Anorthosite

Definition

Anorthosite is a type of intrusive igneous rock that is composed primarily of plagioclase feldspar, typically more than 90% of its composition. This rock type is significant in lunar geology, as it forms the bulk of the Moon's highland crust and provides insights into the Moon's early geological history and differentiation processes.

5 Must Know Facts For Your Next Test

1. Anorthosite is believed to have formed early in the Moon's history during a period of intense volcanic activity, contributing to the formation of its crust.
2. The lunar highlands, primarily made up of anorthosite, are older than the basaltic plains known as the maria, indicating different geological processes over time.
3. Anorthosite samples returned by Apollo missions have provided valuable information about the Moon's composition and its geological evolution.
4. This rock type has a distinctive light color and low density compared to other lunar rocks, which helps scientists identify it in lunar samples.
5. The presence of anorthosite on the Moon supports theories regarding planetary differentiation, where lighter minerals rise to form a crust while heavier minerals sink.

Review Questions

• How does the composition of anorthosite relate to its formation and significance in lunar geology?
  • Anorthosite is composed primarily of plagioclase feldspar, making it unique among igneous rocks. This high content of plagioclase suggests that anorthosite formed through processes involving the crystallization of minerals from a molten state. Its significance lies in its role in shaping the Moon's highland regions and providing insights into the early geological processes that affected the Moon's crust.
• Discuss the differences between anorthosite and basalt in terms of their occurrence on the Moon and their implications for lunar geological history.
  • Anorthosite primarily composes the Moon's highlands, while basalt is found in the darker maria. This difference indicates varying geological processes: anorthosite formed from earlier volcanic activity and crystallization events, whereas basalt represents later volcanic flows. These distinctions help scientists understand the timing and nature of volcanic activities on the Moon and provide a clearer picture of its geological evolution over billions of years.
• Evaluate how the study of anorthosite contributes to our understanding of planetary differentiation and what this means for theories about Earth's early crust.
  • Studying anorthosite enhances our understanding of planetary differentiation by demonstrating how lighter minerals can rise to form a crust while heavier ones sink. This concept has implications for theories about Earth's early crust formation. By comparing lunar anorthosites with terrestrial igneous rocks, researchers can infer processes that shaped both bodies, suggesting that similar differentiation may have occurred on Earth when it was still forming. This comparison enriches our understanding of planetary geology across different celestial bodies.

Related terms

Plagioclase Feldspar: A group of feldspar minerals that are important components of many igneous rocks, including anorthosite, characterized by their sodium and calcium content.

Lunar Highlands: The lighter, elevated regions of the Moon's surface that are composed mainly of anorthosite, distinguishing them from the darker, basalt-rich lowlands.

Igneous Rock: A type of rock formed through the cooling and solidification of magma or lava, with anorthosite being a specific subtype characterized by its high plagioclase content.