CREDIT: perspective view of Mount Shasta, with a Landsat overlay from NASA/JPL

Lab 1: Scope and Nature of Matter, Continent

 

This map shows the geologic map of North America and the surrounding ocean basins. The colors show locations of different ages of rock on the surface. CREDIT: USGS Geologic Map of North America 2005

 

All three major types of rocks are exposed across North America:

 

Igneous -- Molten magma cools underground in cracks or chambers or on the surface around volcanoes to form hard crystalline rocks like granite, diorite, gabbro, rhyolite, andesite, and basalt.

 

Sedimentary -- Any type of rock can be exposed at the surface where rain and ice and weathering break rock into pieces, which are transported by running water, moving ice (glaciers), or wind. Areas such as river beds, river deltas, continental edges of shallow sea floor, and even deep oceans, receive the material, where it accumulates into layers of sediment, that are buried and hardened to sedimentary rock. Examples include sandstone, limestone, shale, and gypsum.

 

Metamorphic -- Any type of rock can be heated up or squeezed or stretched a great deal, which causes physical and/or chemical changes, so that we are left with a different rock type from the original, a metamorphosed rock. Examples include slate, schist, gneiss, and marble.

 

USGS Geologic Map of North America 2005, labeled

 

The oldest part of North America, as for other continents, is called the craton. The craton is divided into a Shield area, where very old igneous and metamorphic rocks are exposed (some of the oldest rocks on Earth are here, at 3.8 billion years old), and a Stable Platform area, where the very old rocks have been buried beneath younger layers of sedimentary rock. Around the fringes of the craton are edges that have at one time or another been collisional boundaries where mountains formed. The Western Cordillera ("core-dee-air-ah") is a wide belt of deformed mountains, fault zones, and volcanic features, where a long history of collision has added significant material to edge of North America. The Ouachita-Applalachian fold belt is a narrower belt along the eastern and southern margins, where collision with South America and Africa formed high mountains 300 million years ago, that have since weathered down to a more gentle mountain belt.