Did You Know?
Staurolite is a unique metamorphic rock. It forms as two intersecting crystals in the shape of a cross. Also known as Fairy Cross, staurolite has many associations in history as a good luck charm. Legend has it that Native American princess Pocahantas gave John Smith a Fairy Cross as a good luck charm. It also has associations with the deportation of the native Cherokee from their homeland in the Blue Ridge Mountains; the staurolite crystals were said to represent the tears of the Cherokee. Today staurolite is the official state rock of Georgia.
The characteristic cross shape of staurolite lead to its name as a “Fairy Cross.”
Metamorphic rocks are new rocks that result from the transformation of preexisting rock as a result of temperature and pressure. Temperatures in excess of 150° C (302° F), but less than the melting point of rock (typically between 625–1,200° C, or 1,157–2,192° F), and pressures on the order of 1,500 times normal atmospheric pressure will result in extensive physical and chemical change in preexisting rocks. Metamorphic rock can be formed two ways:
- Contact Metamorphism: Contact metamorphism refers to the structure and composition of rock changing as a result of exposure to intense heat from magma. This is a localized effect, since only rock close to the magma is affected.
- Regional metamorphism: Rock can also be changed because of interactions that occur over a fairly large geographical area. The intense heat and pressure caused by interacting plates, for example, can result in metamorphic rock being produced. This type of change is known as regional metamorphism.
Try This!
Show Your Age!
The globe in the main view panel is illustrating metamorphic rock deposits in North America.
- Click here to turn on national borders.
- In which country/countries does the vast majority of the metamorphic rock lie? The overwhelming majority of the metamorphic rock lies in Canada.
- Click here to zoom in on the metamorphic rock.
- Click here to see age data for this view.
The scale, known as the geologic time scale, divides up the Earth’s history from its origins 4.6 billion years ago up to the present time.
- Cenozoic Era – 65.5 mya (million years ago) to present
- Mesozoic Era – 251 mya to 65.5 mya
- Paleozoic Era – 542 mya to 251 mya
- Precambrian Time – 4.6 billion years ago to 542 mya
- Are the metamorphic rocks in this part of North America old? Explain.
(Hint: Click here to go back to the location of metamorphic rocks / Click here to return to the geologic time scale view.)
The metamorphic rocks in this part of North America are quite old. They are all from Precambrian Time. This means that they are all at least 542 million years old, and quite probably, much older.
These metamorphic rocks are actually quite old. As a matter of fact, metamorphic rocks from this area have been dated as being among the oldest rocks present on the Earth. Here are two significant dating locations:
- Click here to see the location of rocks dated as being 4.03 billion years old in the rock outcrop known as the Acasta Gneiss.
- Click here to see the location of rocks dated as being 3.8 billion years old in the formation known as the Nuvvuagittuq greenstone belt.