Question

Argon is produced in Earth’s crust via?

• A. Decay of K-40
• B. Volcanoes
• C. Photosynthesis
• D. Cosmic rays

Answer 1

Answer:

A. Decay of K-40

Origin of Argon in Earth's Crust

• Argon, a noble gas, is primarily formed in the Earth's crust through the radioactive decay of Potassium-40 (K-40).
• Potassium-40 is a naturally occurring radioactive isotope of potassium, which is abundant in the Earth's crust and mantle.
• The decay process of K-40 can occur in two main ways:
  1. Electron capture (about 89.3% of decays) leading to the formation of Argon-40 (Ar-40).
  2. Beta decay (about 10.7% of decays) leading to the formation of Calcium-48 (Ca-48).
• The Argon-40 produced gets trapped within the rock matrix where the K-40 decay occurs.
• Over geological timescales, this process has led to the accumulation of significant amounts of argon within the Earth's crust.

Exam-Related Facts:

• Argon constitutes approximately 0.934% of the Earth's atmosphere by volume, making it the third most abundant gas after nitrogen and oxygen.
• While most atmospheric argon is radiogenic (from K-40 decay), a small fraction is primordial (present since Earth's formation).
• The ratio of Argon-40 to Potassium-40 in rocks is used in geochronology to determine the age of rocks, a technique known as Potassium-Argon dating. This method is crucial for understanding Earth's geological history.
• Potassium-40 has a long half-life of approximately 1.25 billion years, which is suitable for dating ancient rocks.