Sedimentary rocks

There are two main types of metamorphism:

• Contact metamorphism—occurs when magma contacts a rock, changing it by extreme heat (Figure 4.14).
• Regional metamorphism—occurs when great masses of rock change over a wide area due to pressure exerted on rocks at plate boundaries.

There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure …

The oldest rocks exposed on Earth are nearly 4.0 billion years old. These metamorphic rocks — the Acasta gneisses — are found in Canada.

Metamorphic rocks were once igneous or sedimentary rocks, but have been changed (metamorphosed) as a result of intense heat and/or pressure within the Earth’s crust. They are crystalline and often have a “squashed” (foliated or banded) texture.

Quartzite and marble are the most commonly used metamorphic rocks. They are frequently chosen for building materials and artwork. Marble is used for statues and decorative items like vases (Figure). Quartzite is very hard and is often crushed and used in building railroad tracks.

Igneous and metamorphic rocks Igneous rocks form from molten rock, and rarely have fossils in them. Metamorphic rocks have been put under great pressure, heated, squashed or stretched, and fossils do not usually survive these extreme conditions. Generally it is only sedimentary rocks that contain fossils.