Spotted Floor EarthCache

This is an EarthCache, which does not have a physical container, but provides a learning exercise and a chance to show that knowledge in this setting.  Placed close to where I work.  If the doors are closed, you can still see the Xenolith through the window.  Please send me the answers to the questions below to validate your log.

1. What is a Xenolith?

A xenolith (from Greek xeno = foreign, lithos = rock) is a fragment of rock that becomes enclosed within another rock during the latter’s formation. In geology, the term is most commonly applied to foreign rock fragments trapped in igneous rocks during magma ascent and solidification

2. How is it Formed?

• Process:
  As magma rises from deep within the Earth, it can tear off pieces of surrounding rock (country rock) from the walls of magma chambers, conduits, or pipes. These fragments are carried upward in the molten material and become embedded as the magma cools and crystallizes

• Types of Xenoliths:

  • Accidental (country rock): Torn from surrounding crustal rocks during magma ascent.
  • Cognate (autoliths): Earlier-formed crystals or rocks from the same magma body.
  • Mantle xenoliths: Pieces of the Earth’s mantle (e.g., peridotite, lherzolite) brought up by magmas like basalt or kimberlite

3. How to Identify a Xenolith?

• Visual Clues:
  • They usually appear as distinct inclusions within igneous rock, often with a different color, texture, or density than the host rock.
  • The color of a xenolith is primarily controlled by its mineral composition and degree of alteration. Since xenoliths are fragments of foreign rock, their colors often contrast sharply with the host igneous rock, making them easier to identify

  • Greenish to Yellow-Green

    • Typical of peridotite or olivine-rich mantle xenoliths.
    • Indicates ultramafic composition (high in magnesium and iron) and origin from the Earth’s mantle.
    • Example: Peridotite xenoliths in basaltic lava flows often appear greenish-yellow

  • Dark Gray to Black

    • Common in basaltic or gabbroic xenoliths.
    • Suggests a mafic composition (rich in pyroxene and plagioclase).
    • These usually come from the lower crust or earlier-formed mafic intrusions

  • Light Gray to White

    • Often seen in granite or felsic xenoliths.
    • Indicates a silica-rich composition (quartz and feldspar).
    • Typically derived from continental crust fragments

  • Reddish or Brownish Tints

    • Caused by oxidation or alteration of iron-bearing minerals (e.g., olivine weathering to iddingsite).
    • Suggests exposure to fluids or surface weathering after emplacement

Sizes range from sand grains to several meters.

Sharp edges on the foreign objecy show the object was engulfed quickly.  Smooth edges show that it was slowly ingested and had time to melt a bit.

What it shows:

• Magma rising through the crust.
• Pieces of surrounding rock (country rock) breaking off and becoming xenoliths.
• Xenoliths embedded in the cooling igneous rock.

Logging Requirements:

There is one large xenolith on the floor area visible.

1. Tell me the size of the large one that you see.  What is the color?  What material do you think it is?
2. Are the edges smooth or jagged?  What does that tell you about the speed of formation?
3. Take a picture at the site without revealing the answers.

Permission to enter entrance common areas to observe has been granted by building management