Question

Explain why silica-rich magma and magma that contains gas erupt explosively.

Answer 1

Silica-rich magma traps water and gas bubbles together which causes huge amount of pressure in the magma.
Resources:
"Silica-rich magma erupts by creating a high viscous magma, which result into releasing high pressurized gases, that cause eruption."

Answer 2

silica-rich magma traps water and gas bubbles together which causes huge amount of pressure in the magma.

I found this on answers.com, "Silica-rich magma erupts by creating a high viscous magma, which result into releasing high pressurized gases, that cause eruption."