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: Setting of a volcanic arc.
A volcanic arc sits above a subduction zone, where one plate sinks below another. The melting there feeds the volcanoes on top.

Step 2: The rock trend to expect.
Arc magma is the calc alkaline kind. As it cools and changes, it moves from low silica to high silica rocks.

Step 3: Order the right rocks.
The normal arc sequence runs basalt first, then andesite, then dacite. Silica rises step by step along this path.

Step 4: Knock out the wrong sets.
Trachyte and rhyolite point to within plate or continental settings, not arcs. Syenite is a coarse intrusive rock and does not belong in a volcanic arc list. Trachybasalt is also off the typical arc trend.

Step 5: Final choice.
So the set that fits a volcanic arc is basalt, andesite and dacite.
\[ \boxed{\text{Basalt-andesite-dacite}} \]