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- [REQUIRED] Post a photo of yourself or a personal item at the site to prove you were there.
- Is the dyke mafic or granitic? (see notes below)
- Compare the dyke and the surrounding rock and say which one looks like it is weathering more?
- Is the dyke older or younger than the surrounding rock? (answer in notes below)
The Town of Tilting on Fogo Island is significant, where the trails cross back and forth over the Fogo Island Batholith, a complex intrusion that underlies 80% of the island. Like several other Silurian bimodal intrusions in Newfoundland, the Fogo Island Batholith contains both dark-coloured mafic and lighter-coloured granitic proportions. It also contains ultramafic rocks, which are more unusual.
The rocks formed as a complex movement between continental masses wrenched the crust. Mafic and granitic magmas polled and gently circulated in a stable chamber just below the earth’s surface in a slow, quiet process.
Here along the Trans-Canada Highway, you can see the mafic and granitic dykes that are part of the bimodal Silurian Dykes that occur over a wide area in central Newfoundland.
Magmatic Dykes
A magmatic dyke is a sheet of igneous rock that cuts across older rock beds. It is formed when magma fills a fracture in the older beds and then cools and solidifies. The magma intrudes into a crack and then crystallizes as a sheet intrusion, either cutting across rock layers or through an unlayered rock mass.
The dyke rock is usually more resistant to weathering than the surrounding rock so erosion exposes the dyke as a natural wall or ridge.
A dyke is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dyke is always younger than the rocks that contain it.
Mafic Rock
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in colour, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite.
Granitic Rock
Granite is a light-coloured igneous rock with grains large enough to be visible to the unaided eye. It forms from the slow crystallization of magma below Earth's surface.
REFERENCES:
Geology of Newfoundland Field Guide: Touring Through Time at 48 Scenic Sites. Martha Hickman Hild. Pages 126-131.
Geology.com What is Granite?
Wikipedia. Granite
Britannica. Mafic Rock
Wikipedia. Mafic
Wikipedia. Dyke