After a volcano erupts, new types of rock can form. Which of the following best explains how this happens?

a) The intense heat from the volcano melts existing rock, which then cools and solidifies to form new rock.
b) The pressure from the volcanic eruption compresses existing rock, transforming it into new types of rock.
c) The volcanic ash and debris settle and harden over time, forming new layers of sedimentary rock.
d) The volcanic gases react with minerals in the surrounding rock, causing chemical changes that result in new rock formations.