Essentials of Geology - Lecture Notes

The Mantle and Magma Formation

Geothermal Gradient

• Geothermal Gradient: The rate of temperature increase with depth.
• Figure 4.17: Illustrates the geothermal gradient for the crust and upper mantle, showing temperature rises with depth.
• The gradient lies in an area suggesting mantle rock is typically solid.
• Slight condition changes at the upper asthenosphere can cause partial melting of mantle rock.

Composition of Earth's Layers

• Earth's crust and mantle are primarily solid, based on earthquake wave studies.
• Outer core is fluid due to its iron-rich composition.
• Magma mainly originates in Earth's upper mantle.

Locations Where Magma is Generated

• Divergent Plate Boundaries: Most magma originates here due to seafloor spreading.
• Subduction Zones: Lesser but significant magma generation.
• Heating of Crustal Rocks: Can cause sufficient melting to generate magma.

Generating Magma from Solid Rock

Temperature and Pressure

• Geothermal Gradient: Average temperature increase is about 25°C per kilometer in the upper crust.
• Under normal conditions, mantle remains solid as its melting point is higher than geothermal gradient.

Decompression Melting

• Pressure Influence: Pressure increases melting temperature; reduction in pressure can trigger melting.
• Decompression melting occurs when hot solid mantle rock ascends to lower pressure regions.
• Oceanic Ridges: Tensional forces promote magma generation here.
• Mantle Plumes: Their ascent can lead to hotspot volcanism.

Role of Water and Volatiles

• Water lowers rock melting temperature, similar to salt on ice.
• Mainly occurs at convergent boundaries where oceanic lithosphere descends.
• Water released from subducting plates can induce mantle melting.

Melting of Crustal Rocks

• Basaltic Magma: Less dense than surrounding materials, rises buoyantly.
• In continental settings, can generate secondary melts of silica-rich felsic magmas.
• Felsic magmas often produce explosive eruptions at convergent boundaries.
• Continental Collisions: Thickening of crust can also cause partial melting.

Summary of Magma Generation

• Decompression Melting: Due to pressure decrease as magma ascends.
• Water Introduction: Lowers melting point of mantle rock.
• Heating of Crustal Rocks: Above their melting temperatures.

Concept Check Questions

1. Define the geothermal gradient and compare it with mantle rock melting temperatures.
2. Explain decompression melting.
3. Discuss the roles of water and volatiles in magma formation.
4. Identify two tectonic settings where magma generation occurs.