Basalt vs Andesite: What Are They, and What's the Difference?

Volcanic eruptions produce some of the most gorgeous and interesting rocks known to humankind.

Depending on the mineral makeup, the cooled lava provides us with solid material for building, construction and other practical uses.

In this article, you’ll learn about the similarities and differences between basalt and andesite.

Basalt vs Andesite: Summary

Basalt and andesite are both igneous rocks that come from volcanoes all over the world. In fact, basalt is one of the most common rocks you’ll find post-eruption. However, andesite occurs in special circumstances because of a separation process from basalt.

So, identifying the two and knowing the differences requires a keen eye.

This is because they are incredibly similar.

However, basalt tends to be glassier and more porous while andesite will have other minerals, like zeolite.

What Is Basalt?

One of the most frequent types of igneous rock on Earth is basalt.

So, you can find basalt in any place where there is or was a volcano.

They riddle coastlines, islands and the crust of land masses, among many others.

Basalt is often what people see on the ocean floor as well as huge plateaus on land.

Characteristics

They usually appear black, green or even brown.

It’s iron and magnesium rich, often comprising other minerals like olivine, pyroxene, augite, and plagioclase.

Because of this, the rock has an overall composition heavy-laden with silica, about 45% to 52%.

These rocks, sitting at 6 on the Mohs Scale of Hardness, are often compact, fine-grained and glassy.

It’s a product of volcanic eruptions and, because of this, there are holes left by gas bubbles.

This gives it a texture that’s coarse and porous.

Types of Basalt

There are many types of basalt available and it depends on the makeup of the minerals of the volcano from whence it came.

These variations allow for different purposes, uses and functions.

Indeed, basalt is a common material used in construction, carbon dioxide detoxification and thermal insulation.

Tholeiitic Basalt: Silica rich and poor sodium content; this is most of what comprises the ocean floor

High and Low Titanium Basalt: As the name suggests, titanium is the main component in this type of basalt

Mid-Ocean Ridge Basalt (MORB): A tholeiitic basalt that erupted only at ocean ridges and is low in elements

High-Alumina Basalt: Silica is either under-saturated or oversaturated with more than 17% alumina. It’s the intermediary between theoleiitic and alkali basalts

Alkali Basalt: Rich in sodium with poor amounts of silica may result in feldspathoids, phlogopite and/or alkali feldspar.

What Is Andesite?

Andesite is an igneous rock, getting its name from the Andes Mountain in South America.

While it is very much found on the west coast of the Andes, it’s also among the Cordillera Mountains in North and Central America as well as in several locations throughout New Zealand.

You can also find it in Popocatépel, Mexico, Mount Shasta, Mount Hood, Mount Adams, Soufriere Saint Vincent in the Caribbean, Krakatoa in Indonesia and Mount Fuji, Japan.

Characteristics

It has a fair amount of silica and has a gray or bluish-gray color, which can appear porphrytic or fine-grained.

Actually, it’s the volcanic equivalent to diorite.

Minerals such as andesine, oligoclase, magnetite, pyroxene and biotite are usually present.

Sometimes, the presence of amygdaloidal andesite means that gas bubbles from the volcanic eruption leave voids that later fill with zeolite.

They can also comprise almandine, hornblende, plagioclase, apatite, ilmenite and/or orthopyroxene.

Andesite sits at 7 on the Mohs Scale of Hardness.

How Andesite Forms

While andesite comes from cooled lava flows, it occurs in convergent plate cages.

For this to happen, basaltic magma crystallizes specific minerals taken out of the melt.

The first of these is olivine and amphiboles, which then separate from the magma and form mafic cumulates.

As this process continues there’s no residual basaltic composition but the silica enriches.

This is how andesite comes to exist. Some of the processes in its formation are:

Mafic parent magma that has fractional crystallization

Crustal material that partially melts

Mixed magma in a reservoir

Uses

Scientists use ancient andesites to map subduction zones in the ocean because these volcanoes form on ocean or continental crusts above these places.

But it has practical uses as well like tiles, cladding and other construction purposes.

It resists slipping and makes an excellent filling material.

How Are Basalt and Andesite Similar?

Because basalt and andesite form in the same areas, they are both volcanic rocks.

Therefore, they have a similar mineral composition and color.

Both contain things like olivine, some variation of feldspar and pyroxene.

Basalt and andesite also have high concentrations of silica.

How Are Basalt and Andesite Different?

What makes basalt different from andesite is the fact that andesite has a higher silica content compared to basalt.

Also, the color of andesite will be lighter and more muted than basalt.

Basalt is more glassy and porous than andesite.

Basalt is the precursor to andesite.

The cooling process removes minerals from the melt to form andesite, in which case the holes contain zeolite (or some other mineral).

Basalt will have defined pores and be very dark.

Why Do People Confuse Basalt for Andesite?

Even though there are clear differences between basalt and andesite, it’s easy to mistake one for the other.

This often happens when you come across a dark rock that appears like basalt.

But, the smoother texture will render it andesite.

The best test you can do if you are unsure is to check hardness.

Since andesite is harder than basalt, you should be able to scratch the surface of basalt with andesite; there will not be any effect if you try to scratch andesite with basalt.

Conclusion

Basalt and andesite are both gorgeous and amazing rocks naturally produced by the earth upon the cooling of lava.

While both are incredibly similar, it’s easy to confuse them.

But with a little practice and discipline, you’ll develop a keen eye to be able to tell the difference quickly.