1. Activity 4 Background information

2. Rock types are not randomly arranged in the Earth’s crust but exist in distinctive bodies called ROCK UNITS

3. Rock Units are formed by the action of some particular process or set of processes: Sediment deposition Magma intrusion or extrusion metamorphism

4. Rock units can consist of sedimentary, igneous, or metamorphic rocks

5. Defining characteristic of a rock unit: Certain processes operate for some period of time to produce a body or mass of rock with fairly uniform rock type, or perhaps a consistent alternation of two rock types

6. Formal Names Many rock units receive formal names The basic sedimentary rock is the formation Formations have 2 part names: – the first part is a place name, like a river, town or mountain – the second part is either the word “Formation” or a rock term like “Sandstone”

7. Volcanic (igneous) and metamorphic rock units have similar 2 part names Formations can be sub-divided into members which have formal or informal names

8. Geologic Cross Sections Vertical cross sections: views of what the geology would look like in an imaginary vertical plane downward from some line on the land surface Outcrop: the bedrock exposed at the Earth’s surface (look along the sides of a highway that cuts through the mountains)

9. Lateral and vertical extent Sedimentary and extrusive igneous rock units tend to be deposited in thin layers that can extend great distances Intrusive igneous and metamorphic rock units vary greatly in size but would not usually be as extensive as sedimentary or volcanic units

10. Sills Sheets of igneous rock that intruded along layers of sedimentary rocks

11. Mount Gould, in Glacier National Park in Montana, is such a prominent and widespread sill that it has its own name, the Purcell Sill The Whin Sill, which is a quartz dolerite, is one of the main natural heritage features of the North Pennines. In fact it is the original sill of geological science. It takes its name from the north of England quarryman's term 'sill' meaning any more or less horizontal body of rock, and 'whin' meaning a hard, rather intractable, black rock

12. The Sill (dark band with baked upper and lower contacts) was injected into the limestone country rock. Therefore, the sill is younger than the limestone since the limestone had to be present before the sill was injected.

13. light-colored igneous sills and dikes Golden Valley Sill - South Africa

14. Dikes Sheets of igneous rocks intruded along fractures that cut through any existing rock Dikes in the Black Canyon of the Gunnison National Park, Colorado, USA Magmatic dikes radiating from West Spanish PeakWest Spanish Peak

15. ShiprockShiprock, New Mexico, the volcanic neck in the distance, with radiating dike on its south side. Photo credit: USGS Digital Data SeriesNew MexicoUSGS Clastic dike (left of notebook) in the Chinle Formation in the Island In the Sky District of Canyonlands National Park, UtahChinle Formation Canyonlands National Park

16. Batholiths Large masses of intrusive igneous rock with irregular shapes These can be seen exposed today because the land surface has worn down due to weathering and erosion Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada batholith. Yosemite National Park Sierra Nevada batholith

17. The mountain slopes around Panguippuli lake seen in this picture belong to the Panguipulli Batholith in southern Chile. Enchanted Rock, Texas Magma mingling products in the Huatulco batholith on the Fort Huatulco beach