarse (tuffs and breccia).
Composition refers to the percentage of certain minerals in your rock. You will need a rock guide to determine what minerals are present in your rock. There are four main composition types for igneous rocks:
- Identification of the composition of igneous rocks can be very difficult if you are not an experienced rock collector or geologist.
- If you have any questions about how to identify a rock, contact a collector or geologist at a local college or university.
- Felsic igneous rocks are light in color. Their mineral composition is primarily feldspars and silicates such as quartz.
- Granite is an example of a felsic rock.
- Felsic rocks have a low density and contain 0-15% mafic crystals. Mafic minerals are olivine, pyroxene, amphibole, and biotite.
- Mafic igneous rocks are dark in color and consist mainly of magnesium and iron. They contain 46-85% mafic mineral crystals and are high in density.
- Basalt is an example of mafic rock.
- Ultramafic igneous rocks are also dark in color and contain higher amounts of minerals found in mafic rocks. These rocks have greater than 85% mafic mineral crystals.
- Dunite is an example of ultramafic rock.
- Intermediate igneous rocks contain 15-45% mafic mineral crystals. They share minerals with both felsic and mafic rocks and are intermediate in color.
- Diorite is an example of an intermediate rock.
The above techniques are very important for the identification of igneous rocks in fields where no other modern techniques are available.
Igneous rocks are the most important ones and their field identification can be a very useful task in the initial surveying of a particular area.