Comprehensive Guide to Magmatism and Magma Composition

Chemical Composition of Magma

• The composition of "maama" (magma) consists of several specific elements and their respective mass percentages:

  • Oxygen: $46.6\,\%$

  • Silicon: $27.7\,\%$

  • Aluminum: $8.1\,\%$

  • Iron: $5.0\,\%$

  • Calcium: $3.6\,\%$

  • Sodium: $2.8\,\%$

  • Potassium: $2.6\,\%$

  • Magnesium: $2.1\,\%$

Fundamental Concepts of Magma and Lava

• Definition of Magma: Magma is a substance composed of semi-liquid hot molten rocks located beneath the Earth's surface. It is specifically found in the melted mantle rock and the oceanic plate.

• Distinction Between Magma and Lava:

  • Both substances are molten rocks.

  • Magma: Located in the underground magma chamber of a volcano.

  • Lava: Located on the Earth's surface once a volcanic eruption has occurred.

• Definition of Magmatism: This is a process occurring beneath the Earth's crust involving the formation and movement of magma.

• Location of Formation: Magmatic processes take place in the lower part of the Earth’s crust and the upper portion of the mantle, a region known as the asthenosphere.

Thermal Processes and Earth's Heat Budget

• Mantle Convection: This is the physical movement of the mantle as heat transfers from the core toward the crust. The temperature of the mantle is variable, depending on its proximity to the crust (cooler) or the core boundary (hotter).

• Earth’s Heat Budget:

  • It serves as the primary driver for most geological processes on Earth.

  • It measures the flow of thermal energy originating from the core, passing through the mantle, and moving up to the atmosphere.

  • This flow is primarily facilitated by mantle convection.

Mechanics of Magmatic Ascent

• Magma rises toward the surface due to two primary factors:

  • Density Contrast: Magma is less dense than the surrounding "country rock." The speed at which magma rises is directly proportional to the magnitude of the density contrast between the magma and the surrounding rock (greater contrast leads to faster rise).

  • Viscosity: This is the measure of a fluid's resistance to flow. Magmas with low viscosity flow more easily than magmas with high viscosity.

Factors Affecting the Viscosity of Magma

• The internal resistance to flow in magma is determined by three specific factors:

  • Temperature: An increase in temperature leads to a decrease in viscosity ($\text{High Temperature} \rightarrow \text{Low Viscosity}$).

  • Silica Content ($SiO_2$): An increase in silica content leads to an increase in viscosity ($\text{High Silica Content} \rightarrow \text{High Viscosity}$).

  • Dissolved Water ($H_2O$): An increase in dissolved water leads to a decrease in viscosity ($\text{High Dissolved Water} \rightarrow \text{Low Viscosity}$).

Conditions for Mantle Melting

• Melting in the mantle occurs under three specific conditions:

1. Increase in Temperature (Heat Transfer Melting):

   • Conduction: Heat is transferred from hotter molten rocks to the Earth’s cold crust.

   • As magma rises, it is often sufficiently hot to melt the country rock it comes into contact with.

   • This occurs at convergent boundaries, where tectonic plates crash together. For example, wet partial melting of the oceanic crust creates magma at subduction zones near trenches.

2. Decrease in Pressure (Decompression Melting):

   • Mantle rocks typically remain solid under high pressure. However, during convection, these rocks may move upward to shallower levels.

   • The reduction in pressure triggers melting. This process occurs at the Mid-Ocean Ridge, an underwater mountain system where upwelling mantle rocks undergo decompression.

3. Addition of Volatiles (Flux Melting):

   • When water ($H_2O$) or carbon dioxide ($CO_2$) is added to hot rocks, the melting points of the minerals within those rocks decrease.

   • If a rock is already near its melting point, the addition of volatiles can trigger partial melting.

   • This occurs at subduction zones where volatiles are released from the subducting plate into the overlying asthenosphere.

Practical Illustrations and Analogies

• Sugar Crystal Analogy:

  • Sugar Crystals alone: Solid.

  • Sugar Crystals + Heat: Begins the process of melting.

  • Sugar Crystals + Heat + Water: Results in a fully melted state, analogous to the effect of volatiles and temperature in magmatism.

Questions & Discussion

• Which element has the highest amount in magma? Oxygen ($46.6\,\%$).

• Which element has the lowest amount according to the list? Magnesium ($2.1\,\%$).

• What are the top two compositions of magma? Oxygen and Silicon (combined $\approx 74.3\,\%$).

• What process describes the formation of intrusive igneous rocks or plutonic rocks from the solidification of magma? Magmatism/Plutonism.

• What process describes the activity or motion of magma? Magmatism.

• What causes the transfer of heat from core to crust? Mantle convection.

• What is the molten rock on the Earth's surface? Lava.

• In which boundary does decompression melting occur? Divergent boundaries (specifically Mid-Ocean Ridges).

• What are the two factors that make magma rise to the surface? Density contrast and Viscosity.

• What are the three conditions that allow magma to form? Increase in temperature, decrease in pressure, and addition of volatiles.

• What are the three factors that affect viscosity? Temperature, Silica content ($SiO_2$), and dissolved water ($H_2O$).