Metamorphic Rocks - Formation

Metamorphic Rocks

Rocks that have been subjected to tremendous heat and/or pressure, causing them to change into another type of rock.

E.g. Feldspar & Mica → Limestone & Gneiss → Marble

(Clay mineral-rich rock → Sedimentary rock → Metamorphic)

Formation

• Occur at plate boundaries

  Metamorphism (the changing of rock) can occur at destructive plate boundaries

• The oceanic plate is forced under the lighter continental plates.

Metamorphic rocks can result because…

1. Temperature increases

2. Pressure caused by increasing weight

3. Pressure caused by folding

Temperature Increases

As a oceanic plate descends below the edge of a continental plate (subduction) friction between the plates will cause a temperature increase.

This heat may cause metamorphism so occur.

What happens at the same time

Oceanic crust is subducted deeper into the mantle.

The temperature would increase by 25°^C for every KM.

Thermal Metamorphism

The deeper a layer of rock descends, the hotter the temperature and the more likely it is to be metamorphosed into another rock.

Pressure caused by increasing weight

Subduction will cause layers of rock to be buried deeper under the Earth.

The pressure will increase from the growing weight of more rock layers.

Can also lead to metamorphic rock creation

Pressure caused by folding

When two continental plates →← they are forced up and form fold mountains.

The pressure of these folding plates is powerful enough to change the nature and structure of rock layers into metamorphic rock - Regional Metamorphism.

Foliated Metamorphic Rocks

A striped or banded appearance.

Occur when mineral grains are squeezed into flat layers or elongated by pressure.

Non-Foliated Metamorphic Rocks

Do not have bands or strips of minerals.

Usually occur near hot igneous rocks

These rocks changes the mineral structure without added pressure.

In Ireland

Metamorphic rocks are the oldest in Ireland.

E.g. Malin Head Co. Donegal.

• Gneiss rock found here

• Approx 490MYA

• Caledonian Orogeny

• Laurentia & Avalonia →←

• Resulted in heat and pressure

• Metamorphosed sandstone → quartzite

  (Sugarloaf Mountain Co. Wicklow)

• Connemara Marble in Clifden Co.Galway