Planet Geo: Debunking the Continental-Crust Granitic Myth and Exploring Lower-Crust Realities

Planet Geo challenges the common view that continental crust is purely granitic, arguing the continental average lies between felsic and mafic rock types. The episode explains how the upper crust is largely sedimentary and granitic, while deeper crust is inferred from exhumed terrains and xenoliths. It also explores competing models of crust structure (two-layer vs three-layer) and the role of water and subduction-like processes in crust formation.

Introduction: Rethinking the Continental Crust

In this discussion, the hosts challenge the long-held simplification that continental crust is granite. They explain that the crust's average composition sits between felsic and mafic rocks, and that actual crustal rocks on continents include basalt, granite, and sediments. The conversation emphasizes how little we know about the lower crust and how this uncertainty shapes our understanding of crust formation and stabilization.

"No, the continental crust is not granite, the average sits in between" - Planet Geo

Finding the Deep: Exhumed Terrains and Xenoliths

The episode examines how scientists access the middle and lower crust, using exhumed terrains from former mountain belts and xenoliths carried up by magmas. Exhumed rocks often preserve granulite facies signatures, revealing upper crustal processes while hinting at deeper compositions. The discussion covers specific regions where deep crustal rocks are exposed, offering clues about the lower crust’s makeup and its history.

"Exhumed terrains and xenoliths give us a window into the lower crust" - Planet Geo

Three-Layer vs Two-Layer Models: What Do We See?

The hosts compare a traditional three-layer crust model (felsic upper crust, intermediate middle crust, mafic lower crust) with a two-layer model (granite-dominated upper crust and an intermediate lower crust). They highlight biases in xenolith records and exhumed terrains, noting the absence of abundant andesite in these datasets and the implications for crust models.

"Three-layer models come from xenolith records, but andesite is oddly missing" - Planet Geo

Why It Matters: Water, Stabilization, and Earth’s History

The conversation turns to the role of water in granite formation and continental crust growth, referencing Campbell and Taylor’s idea that water is essential for granite production, and by extension for crust formation and oceans. The hosts emphasize that even with composition data, timing and growth history remain debated, underscoring the complexity of crustal stabilization and recycling over billions of years.

"Water is a key ingredient in making granites, and thus continents" - Planet Geo