Earth, Our Environment - Class Notes
Chapter 4, The Cornerstones of Geology: Rocks - 08
Magma Movement and Density

Magma Movement

Once Earth's heat melts igneous material, the driving force behind magma movement is gravity. The density of magma is usually lower than that of the surrounding rock. The density contrast gives the magma the ability to rise through the crust.

Calculating Density:

Density = Mass ÷ Volume

When you calculate, be careful that the physical units of the numbers are consistent.

The units of density are grams per cubic cm (g/cm^3) or kilograms per cubic meter (kg/m^3).


Density (g/cm3)

Density (kg/m3)




Average Earth



Typical Continent



Typical Mantle



The shallow crust is brittle and often contains cracks. Magma injected into these cracks forms dikes and sills. These features are usually small and cool quickly.

The lower crust is less brittle and does not contain cracks, so dikes and sills are features formed primarily in the upper crust.

To move through the lower crust, magma either rises as a diapir, pushing the crust out of its way. Or it assimilates the surrounding crust by melting its way upward.

[Study Figure 4.8 on page 81 of the text]

Time-Dependent Material Behavior

The behavior of many materials depends on the time scale that we study the behavior and the temperature and pressure conditions.

Glaciers are made of ice, over the years they flow like an "ice river"

The mantle rocks behave like a solid on short time scales, but over the long times of plate tectonic processes, the mantle flows.

Thus, the mantle is not a simple convecting fluid on short time scales. If we could reach into Earth and bring a piece of the mantle to the surface, it would be solid.

Our common experience is not the only reference frame important in natural processes, and we must be careful not to let our intuition blind us in this regard.

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Prepared by: Charles J. Ammon
February 1997