What happens to magma viscosity as temperature drops?
Viscosity increases.
How does crystallization affect magma viscosity?
Crystallization increases viscosity.
What happens to magma composition as it rises toward the surface?
It becomes more silicic (polymerized), increasing viscosity.
What happens to gases in magma as pressure drops?
Gases exsolve, concentrating bubbles and increasing viscosity.
What are the main forms of lava flows?
Pahoehoe, ‘A’a, coulées, domes, plugs, and spines.
What two factors are most important in lava rheology?
Viscosity and yield strength.
What controls flow direction of lava?
Topography.
What are typical effusion rates measured in?
Cubic meters per second (m³/s).
Describe pahoehoe surface morphology.
Shiny, smooth, glassy, and advances by “toes.”
Typical thickness of pahoehoe flows?
1–3 meters.
Describe the surface of an ‘A’a flow.
Rough, clinkery, brecciated with sharp blocks.
Typical thickness of ‘A’a flows?
3–20 meters.
How do blocky flows differ from ‘A’a?
Larger surface blocks (meters vs. cm–dm) and thicker (>20 m).
What lava compositions form blocky flows?
Andesites, dacites, and some rhyolites.
Where do pillow lavas form?
In subaqueous or subglacial environments.
What creates the “pillow” shape?
Rapid quenching of a lava skin forming a balloon around liquid lava.
At what effusion rates do pillows form?
Low effusion rates.
Why do rhyolitic coulées have very high aspect ratios?
Their high viscosities prevent horizontal flow.
What volcanic landform represents extremely viscous lavas?
Lava domes and spines.
Define viscosity.
Internal resistance of a fluid to flow under stress.
List four main factors affecting magma viscosity.
Temperature, pressure, crystals, bubbles, and silica content.
Which has higher viscosity: basalt or rhyolite?
Rhyolite.
What are the three main mechanisms of cooling lava flows?
Conduction, convection, and radiation.
What is the Reynolds number used for?
To determine laminar vs. turbulent flow in Newtonian fluids.
