An ophiolite is primarily composed of three distinct rock types: pillow basalts, sheeted dikes, and gabbro.
Understanding Ophiolites
An ophiolite is a unique geological formation that represents a segment of ancient oceanic lithosphere—comprising oceanic crust and the underlying upper mantle—that has been thrust onto or uplifted into continental crust. These formations are crucial for geologists studying the processes of seafloor spreading and plate tectonics, as they provide a direct look at rocks that typically lie deep beneath the ocean.
Ophiolites are not just random collections of rocks; they are a suite of rocks, a set of three distinct rock types that form together as part of a single process at mid-ocean ridges. A notable example of such a formation is the Coast Range Ophiolite.
The Three Primary Rock Types of an Ophiolite
The characteristic sequence of rocks in an ophiolite, from top to bottom, reflects the layers of oceanic crust and the uppermost mantle.
Pillow Basalts
- Description: These are fine-grained, dark extrusive igneous rocks that form when hot lava erupts underwater and cools very rapidly. The quick cooling causes the lava to form rounded, sack-like structures that resemble stacked pillows, hence the name.
- Role: Pillow basalts represent the uppermost volcanic layer of the oceanic crust, formed directly on the seafloor.
Sheeted Dikes
- Description: Beneath the pillow basalts lies a layer of sheeted dikes. These are vertical, parallel intrusions of basaltic rock, often appearing as a series of interconnected, tabular bodies. They form as magma continuously rises through cracks in the crust to feed the eruptions above.
- Role: Sheeted dikes act as the conduits through which magma flowed from deeper magma chambers to create the pillow basalts on the seafloor.
Gabbro
- Description: Gabbro is a coarse-grained, dark intrusive igneous rock. It has a similar chemical composition to basalt but forms when magma cools slowly at greater depths within the crust, allowing larger crystals to grow.
- Role: The gabbro layer represents the deeper, slower-cooled part of the oceanic crust, forming the solidified magma chambers beneath the sheeted dikes.
Below these three primary rock types, ophiolites typically include rocks from the Earth's upper mantle, most commonly peridotite, which is often serpentinized (altered by water). However, the core definition of the rock types making up the crustal component of an ophiolite specifically refers to the pillow basalts, sheeted dikes, and gabbro.
Summary of Ophiolite Rock Types
The following table summarizes the key characteristics of the rocks found in an ophiolite sequence:
| Rock Type | Description | Formation Environment | Position in Ophiolite Sequence |
|---|---|---|---|
| Pillow Basalts | Fine-grained, dark extrusive igneous rock, characterized by pillow-like shapes. | Rapid cooling of lava from underwater volcanic eruptions. | Uppermost layer (volcanic top) |
| Sheeted Dikes | Interconnected, nearly parallel, vertical intrusions of basaltic rock. | Magma conduits feeding surface eruptions at spreading centers. | Below pillow basalts |
| Gabbro | Coarse-grained, dark intrusive igneous rock. | Slow cooling of magma in deeper crustal magma chambers. | Below sheeted dikes |
