Question

A volcanic rock consisting of alkali feldspar (70%), sodic plagioclase (10%) and nepheline (20%) is named as

• A. Phonolite
• B. Tephrite
• C. Trachyte
• D. Andesite

Hint:

Phonolite is a volcanic rock rich in alkali feldspar and nepheline, often found in alkaline volcanic regions.

Answer 1

The Correct Option is A

Step 1: Understanding the composition of volcanic rocks.
Phonolite is a volcanic rock with a composition rich in alkali feldspar and nepheline. It typically contains more than 60% alkali feldspar and a substantial amount of nepheline.

Step 2: Analyzing the options.
(A) Phonolite: Correct — Phonolite is characterized by a composition of alkali feldspar and nepheline, as described.
(B) Tephrite: Tephrite is a basaltic rock that contains a higher proportion of plagioclase than phonolite.
(C) Trachyte: Trachyte is similar to phonolite but contains less nepheline.
(D) Andesite: Andesite is an intermediate volcanic rock, not matching the given composition.

Step 3: Conclusion.
The correct answer is (A) Phonolite.