Identifying Metamorphic Rocks -- Earth Rocks!

Summary

This educational script delves into the fascinating world of metamorphic rocks, explaining their formation from parent rocks through processes influenced by temperature, pressure, and chemical fluids. It outlines how different metamorphic conditions result in varied rock types, such as marble and quartzite, and discusses key characteristics like crystal size and foliation. The script also explores specific rock formations like skarn, hornfels, slate, and gneiss, linking their textures and compositions to their geological origins and settings, providing a foundational guide to identifying and understanding these complex geological wonders.

Takeaways

• 🌋 Metamorphic rocks are created from parent rocks through processes involving changes in temperature, pressure, and potentially the addition of chemically active fluids.
• 🔍 Different parent rocks under varying metamorphic conditions can result in a range of metamorphic rocks, providing clues to the original geological setting.
• 📚 By the end of the learning module, one should be able to identify common metamorphic rocks, recognize their textures and compositions, and understand their formation environments and histories.
• 🔬 For certain parent rocks like calcite-rich limestone, the resulting metamorphic rock, marble, maintains the original composition but shows changes in crystal size as an indicator of metamorphic intensity.
• 📏 The size of crystals in metamorphic rocks like marble and quartzite is a key indicator of the degree of metamorphism experienced, without providing specific information about the setting.
• 📈 Foliation, the alignment of crystals within a rock, is a textural feature that indicates the rock has been subjected to increasing pressure.
• 🛠 The presence of certain minerals and their stability under specific temperature and pressure conditions can provide insights into the metamorphic setting and history of a rock.
• 🌡 Minerals like calcite and quartz are stable across a wide range of conditions and do not typically form new minerals unless in the presence of other minerals or chemicals.
• 💧 Metamorphic changes occur without melting the rock, as increased temperatures allow atoms to rearrange and form larger crystals or more stable compounds.
• 🏔 Nonfoliated rocks like marble, quartzite, skarn, and hornfels can form in contact metamorphic settings, with transformations depending on the original parent rock.
• 🌊 Foliated rocks with clear mineral alignment indicate higher pressures, and their mineralogy and textures can reveal the metamorphic grade and environment, such as burial, convergence, or subduction zones.

Q & A

What causes metamorphic rocks to form?

Metamorphic rocks form when parent rocks undergo chemical and/or physical transformations due to increased temperature, pressure, and possibly the addition and interaction of chemically active fluids, typically hot waters rich in dissolved ions.

How do metamorphic rocks provide clues about their original geologic formation setting?

Different metamorphic conditions result in very different metamorphic rocks from the same parent rock, thus the characteristics of the metamorphic rock can offer clues about the original geological setting.

What is the significance of crystal size in metamorphic rocks?

The size of crystals in metamorphic rocks, such as marble and quartzite, is an indication of metamorphic intensity, reflecting how high the temperature and pressure got during the metamorphic process.

What is foliation and how does it form in metamorphic rocks?

Foliation is the alignment of crystals within a rock, which occurs when a rock is subjected to increasing pressure. Highly foliated rocks display a high alignment of crystals, which can indicate the metamorphic setting.

Why can't calcite and quartz crystals form foliation?

Calcite and quartz crystals, which tend to be uniformly shaped without a long axis, are more like rounded balls or pebbles, making it impossible for them to align and form foliation.

How does the presence of foliation in a rock relate to the rock's metamorphic setting?

The presence of foliation, combined with the types of minerals present, can provide information about the metamorphic setting. For example, foliation can only occur under increasing pressure, suggesting specific geological conditions.

What is the difference between skarn and hornfels, and what do they indicate about the rock's formation environment?

Skarns are rocks that form in a contact metamorphic environment with high temperatures and likely high amounts of chemically active fluids but with little to no pressure. Hornfels, also nonfoliated, forms from mudstone or basalt under similar settings but indicates a different parent rock.

How do the mineral compositions in skarns provide information about the temperature and fluid availability during metamorphism?

The mineral compositions and sizes in skarns can indicate the temperature and types of fluids available during metamorphism, with minerals stable at the highest temperatures found closest to the magma chamber.

What is the sequence of metamorphic rock formation from mudstone under increasing metamorphic grades?

The sequence of metamorphic rock formation from mudstone under increasing grades is mudstone to shale to slate to phyllite to schist to gneiss or migmatite, depending on the pressure and temperature conditions.

How does the formation of serpentinite relate to its geological setting and what does it indicate about the parent rock?

Serpentinite forms through hydrothermal metamorphism of peridotite at mid-ocean ridges, indicating that the parent rock was rich in olivine and that the setting was one where seawater interacted with the mantle rock near the surface.

What can the identification of a metamorphic rock tell us about its formation history?

Identifying a metamorphic rock can reveal its formation history, including the parent rock, the geological setting, the temperature and pressure conditions, and the presence of any chemically active fluids during metamorphism.