Question

Which one of the following rock combinations is characteristic of volcanic arcs?

• A. Basalt-andesite-dacite
• B. Basalt-trachyte-ryolite
• C. Andesite-syenite-dacite
• D. Trachybasalt-basalic andesite-andesite

Hint:

Basalt, andesite, and dacite are the characteristic rocks of volcanic arcs due to their formation from subduction zone magmatism.

Answer 1

The Correct Option is A

Step 1: Understanding volcanic arcs.
Volcanic arcs are formed at convergent plate boundaries where one tectonic plate is forced under another, leading to magma formation. The characteristic rocks of these arcs are often basalt, andesite, and dacite. These rocks form as magma rises and cools in the crust.

Step 2: Characteristics of basalt.
Basalt is the most common rock found in oceanic crust and is associated with volcanic activity, especially at mid-ocean ridges and volcanic arcs. It is rich in iron and magnesium.

Step 3: Characteristics of andesite.
Andesite is formed from the partial melting of basalt and is the most common rock type in volcanic arcs. It has intermediate silica content and is formed in subduction zones.

Step 4: Characteristics of dacite.
Dacite is rich in silica and forms from more evolved magma. It is often found in volcanic arcs, where it represents the final stages of magma differentiation.

Step 5: Analysis of each option.
- (A) Basalt-andesite-dacite: This combination is typical of volcanic arcs, where basalt forms the initial stage of magma, and andesite and dacite form through differentiation.
- (B) Basalt-trachyte-ryolite: This combination is more typical of continental volcanic activity but not specifically volcanic arcs.
- (C) Andesite-syenite-dacite: Syenite is an intrusive rock and is not typically associated with volcanic arcs.
- (D) Trachybasalt-basalic andesite-andesite: While basalt and andesite are common, trachybasalt is less common in volcanic arcs.

Step 6: Conclusion.
The correct rock combination typical of volcanic arcs is basalt-andesite-dacite, as these are the primary rocks formed in subduction zone volcanism.
\[ \boxed{\text{Basalt-andesite-dacite}} \]