Metamorphic Rock Is Changed Into Igneous Rock By

Metamorphic Rock Transformed: The Journey to Igneous Rock

Metamorphic rocks, formed under immense pressure and heat deep within the Earth's crust, represent a fascinating stage in the rock cycle. But their journey doesn't end there. These rocks, with their altered mineral structures and textures, can be transformed once more, this time into igneous rocks. This transformation is a testament to the dynamic processes shaping our planet, a cycle of creation and destruction driven by geological forces. Understanding how metamorphic rock changes into igneous rock requires examining the crucial processes involved, the specific conditions needed, and the resulting igneous rock types.

The Path to Igneous Transformation: Melting and Magma Formation

The key to understanding the metamorphosis of metamorphic rock into igneous rock lies in the concept of melting. Metamorphic rocks, while incredibly resilient, are not impervious to the extreme conditions found deep within the Earth. When subjected to sufficiently high temperatures and, often, changes in pressure, they begin to melt. This melting process isn't uniform; it often occurs partially, creating a mixture of molten rock (magma) and solid rock fragments.

The Role of Temperature

Temperature is the primary driving force behind melting. The melting point of a rock depends on its mineral composition and the pressure it experiences. However, generally, temperatures exceeding 700-1200°C (1300-2200°F) are needed to initiate significant melting of most metamorphic rocks. This intense heat can be generated by several processes:

• Subduction Zones: At convergent plate boundaries, one tectonic plate slides beneath another, dragging metamorphic rocks into the Earth's mantle. The immense friction and heat generated during subduction create the necessary temperatures for melting. The introduction of water from the subducting slab can also lower the melting point of the metamorphic rocks, facilitating the process.

• Mantle Plumes: These upwellings of hot mantle material can inject intense heat into the crust, causing melting of overlying metamorphic rocks. The heat from these plumes can significantly increase regional temperatures, leading to widespread melting.

• Magma Intrusions: The intrusion of magma from deeper within the Earth into existing metamorphic rock formations can also cause melting. The heat conducted from the intruded magma can be sufficient to melt the surrounding metamorphic rock, creating a hybrid magma body.

Pressure and its Influence

While temperature is the primary driver, pressure also plays a significant role. Pressure, particularly confining pressure (pressure from all sides), affects the melting point of rocks. Increasing pressure generally increases the melting point; however, a decrease in pressure can trigger melting, particularly in the case of decompression melting, often associated with mantle plumes.

The Journey from Solid to Molten: Partial Melting and Magma Composition

The melting process of metamorphic rocks is rarely complete. Instead, it often involves partial melting. This means that only a portion of the metamorphic rock melts, leaving behind solid residual material. The composition of the resulting magma will differ from the original metamorphic rock, as certain minerals melt at lower temperatures than others.

The minerals that melt first, typically those with lower melting points, will be enriched in the magma. This selective melting leads to a magma composition that is different from the original metamorphic rock. This explains why the igneous rocks formed from the melting of metamorphic rocks can have diverse compositions, even when originating from the same parent material. The resulting magma can be felsic (rich in silica), intermediate, or mafic (rich in magnesium and iron), depending on the composition of the parent metamorphic rock and the degree of partial melting.

The Cooling and Crystallization Process: From Magma to Igneous Rock

The magma generated from the melting of metamorphic rocks will eventually cool and crystallize. The rate of cooling profoundly affects the resulting igneous rock texture.

Intrusive Igneous Rocks: Slow Cooling, Large Crystals

If the magma cools slowly beneath the Earth's surface, it forms intrusive igneous rocks. Slow cooling allows ample time for large crystals to grow, resulting in a phaneritic (coarse-grained) texture. Examples of intrusive igneous rocks formed from the melting of metamorphic rocks include granite and diorite. The slow cooling also facilitates chemical differentiation, resulting in rock bodies with varied compositions.

Extrusive Igneous Rocks: Rapid Cooling, Small Crystals

If the magma reaches the surface and erupts as lava, it cools rapidly, forming extrusive igneous rocks. Rapid cooling leaves little time for crystal growth, resulting in an aphanitic (fine-grained) texture or even a glassy texture if cooling is extremely rapid. Examples of extrusive igneous rocks formed from the melting of metamorphic rocks include rhyolite and andesite. The fast cooling often traps gases within the rock, leading to the formation of vesicular textures.

Specific Examples: Metamorphic Parent Rocks and their Igneous Offspring

Several metamorphic rock types can undergo this transformation, each producing characteristic igneous rocks.

• Gneiss: This high-grade metamorphic rock, often formed from the metamorphism of granite or shale, can melt to form granitic or granodioritic magmas. These magmas, upon cooling, will produce intrusive rocks like granite or extrusive rocks like rhyolite.

• Schist: This medium-grade metamorphic rock, often formed from mudstones or shales, can melt to form andesitic or dacitic magmas, yielding intrusive rocks like diorite or extrusive rocks like andesite.

• Marble: This metamorphic rock, derived from limestone or dolomite, typically melts at higher temperatures. The melt may contribute to the formation of more mafic igneous rocks, though the limestone component may also react with other magma components to produce diverse mineral assemblages.

The Wider Context: The Rock Cycle and Plate Tectonics

The transformation of metamorphic rock into igneous rock is a crucial part of the rock cycle. It demonstrates the interconnectedness of different rock types and the continuous recycling of materials within the Earth's system. This process is intimately linked to plate tectonics, as the movement of tectonic plates drives the subduction zones, mantle plumes, and magma intrusions that are essential for the melting of metamorphic rocks. The resulting igneous rocks then become potential sources for sedimentary and metamorphic rocks in subsequent cycles.

Conclusion: A Continuous Cycle of Transformation

The journey of metamorphic rock to igneous rock showcases the dynamic and ever-changing nature of our planet's geological processes. This transformation, driven by the intense heat and pressure within the Earth, highlights the interconnectedness of different rock types and their role in the continuous rock cycle. Understanding this process allows us to better appreciate the complex forces shaping our planet and the remarkable resilience and transformation capabilities of the Earth's materials. The diverse igneous rocks that result from the melting of metamorphic rocks contribute to the incredible variety and complexity of the Earth's crust, showcasing the beauty and power of geological processes over millions of years.