MEGA XENALITH EarthCache

Law of included fragments

The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.

For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found.

These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

In a more generalized understanding, xenoliths represent segments of the “country rock” that is incorporate into magma while the magma is still fluid. Xenoliths can either be settled in position once plucked from their “country rock” or can transported away from their origin.

If the “Country rock” density is greater than that of the new If the “magma” magma the xenolith will settle deep within the “intrusion.”

If the “Country rock” density is less than that of the new magma the xenolith will be transported further up the intrusion.

Intrusive Environments

Magma that cools at depth form bodies of rocks called intrusive bodies or plutonic bodies called plutons, from Greek god of the underworld - Pluto.

When magma intrudes it usually affects the surrounding rock and is also affected by the surrounding rock.  It may metamorphose the surrounding rocks or cause hydrothermal alteration. 

The magma itself may also cool rapidly near the contact with the surrounding rock and thus show a chilled margin next to the contact.  

It may also incorporate pieces of the surrounding rocks without melting them.  These incorporated pieces are called xenoliths (foreign rocks).  

Magma intrudes by injection into fractures in the rock and expanding the fractures.   The may also move by a process called stoping, wherein bocks are loosened by magma at the top of the magma body with these blocks then sinking through the magma to accumulate on the floor of the magma body. 

A QUICK SELF TEST TO ASSESS YOUR UNDERSTANDING:

In each diagram, determine the origin of the inclusions and whether they are older or younger than the rocks that contain them.

Xenoliths provide important information about the composition of the otherwise inaccessible mantle.

Your clue to the question above.

Xenolith, a rock fragment within an intrusive igneous body that is unrelated to the igneous body itself.

Xenoliths, which represent pieces of older rock incorporated into the magma while it was still fluid, may be located near their original positions of detachment or may have settled deep into the intrusion, if their density is greater.

Xenoliths can be contrasted with autoliths,, or cognate xenoliths, which are pieces of older rock within the intrusion that are genetically related to the intrusion itself. The general term for all such incorporated bodies is inclusions.

Xenoliths are usually reconstituted through the processes of contact metamorphism contact metamorphism, in which heat and fluids cause mineralogic and chemical changes in the parent rock of the xenolith; a study of these changes can give information on the temperature and composition of the magmatic body.

ANSWER TO THE QUESTION ABOVE.

PLEASE CLICK ON ANY IMAGE FOR FURTHER INFORMATION

XENOLITH	DYKE

INCLUSIONS	CLAST

Earthcache Task

LOGGING REQUIREMENTS

Answers can be found in a link on this cache page and at the Earthcache site.

EXAMINE AND DESCRIBE FORMATIONS

1. Examine both the Xenolith and Country rock describe the difference.

EXAMINE AND DESCRIBE FORMATIONS

2. Stand facing the Xenolith and from your observations what is the roughly triangular feature at the left hand end between the two rock types?

EXAMINE AND DESCRIBE FORMATION.

3. What type of rock is the xenolith here and what type of rock is it incorporated into?

A. Columnar Basalt xenolith incorporated into Ross on Mull Granite.
B. Limestone xenolith incorporated into Ross on Mull Granite.
C. Hercynite xenolith in Ross on Mull Granite.
D. Moine Psammite xenolith incorporated into Ross on Mull Granite.

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Click hereTO VIEW THE SNH FOSSIL CODE.

Click hereTO VIEW COMMON FOSSILS, ROCKS & MINERALS.

Click hereTO VIEW GEOLOGICAL FIELDWORK CODE.

FURTHER READING AND REFERENCE:

The Earth Through Time Eighth Edition
by Harold L. Levin

http://www.tulane.edu/~sanelson/eens1110/igneous.htm

Click here FOR WALK / ROUTE DESCRIPTION.

Click hereFOR ERRAID HISTORY.

Click hereTo read: The Field Description of Igneous Rocks:Jerram and Petford

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