What is made out of igneous rock?

Igneous Rocks: Earth’s Fiery Fingerprints

Ever wonder what makes up the very ground beneath our feet? Well, a huge chunk of it is igneous rock, one of the three main rock types – think of them as the geological building blocks of our planet. These rocks are born from fire, quite literally! The name “igneous” even comes from the Latin word “ignis,” meaning fire. They’re essentially the cooled-down offspring of molten rock, either magma bubbling beneath the surface or lava spewing out in dramatic volcanic eruptions.

Now, when we talk about how much of the Earth is made of igneous rock, we’re not talking small potatoes. They make up a whopping 90-95% of the top ten miles or so of the Earth’s crust! That’s a whole lot of fire-born stone. They also make up around 15% of the land surface we see every day, and practically all of the ocean floor is made of it.

What’s Inside These Fiery Rocks?

So, what exactly goes into making an igneous rock? Well, the main ingredients are silicate minerals – they make up over 90% of the whole shebang. Imagine a molten soup rich in elements like silicon, oxygen, aluminum, and a bunch of others. As this soup cools, these elements combine to form different minerals.

These minerals can be broadly divided into two groups: felsic and mafic. Felsic minerals, like quartz and feldspar, are generally light in color – think of them as the “lightweights” because they don’t have much iron or magnesium. Mafic minerals, on the other hand, are the heavy hitters. Minerals like olivine and pyroxene are packed with iron and magnesium, giving them a dark color. You’ll also find accessory minerals in the mix, like zircon, but they’re usually just present in small amounts.

Classifying the Fiery Family

Geologists like to classify igneous rocks based on their silica content – that’s the amount of SiO2 they contain. It’s like sorting them into different categories based on how much “silica spice” they have!

• Felsic rocks are the silica-rich ones, with over 63% SiO2. Granite and rhyolite are good examples. They’re usually light-colored and not too dense.
• Intermediate rocks have a silica content between 52% and 63%. Diorite and andesite fall into this category.
• Mafic rocks are lower in silica, with 45% to 52% SiO2. Basalt and gabbro are common examples. They’re darker and denser than felsic rocks.
• Ultramafic rocks are the real silica-lightweights, with less than 45% SiO2. These rocks are made up of over 90% mafic minerals, like dunite.

The Great Divide: Intrusive vs. Extrusive

Igneous rocks also get sorted based on where they cool down. It’s like the difference between baking a cake in the oven (intrusive) and letting it cool on the counter (extrusive).

• Intrusive rocks, also called plutonic rocks, form when magma cools slowly inside the Earth. This slow cooling gives crystals time to grow nice and big, resulting in a coarse-grained texture. Granite is a classic example.
• Extrusive rocks, or volcanic rocks, form when lava cools quickly on the Earth’s surface. This rapid cooling doesn’t give crystals much time to form, so you end up with a fine-grained or even glassy texture. Basalt is a common extrusive rock.

A Few Rock Stars: Examples of Igneous Rocks

Let’s take a closer look at some famous igneous rocks:

• Granite: This is the rock you see in fancy countertops and monuments. It’s a felsic intrusive rock made of quartz, feldspar, and a bit of dark minerals like biotite.
• Basalt: This dark, fine-grained rock makes up most of the ocean floor. It’s a mafic extrusive rock composed of plagioclase and pyroxene.
• Diorite: Think of this as granite’s less flashy cousin. It’s an intermediate intrusive rock made mostly of plagioclase feldspar, amphibole, and pyroxene.
• Gabbro: This is the mafic version of diorite. It’s a mafic intrusive rock made of pyroxene, olivine, and plagioclase feldspar.
• Rhyolite: This is basically the extrusive version of granite. It’s a felsic extrusive rock with a fine-grained texture.
• Obsidian: This is volcanic glass! It cools so quickly that crystals don’t even have time to form.
• Dunite: This rock is almost entirely made of olivine. It’s an ultramafic plutonic rock.

Texture Talk: How Igneous Rocks Look and Feel

The texture of an igneous rock tells you a lot about how it formed. It’s all about the size and arrangement of the crystals.

• Phaneritic: Big crystals you can see with the naked eye – that’s a coarse-grained texture. Think intrusive rocks.
• Aphanitic: Tiny crystals you can’t see without a microscope – that’s a fine-grained texture. Think extrusive rocks.
• Porphyritic: Big crystals floating in a sea of tiny crystals. This tells you the rock had a two-stage cooling history.
• Pegmatitic: Super huge crystals! These form when magma cools incredibly slowly.
• Glassy: No crystals at all! This happens when lava cools so fast that crystals don’t have a chance to form.
• Vesicular: Full of holes! These are gas bubbles that got trapped as the rock cooled. Pumice is a good example.

So, there you have it – a peek into the fascinating world of igneous rocks. By understanding their composition and formation, we can unlock secrets about the Earth’s history and the fiery processes that shaped our planet.