Why Do Rocks Smell?

Objectives:

To become familiar with some of the rocks that may act as petroleum source rocks and their properties.
To identify some of the physical properties of sedimentary rocks.
To discover that organic matter is normally confined to the fine grained sedimentary rocks.
To discover that rocks do have odours and deduce one reason why.

Background Information: Most sediments contain organic debris consisting of the remains of plants and animals. As the sediments are slowly buried they become compacted and the particles are cemented together to form sedimentary rocks. If conditions were right for the preservation of the organic matter, such as rapid burial or the absence of oxygen, then deep burial also serves to mature the organic matter. With continued burial and heating over time the organic matter may well produce hydrocarbons. Some of the hydrocarbons may migrate out of the source rocks but the process is never completed. As a result, residual organic material and its by-products are usually retained, especially in very fine grained, dark shales or limestones. Breaking these rocks will expose fresh surfaces and allow the hydrocarbons to escape into the atmosphere.

An effective method to reinforce, or even introduce this concept is to bring into the classroom a quantity of mud (muck, ooze!) from a near-by swamp, lake bottom, or other permanent standing body of water, and if not already rich enough in odour, allow it to 'ferment` for a week or so (with a loosely closed cover). The students should readily identify the odour and likely cause.

You may wish to see if the students have had an introduction to the three basic rock groups, although this is not necessary. The experiment will lead them to some sort of crude classification system of their own and, if their powers of observation are fairly keen, they may be able to classify the rocks according to texture which will be a lead-in to the rock groups.

Materials: Hand-sized pieces of black shale, sandstone, limestone, granite, basalt and various metamorphic rocks

Rock hammer or ball-peen hammer

Gloves

Record sheets & pencil.

Method:

Have students experience the "hands-on" approach and test their senses and ability to record physical properties such as colour, texture, hardness, smell, feel, density, etc.
Ensure their observational records allow for the variables above.
Ensure that the safety equipment is used and safety recommendations are followed. Carefully monitor the students' efforts to break the rocks. The efforts need not be too vigorous if your specimens are of reasonable size. The harder rocks will cause small sharp pieces to fly off when the rocks are struck.
Examine each sample carefully before breaking it. Record your observations as to:

colour
texture (means how you would describe the feel of the rock: smooth, rough, fine, coarse, slippery, wet, dry, warm, cool, etc.)
density (heavy, light)
layered looking or massive
any other features you observe
Can you identify the rock? If so, what is it and to what rock group does it belong (sedimentary, igneous, metamorphic)?
Break one rock at a time and record your findings. Begin immediately with any odour you may notice. Note, you will have to quickly bring a freshly broken surface very close to your nose. Was the rock easy or difficult to break? How did it break (smoothly, with rough edges, with splinters, with sparks)?
What conclusion can you make?

Expected Responses

Last Modified: 2004-12-10