In practice, "pluton" usually refers to a distinctive mass of igneous rock, typically several kilometers in dimension, without a tabular shape like those of dikes and sills. Batholiths commonly are aggregations of plutons.
The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz diorite. Generally light colored, coarse-grained plutons of these compositions are referred to as granitoids.
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, there are two categories of igneous rocks - volcanic and plutonic rocks, how they differ from one another ?.
Defining Volcanic and Plutonic 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
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.
Plutonic Rocks
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 colours, from pink to Gray, and this is a characteristic of plutonic rocks, meaning that they come in many different sizes and colours.
Pit gnammas
Pit gnammas are usually deeper than 50cm and typically form in granite, often at the base of an outcrop. Round pit gnammas in WA measure up to 12m in diameter, with canoe-shaped ones as long as 15m. They can hold water for a long time
Pit gnammas are mostly found in the WA Wheatbelt, with some out in the Goldfields. There are a few in South Australia on the Eyre Peninsula. There are none in eastern Australia
Pan gnammas
Pan gnammas are typically 5-20cm deep, 1-3m in diameter and usually form in granite but can occur in other rock types. They hold seasonal rain water for a short time. Pan gnammas are the most common kind.
Pipe gnammas
Rare pipe gnammas have narrow shafts 30-60cm diameter up to 2m deep. They typically form in lateritic rock further east in the Gibson and Victoria Deserts.
Armchair gnammas
Armchair gnammas are uncommon too. They are like seats built into sloping rock, with steep back and side walls with either a flat or deep bottom.
You are welcome to log a "Found it" log when you message or email the answers for the following questions. any "Found it" logs without the answers being sent will be deleted.
Q1 You are standing on the top of a pluton. Do you think the shape indicates that the pluton is larger or smaller underground.
Q2 Estimate the grain size of the granite. What do you think this tells you about the pluton cooling process?
Q3 What is the unique shape of the Gnamma at GZ and is it a Pit, Pan, Pipe or Armchair Gnamma?