An ocean floor on land
In the mountains of Denizli Province is a geological large geological feature: a complete, well-preserved segment of oceanic crust and upper mantle. Known as the Çomakdağ Ophiolite, this is a piece of the ancient Neo-Tethys Ocean floor, created during the Late Cretaceous period. The sequence is part of a larger geological structure called the Lycian Nappes, a collection of massive rock sheets that were thrust over the continental margin of the Anatolide-Tauride block.
The process that placed this section of ocean floor high in the mountains is called obduction. As the African and Eurasian tectonic plates converged, the Neo-Tethys Ocean between them began to close. Instead of one plate sliding neatly under the other (subduction), a slab of the dense oceanic lithosphere was scraped off and pushed up and onto the lighter continental crust. This process began in the Santonian, a stage of the Late Cretaceous about 86 million years ago. The result is an opportunity for scientists to walk across rocks that are normally found kilometers beneath the ocean surface.
A geologist's layer cake
The Çomakdağ Ophiolite is scientifically valuable because its layered structure, or pseudostratigraphy, is almost fully intact. It allows for direct study of the processes that occur at mid-ocean ridges where new oceanic crust is born.
The sequence, from deepest to shallowest, includes:
- Mantle Tectonites: At the base are peridotites, primarily a rock type called harzburgite. These are rocks from the Earth's upper mantle that have been deformed by the immense pressures of mantle convection. Much of the peridotite has been altered by water into a greenish, waxy rock called serpentinite.
- Gabbros: Above the mantle rocks are gabbros. These coarse-grained igneous rocks formed from the slow cooling of magma in a chamber beneath the seafloor. The layering visible in some of these rocks records the gradual crystallization and settling of minerals.
- Sheeted Dike Complex: This layer consists of hundreds of near-vertical, parallel sheets of basaltic rock (diabase). These dikes were the conduits that fed magma from the chamber below to the ocean floor above.
- Pillow Basalts: The top layer of the igneous crust is composed of pillow basalts. These distinctive, rounded shapes formed as lava erupted directly into the cold seawater, causing its surface to quench and solidify almost instantly.
- Pelagic Sediments: In some areas, the volcanic rocks are capped by deep-sea sediments, such as chert and limestone, that slowly accumulated on the ocean floor after it formed.
Studying the chemical composition and structure of these layers shows the mechanics of seafloor spreading and the evolution of magma deep beneath the waves.