Activity 4

What are ROCK UNITS?• Rocks formed in large volumes

during a specific process• Many different kinds of rock

deposits in an area that may have formed at the same time

How are Rock Units formed? • Sediment deposition

(sedimentary rocks)•Magma intrusion or extrusion

(igneous rocks)• Metamorphism

(metamorphic rocks)

What is a Contact?• the change from one

rock unit to another• usually abrupt and

across some surface or narrow zone of change

What is the defining characteristic of a rock

unit?A body of rock that consists

dominantly of a certain rock type, or a combination

of types

Rock units can consist of sedimentary,

igneous, or metamorphic rocks

Why do Rocks have 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”

• Volcanic (igneous) and metamorphic rock units have similar 2 part names

• Formations can be sub-divided into members which have formal or informal names

What are 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)

How are rocks deposited?

• Sedimentary and extrusive igneous rock units tend to be deposited in thin layers that can extend great distances – lateral extent (horizontal)

• Intrusive igneous and metamorphic rock units vary greatly in size but would not usually be as extensive as sedimentary or volcanic units – vertical extent

What are Sills?

• Sheets of igneous rock that intruded along layers of sedimentary rocks

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

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.

light-colored igneous sills and dikes

Golden Valley Sill - South Africa

What are 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 Peak

Shiprock, New Mexico, the volcanic neck in the distance, with radiating dike on its south side. Photo credit: USGS Digital Data Series

Clastic dike (left of notebook) in the Chinle Formation in the Island In the Sky District of Canyonlands National Park, Utah

What are 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.

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