Three Types of Rock at Peggy’s Cove

We all love Peggy’s Cove for how beautiful it is but how it formed also offers a great geology lesson about the three types of rock:

IGNEOUS ROCK - Rocks formed by molten magma are called igneous rocks and can be fine-grained or coarse-grained depending on how close to the earth’s surface the cooling occurred. Granite is an example of a coarse-grained igneous rock which cooled relatively slowly beneath the earth’s surface. Granite is often exposed at the surface as overlying rocks are eroded.

SEDIMENTARY ROCK - Sedimentary rocks can be formed by two processes. The first process is the laying down, over time, of weathered material on the Earth’s surface or under the oceans. The second process is the evaporation of seawater to leave behind deposits of minerals like salt and gypsum, called evaporites.

METAMORPHIC ROCK - A third type of rock, called metamorphic rock, may be formed from sedimentary or igneous rocks. Through heat and pressure, new minerals form in the rocks, transforming the rock’s original appearance and both chemical and structural properties. The rocks may be so altered that the original rock type may be difficult to identify.

The rocks at Peggy’s Cove began 470 million years ago as mud and sand in a deep ocean basin near ancient Africa. They built up and were eventually compacted into shale and sandstone (sedimentary rocks).

As tectonic plate movement caused North Africa and North America to collide 400 million years ago, the shale and sandstone were squeezed between the two continents and heat and pressure transformed them into slate and quartzite (metamorphic rocks).

Heat from the continental collision also melted rock at the base of earth’s crust 375 million years ago. The molten granite was less dense than the solid rocks around it so it rose through the earth’s crust like a piece of wood rises through water. As it rose and got further from the heat deep inside the earth, it cooled and solidified several kilometres below earth’s surface.

Uplift (upward pressure) from the collision continued to push the granite toward surface, while weathering and erosion removed the slate and quartzite above. Eventually erosion exposed the granite (an igneous rock) at surface, where it is now.

Here is another bit of geological trivia from Peggy's Cove:

One of the pictures below shows a rock within another rock at Peggy's Cove. It is what we call a xenolith - a piece of rock engulfed in an igneous rock while it was molten.

As the molten granite rose, some of the surrounding wall rock broke off and was absorbed into the molten granite. (Think of the molten granite as liquid passing through a pipe and the wall rock as the pipe. Xenoliths are pieces of the pipe that break off and get carried along by the liquid.)

Some pieces of broken wall rock are melted by the molten granite and become part of it, often appearing as wisps of different-coloured rock within the granite.

In this case, the xenolith became trapped in the granite as the granite rose further from the heat deep inside the earth, cooled and solidified.