Question

What does the mixture of grain sizes in a porphyritic igneous rock indicate about its cooling history? (D)

Short answer

The mixture of grain sizes in a porphyritic igneous rock indicates a complex cooling history, involving both slow and rapid cooling stages. The larger phenocrysts developed during a slow cooling stage deep within the Earth's crust, while the finer-grained groundmass formed later when the magma cooled rapidly near the Earth's surface or shallow within the crust. This information provides insights into the geological processes and tectonic environment responsible for the rock's formation.

Step-by-step solution

Definition of porphyritic igneous rock

A porphyritic igneous rock is a type of rock that contains larger mineral crystals, called phenocrysts, embedded in a finer-grained matrix, called the groundmass. These rocks form when magma cools and solidifies at different rates, resulting in a mixture of grain sizes.

Factors affecting cooling history

The cooling history of an igneous rock is influenced by various factors such as the depth at which the magma cools, the rate of cooling, and the composition of the magma. These factors can lead to different textures and grain sizes in the rock.

Relationship between grain size and cooling history

In general, the size of mineral grains in igneous rocks is directly related to the cooling rate of the magma. When magma cools slowly, minerals have more time to grow and form larger crystals. Conversely, when magma cools rapidly, minerals have little time to grow, resulting in finer-grained rocks with smaller crystals.

Cooling history in porphyritic igneous rocks

In the case of porphyritic igneous rocks, we observe a mixture of grain sizes: larger phenocrysts surrounded by a finer-grained groundmass. This mixture indicates that the rock experienced two distinct cooling stages: 1. Slow cooling stage: The phenocrysts formed during a slow cooling stage within the magma chamber. During this time, the magma cooled slowly, allowing the larger phenocrysts to grow and develop. This stage occurs deep within the Earth's crust, where the temperature remains high for a longer time. 2. Rapid cooling stage: At some point, the magma containing phenocrysts was expelled from the magma chamber and erupted onto the Earth's surface or intruded into the crust. This event caused the remaining magma to cool quickly, resulting in the finer-grained groundmass. The rapid cooling happened either near the surface or shallow within the crust.

Conclusion

The mixture of grain sizes in a porphyritic igneous rock indicates a complex cooling history, with the rock experiencing both slow and rapid cooling stages. The presence of phenocrysts shows that at least part of the cooling process occurred at a slow rate, while the finer-grained groundmass reveals a subsequent rapid cooling event. This information helps geologists to understand the geological processes that shaped the formation of the rock and the associated tectonic environment.

Key concepts

Igneous Rock Textures

The texture of an igneous rock is a crucial clue to its formative history and characterizes the overall appearance of the rock based on the size, shape, and arrangement of its mineral grains.

Understanding these textures can help decipher the conditions under which the rock formed. For example, rocks with large, well-formed crystals indicate a slow cooling process, whereas rocks with small crystals suggest rapid cooling. This is important in studying porphyritic textures, where the rock displays both large, conspicuous crystals and a fine-grained background.

Since texture directly reflects the cooling history, identifying whether an igneous rock has a porphyritic, glassy, aphanitic, or phaneritic texture provides valuable insights into the volcanic or plutonic origin of the rock.

Cooling History of Magma

The cooling history of magma is a narrative of the journey from a molten state to solid igneous rock. This history is complex, as it involves the rate at which magma loses heat and the influence of external factors on this process.

Slow cooling often occurs deep underground, giving minerals ample time to grow and resulting in larger crystals. On the other hand, magma that reaches the surface cools rapidly due to the lower temperatures, leading to smaller crystals or even glassy textures in the case of extreme cooling rates.

The presence of different sized grains in porphyritic rocks is a testament to this variable cooling rate — initial slow cooling allows the formation of larger crystals (phenocrysts), followed by rapid cooling that forms the fine-grained groundmass encapsulating them.

Phenocrysts and Groundmass

In porphyritic igneous rocks, the contrasting sizes of crystals define the rock's unique characteristics. Phenocrysts are the prominent larger crystals that initially grow within a slowly cooling magma. The groundmass, or matrix, is the finer-grained material that forms when the remaining magma cools more quickly.

These two components represent different stages of the cooling process and allow geologists to infer the cooling rate and environment that the magma experienced. The phenocrysts are essentially 'frozen in time', showcasing the conditions of the deeper magmatic chamber, while the groundmass records the subsequent rapid cooling and solidification that the rock underwent.

Geological Processes

The formation of porphyritic igneous rocks is a result of geological processes operating on different scales and contexts within the Earth. These processes involve the interaction between the internal thermal dynamics of the Earth, the chemistry of the magma, and the tectonic settings that facilitate magma movement.

Geological events like volcanic eruptions, magma intrusion into surrounding rock, and tectonic plate movements all play roles in determining the rock's texture and composition. By interpreting these rocks, geologists can reconstruct past geological events, contributing to our understanding of the Earth's dynamic history and the processes that shape our planet's surface.