Geologists observe two rocks that were originally formed from lava. Rock A is dark-colored and Rock B is light-colored. Which rock formed from lava with the higher viscosity?

Geologists observe two rocks that were originally formed from lava. Rock A is dark-colored and Rock B is light-colored. Which rock formed from lava with the higher viscosity?

• A. Rock A
• B. Rock B
• C. Both
• D. Neither

Answer: (B)

Rock B is the higher-viscosity lava origin. Lava viscosity tracks silica content and temperature: felsic, light-colored magma has more silica and is more viscous, while mafic, dark-colored magma has less silica and flows more easily. Since Rock B is light-colored, it reflects a felsic composition and higher viscosity, whereas Rock A is dark-colored and corresponds to a lower-viscosity, mafic magma.

Rock B is the higher-viscosity lava origin. Lava viscosity tracks silica content and temperature: felsic, light-colored magma has more silica and is more viscous, while mafic, dark-colored magma has less silica and flows more easily. Since Rock B is light-colored, it reflects a felsic composition and higher viscosity, whereas Rock A is dark-colored and corresponds to a lower-viscosity, mafic magma.