Difference Between Basalt and Granite

Basalt and granite are two common types of igneous rocks, but despite sharing a similar composition, they have many differences. In this essay, we will discuss the differences between basalt and granite in terms of their origins, appearance, texture, mineral composition, and uses.

What is Basalt?

Basalt is an igneous, mafic, and volcanic rock which is produced by lava flows in many different volcano types. It contains mainly volcanic glass, pyroxene and plagioclase feldspar and is fine-grained. Basalt is one of the most common types of rock on Earth as well as other planetary bodies in the Solar System.

• Composition of Basalt − Since basalt is mafic, it contains minerals with significant iron and magnesium. The minerals making up basalt include pyroxene, plagioclase feldspar, amphibole, and some olivine. Volcanic glasses are also present. Some of the minerals composing basalt, such as olivine, are very sensitive to chemical weathering at Earth’s surface because of the presence of water.

• Formation of Basalt − Basalt forms at the surface where it will harden from lava. Places where basalt is abundant include mid oceanic ridges, hot spots, and rift basins. Because it forms at the surface, basalt will cool relatively quickly within a few days to a few months and, as a result, the minerals grains in basalt are fine grained and difficult to see with the unaided eye.

• Mid-oceanic ridges − The mid oceanic ridges are a type of boundary between two tectonic plates composed of oceanic crust. It is at the mid oceanic ridges where new oceanic crust is formed. The upper 1-2 km of the oceanic crust is basalt. The basalt that forms at the mid oceanic ridges has a specific composition that makes it distinctive, as a result, basalt deposits that form at the mid oceanic ridges are called MORB (Mid-Oceanic-Ridge-Basalt) deposits or MORBs.

• Hotspots − Hotspots are regions near the base of the crust where a plume of hot mantle material causes volcanic activity at the surface. When hotspots occur beneath oceanic crust, the resulting molten rock will often produce basaltic lavas. Examples of basalt deposits that form at hotspots include the basalt bedrock of the Hawaiian Islands. The Martian volcanoes of Tharsis, Olympus Mons, Ascreaus Mons, and Arsia Mons, are likely examples of hotspot volcanism on a much larger scale than terrestrial cases.

• Rift basins − Basalt is also commonly formed at continental rifts. Mantle plumes can form beneath the continental crust causing extension of the lithosphere and the production of significant melt in the crust. If the melt extrudes onto the surface, this can lead to extensive basalt flows that form what are called flood basalts where hundreds of square kilometers of basaltic lava can be produced.

What is Granite?

Granite is an intrusive igneous rock with a felsic composition. Granite makes up the cores of continents and much of the bulk of major mountain ranges around the world. Furthermore, many of the continental rock formations are ultimately derived from granite that either was destroyed by weathering and erosion or metamorphosed. Granite is also one of the most recognizable rocks to most people.

Composition of Granite

Granite is classified as a felsic rock, meaning that it has significant amounts of feldspar and quartz. The primary minerals making up granite include quartz, feldspars, micas, and sometimes pyroxene, but mostly quartz and feldspar. Because granite often has excess potassium from alkali feldspars, granite is slightly radioactive because radioactive potassium (40K) is relatively common.

Not all rocks that resemble granite are true granites. These rocks that resemble granite physically, chemically, and mineralogically but that are not in fact granite are referred to as granitoids.

Formation of Granite

Granite is considered a plutonic rock since it forms deep beneath the surface. Plutonic rocks are contrasted with volcanic rocks which form at the surface. Granite tends to form at continental subduction zones where oceanic crust is subducting beneath continental crust. It will also form at continental collisions zones.

During the process of plate subduction or continental collision, large chambers of magma will form within the crust which will harden into masses of rock called plutons. As tectonic plates collide, they are compressed, and the plutons are uplifted and exhumed at the surface. Over time, the surrounding rock will erode away leaving the plutons as enormous masses of exposed granite. The granite peaks of many of the world’s major mountain ranges are examples of plutons around which surface rock has eroded to expose these ancient subterranean stone giants.

Similarities between Basalt and Granite

Basalt and granite are both silicate rocks which contain common minerals such as feldspar and pyroxene. They also are both very common rocks on Earth. Furthermore, they are both igneous, meaning that they form from direct crystallization of molten rock.

Differences basalt and granite

• Origins − Basalt is formed from the rapid cooling and solidification of lava that has erupted from a volcano. As a result, it is most commonly found in areas of recent volcanic activity, such as the Hawaiian Islands or Iceland. Granite, on the other hand, is formed from the slow cooling and solidification of magma beneath the Earth's surface. It is typically found in areas where there has been extensive tectonic activity, such as mountain ranges like the Rocky Mountains or the Himalayas.

• Appearance − Basalt and granite have distinct differences in their appearance. Basalt typically has a dark color, ranging from dark gray to black, and has a fine-grained texture. It often appears shiny due to the presence of small crystals of feldspar or pyroxene. Granite, on the other hand, can vary widely in color, from light gray to pink, red, or even blue. It has a coarse-grained texture, with visible crystals of quartz, feldspar, and mica.

• Texture − Basalt has a fine-grained texture due to the rapid cooling of lava. This means that the individual crystals that make up the rock are too small to see with the naked eye. Granite, on the other hand, has a coarse-grained texture due to the slow cooling of magma. This means that the individual crystals that make up the rock are visible to the naked eye, giving it a distinctive appearance

• Mineral Composition − The mineral composition of basalt and granite is also different. Basalt is composed mainly of mafic minerals, such as pyroxene and olivine, and plagioclase feldspar. Granite, on the other hand, is composed mainly of felsic minerals, such as quartz, orthoclase feldspar, and mica. These differences in mineral composition contribute to the different physical properties of basalt and granite.

• Uses − Basalt and granite are both used in a variety of applications due to their different physical properties. Basalt is often used as a building material due to its durability and strength. It is also used as an aggregate in concrete and road construction. Granite is often used as a decorative stone due to its unique appearance and durability. It is commonly used as a building material for countertops, flooring, and monuments

The following table highlights the major differences between Basalt and Granite −

Conclusion

In conclusion, while basalt and granite share similarities in their composition, they have many differences in terms of their origins, appearance, texture, mineral composition, and uses. Understanding these differences is important in order to make informed decisions about their use in various applications.

Vineet Nanda

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Updated on: 21-Aug-2023

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