Question

What is the percentage of silica in Granitic magma? A. less than \(45 \%\) B. \(45-52 \%\) C. \(53-65 \%\) D. greater than \(65 \%\)

Short answer

D. Greater than 65%

Step-by-step solution

Understand Granitic Magma Composition

Granitic magma is primarily composed of silica, which is a major component in rocks that are rich in quartz and feldspar. This type of magma is known for having a high silica content compared to other magmas.

Determine Silica Percentage Range for Granitic Magma

Granitic magma typically contains a high percentage of silica. The range for silica content in granitic magma is generally greater than 65%.

Match Percentage Range with Provided Options

Compare the known range of silica in granitic magma (>65%) with the options provided: A. less than 45%, B. 45-52%, C. 53-65%, D. greater than 65%. The correct match is D: Greater than 65%.

Key concepts

Silica Content

Silica content plays a critical role in determining the characteristics of different types of magma, including granitic magma. Silica, mainly composed of silicon and oxygen, is a significant component in granitic magma, which is known for having the highest silica content when compared to other magma types. In general, granitic magma contains more than 65% silica, which differentiates it from other magmas such as basaltic or andesitic magmas. High silica content leads to thicker, more viscous magma, which influences the magma's flow and eruption style. Understanding this property helps geologists predict volcanic behaviors and the types of rock that will form when the magma cools and solidifies.

Magma Composition

The composition of magma is primarily defined by the content of silica, along with other elements. Granitic magma is rich in silica along with elements like aluminum, potassium, sodium, and calcium.

• High silica – influences magma's viscosity and color
• Presence of quartz and feldspar – important minerals in granitic magma

The abundance of these elements affects the magma's physical properties, like its viscosity and melting point. Granitic magma, being high in silica, tends to be more viscous and cooler compared to others such as basaltic magma. This property not only affects how magma flows but also the type of rock it eventually forms. The minerals that crystallize as the magma cools can greatly influence the texture and mineralogy of the resulting igneous rock.

Quartz

Quartz is a key mineral found in granitic magma, which significantly affects its characteristics. Quartz is made up of silicon and oxygen, forming a crystalline structure that is highly resistant to weathering and erosion. This mineral is one of the most abundant in the Earth's crust, and its presence in granitic magma contributes to the rock's overall silica content.
The structure of quartz crystals makes it a durable mineral, which is why granitic rocks are typically hard and resistant to wear. Quartz also helps in determining the hardness and chemical behavior of the granitic rocks formed from this type of magma. When granitic magma cools slowly, large quartz crystals can form, contributing to the grainy texture of rocks like granite. Studying quartz in granitic magma provides insights into the cooling history and conditions during formation.

Feldspar

Feldspar is another major component of granitic magma, significantly influencing its final rock form. This mineral group consists of potassium feldspar (orthoclase) and plagioclase feldspar, both contributing to the overall silica content. Feldspar is the most abundant mineral in the Earth's crust, and in granitic magma, it plays a crucial role in the texture and composition of the resulting rock.

• Potassium feldspar – often gives a pink or tan hue to granite
• Plagioclase feldspar – can vary from white to grey

The crystallization of feldspar as granitic magma cools contributes to the coarse-grained texture of igneous rocks such as granite. The presence of feldspar also affects the mineralogical composition of granitic rocks; it's often used in identifying and classifying these rocks in geological studies. Understanding feldspar's role in granitic magma helps in assessing the rock's durability and how it might be used in construction or other applications.