Final answer:
Igneous rocks with an andesitic composition are typically formed at convergent plate boundaries and are characterized by a medium amount of silica. They have a coarse-grained texture comprised mostly of feldspar and quartz, with some hornblende and biotite. Areas like the Central Andes showcase these rocks formed during the Neogene period.
Step-by-step explanation:
Igneous rocks with an andesitic composition are igneous rocks that have formed from magma with a specific chemical content, typically characterized by a medium amount of silica and often found in volcanic arc settings like the Andes Mountains. These rocks are typically formed at convergent plate boundaries, where an oceanic plate is subducted under a continental plate, leading to magma formation that rises and solidifies to form rocks with an andesitic composition. This magma usually cools slowly beneath the surface, resulting in a coarse-grained texture composed mostly of feldspar and quartz, with minor amounts of hornblende and biotite - a composition that helps define andesitic igneous rocks. Furthermore, the Central Andes Mountains are a prime example where such rocks have formed during the Neogene period between 23 and 2.5 million years ago, including both intrusive and extrusive igneous rocks. An andesitic composition in igneous rocks indicates a particular origin and mineral composition, which is crucial for understanding the geological history and processes that formed them.