Volcanic and plutonic rocks are types of igneous rock. Volcanic rocks form when lava cools and solidifies on Earth, and plutonic rocks form when magma cools and solidifies below Earth. Learn about different characteristics of these igneous rocks.
Did you know that some rocks are formed from a liquid? Of course, I’m not talking about just any liquid; I’m talking about magma, which is hot molten rock from deep within the earth.
Magma has properties of a liquid. When magma is allowed to cool, it crystallizes into a solid rock form. The rocks made from the cooling and solidifying of molten rock are called igneous rocks, and in this lesson, you will learn about the two categories of igneous rocks – volcanic and plutonic rocks – and how they differ from one another.
Defining Plutonic and Volcanic Rocks
The term ‘igneous’ comes from the Latin language and is the word for ‘fire,’ which is a very appropriate name due to the fact that igneous rocks originate from fiery, red-hot magma. We associate magma with volcanoes, yet you don’t actually see magma because by definition, magma is located below the surface of the earth.Magma forms in the deeper layers of the earth where temperatures are so hot that rocks melt. This melted rock, or magma, is less dense than the rock surrounding it, allowing it to rise toward the earth’s surface.
Some of this rising magma finds its way to the inside of volcanoes, yet you do not see it until it comes out of the volcano as lava. In other words, magma is hot molten rock in the middle of a volcano, and lava is hot molten rock leaving a volcano.This differentiation between magma and lava is important when we consider the two classifications of igneous rock. This is because plutonic rocks are rocks formed when magma cools and solidifies below the earth’s surface, and volcanic rocks are rocks formed when lava cools and solidifies on the earth’s surface.
Volcanic rocks are also known as ‘extrusive igneous rocks.
‘ We see from their definition that volcanic rocks form on the surface, or ‘exterior,’ of the earth. If you remember that ‘extrusive’ and ‘exterior’ both start with the letters ‘ext,’ it may help you recall this term.Yet, the term ‘extrusive’ also helps us understand how volcanic rocks form because they form from the ‘extrusion,’ or eruption of lava from a volcano. When lava meets the cooler temperatures of the atmosphere, it cools rapidly and solid crystals form. However, because of the rapid cooling, this crystallization happens too fast for the crystals to grow very big.
In fact, the crystals of volcanic rocks are so small that you can only see them with a microscope.This fast rate of cooling makes for one of the most notable differences between volcanic rocks and plutonic rocks. As we will learn in a moment, plutonic rocks cool much slower and under higher pressure because they are in the ground. Therefore, their crystals have the right conditions to grow large.The volcanic rocks, in comparison, contain small crystals and may even develop a glassy appearance or even cool so fast that they trap gas bubbles in the rock.
This gives us very light, porous volcanic rocks, such as pumice. You may be familiar with pumice stones and may have even used one to smooth dry skin on the heels of your feet.
Let’s take a closer look at plutonic rocks. We see from the definition that plutonic rocks form inside the earth. For this reason, plutonic rocks are also known as ‘intrusive igneous rocks.’ You can recall this term by remembering that both ‘inside’ and ‘intrusive’ begin with ‘in.’You can also connect the word ‘plutonic’ with the word ‘in’ if you recall that plutonic rocks are named for the Greek god Pluto, who was the god of the underworld located deep ‘inside’ the earth.
However, the word ‘intrusive’ can also apply to how plutonic rocks are formed. We remember that magma has properties of a liquid, so it can squeeze into cracks and crevices, as if it is an ‘intruder’ who is invading the rocks. When magma pushes its way into rock crevices, it finds itself under high pressure and slowly cools.Because magma is under high pressure and takes a long time to cool, it allows time for the formation of large crystals. Therefore, plutonic rocks have coarse-grained crystals. A good example is granite, which is a very hard plutonic rock. The countertops in your kitchen might be made of granite, as this rock is often used in building.
Another thing you might know about granite is that it comes in many different colors, from pink to gray, and this is a characteristic of plutonic rocks, meaning that they come in many different sizes and colors.
Pluton is the term used to describe a mass of plutonic rock. Plutons range in size. They can be massive, as we see with batholiths, which are very large plutons that form when many plutons join to form a huge expanse of rock.
Batholiths first develop deep below the earth’s surface, yet have massive areas that are exposed to the surface due to years of erosion that weathers away overlying rock. It’s almost as if Mother Nature painstakingly excavates batholiths to uncover them to the world.The exposed area of batholiths spans at least 40 square miles, with some batholiths being so massive that they span across states. For example, California’s Sierra Nevada Range is a batholith.
Plutons: Sills and Dikes
In contrast to the large batholiths, plutons that are formed when magma is forced into cracks and crevices between existing rocks are considerably smaller, more in the range of 20 meters wide.
Sills are sheet-like plutons that form parallel to existing rock. Because sills are usually found lying horizontally in between layers of existing rock, it may help you to remember this term by thinking of the windowsill that lies in a horizontal position under your window.In contrast to sills, which lie between preexisting rock structures, dikes are sheet-like plutons that form in cracks of existing rocks and cut across preexisting structures. Dikes form when magma is injected into rocks, as opposed to between beds of rock, like we see with the sills. So, dikes may be found lying in a vertical or slanted position.Because plutonic dikes can be found standing upright, like a wall, it might help you to recall this term if you think of them as being similar to a dike that is a wall designed to hold back water. And maybe you remember the story from your childhood of the little Dutch boy who saved Holland by sticking his finger in the dike to stop the country from being flooded.
Let’s review. Igneous rocks are rocks made from the cooling and solidifying of molten rock. There are two categories of igneous rocks. Volcanic rocks are rocks formed when lava cools and solidifies on the earth’s surface. Volcanic rocks are also known as ‘extrusive igneous rocks’ because they form from the ‘extrusion,’ or eruption, of lava from a volcano.
Lava cools rapidly when it exits a volcano, giving crystals insufficient time to form before the rock solidifies. Therefore, volcanic rocks contain small crystals and may even develop a glassy appearance or be light, porous volcanic rocks, such as pumice.Plutonic rocks are rocks formed when magma cools and solidifies below the earth’s surface. Plutonic rocks are also known as ‘intrusive igneous rocks’ because they form when magma squeezes into cracks and crevices, as if it is an ‘intruder’ who is invading the rocks. This magma is under high pressure and takes a long time to cool, allowing time for the formation of large crystals.
Therefore, plutonic rocks have coarse-grained crystals. A good example is granite, which is a very hard plutonic rock.Pluton is the term used to describe a mass of plutonic rock. Plutons range in size.
Batholiths are very large plutons that form when many plutons join to form a huge expanse of rock. California’s Sierra Nevada Range is a batholith.Sills are sheet-like plutons that form parallel to existing rock.
In contrast to sills, which lie between preexisting rock structures, dikes are sheet-like plutons that form in cracks of existing rocks and cut across preexisting structures.
After this lesson, you should have the ability to:
- Define igneous rock
- Explain why volcanic rocks are considered extrusive igneous rocks and plutonic rocks are considered intrusive igneous rocks
- Summarize why magma and lava form either coarse-grained crystals or small crystals
- Define pluton and batholith and identify an example of a batholith
- Differentiate between sills and dikes