Gabbro

Definition

Gabbro is a coarse-grained, intrusive igneous rock primarily composed of plagioclase feldspar and pyroxene. It forms from the slow crystallization of magma beneath the Earth's surface, which allows large crystals to develop. This rock is crucial in understanding oceanic crust formation and evolution, particularly as it constitutes a significant portion of the oceanic crust beneath mid-ocean ridges.

5 Must Know Facts For Your Next Test

1. Gabbro forms at depths of several kilometers below the surface, making it less accessible compared to its volcanic counterpart, basalt.
2. The texture of gabbro is characterized by its interlocking crystals, typically visible to the naked eye, which results from slow cooling rates in the Earth's crust.
3. Gabbro is an essential component of oceanic crust, often found in areas where tectonic plates are diverging, like mid-ocean ridges.
4. It contains minerals such as olivine, amphibole, and biotite in addition to plagioclase feldspar and pyroxene, which contribute to its dark coloration.
5. The study of gabbro helps geologists understand the processes of magma evolution and the history of oceanic crust formation.

Review Questions

• How does the formation process of gabbro differ from that of basalt?
  • Gabbro forms through the slow crystallization of magma beneath the Earth's surface, allowing large crystals to develop due to a longer cooling period. In contrast, basalt is formed when lava erupts onto the surface and cools rapidly, resulting in a fine-grained texture. This difference in cooling rates explains why gabbro has a coarse texture with visible crystals while basalt appears smooth and glassy.
• Discuss the role of gabbro in understanding mid-ocean ridges and oceanic crust dynamics.
  • Gabbro plays a critical role in understanding mid-ocean ridges because it constitutes a significant portion of the oceanic crust formed during seafloor spreading. As tectonic plates diverge at these ridges, magma rises and solidifies into gabbro at depth. Studying gabbro samples from these regions provides insights into the composition and processes that shape oceanic crust dynamics, including magma evolution and tectonic activity.
• Evaluate how studying gabbro can enhance our knowledge of geological processes on Earth.
  • Studying gabbro can greatly enhance our understanding of geological processes because it serves as a record of magma's behavior and composition beneath the surface. By analyzing gabbro samples, geologists can learn about the conditions under which oceanic crust forms and evolves over time. Additionally, this knowledge helps interpret broader tectonic processes and contributes to models predicting future geological changes related to plate tectonics and volcanism.

Related terms

Basalt:

Basalt is a fine-grained, extrusive igneous rock that forms from the rapid cooling of lava at or near the Earth's surface, often found in volcanic regions.

Mid-ocean Ridge: A mid-ocean ridge is an underwater mountain range formed by plate tectonics, where new oceanic crust is created by volcanic activity.

Plutonic Rock: Plutonic rock refers to igneous rocks that crystallize from magma beneath the Earth's surface, including gabbro, which cools slowly to form large crystals.