Basalt: A Hot Rock! EarthCache

What You Need and “Things To Be Aware Of”

You will need: a thermometer, set of scales, calculator (all as accurate as possible!), a pot, cardboard and a sample of basalt gravel that you will obtain from GZ. This is more technical than the average earthcache but if you follow the instructions closely you will get it right!

Background

Basalt is an extrusive igneous rock. This means that it has formed from molten rock erupting out of a volcano rather than cooling slowly underground. As a result of this relatively rapid cooling in air it usually has only very small crystals of minerals (try looking for these with a hand lens at GZ). The Blackhead quarry area was a peripheral vent of the Dunedin Volcanic Group which was active 10-13 million years ago.

Basalt is the earth's most common igneous rock. Its uses include construction (basalt building blocks, pavers, bench tops) highways (as aggregate in asphalt, and volcanic chip highways), statues, hangi stones and when heated and extruded it can be turned into stone wool which is a useful insulator.

Basalt's use as hangi stones or building blocks stems from its relatively high specific heat capacity. Materials with a high specific heat capacity take a long time to heat up - thus a house made of basalt stays cool well into the summer. They also store a lot of heat energy (useful in hangis) and take a long time to release the heat - thus your basalt house releases its heat gained during the summer well into autumn keeping the house warm.

Your task is to carry out an experiment to calculate the specific heat capacity of a sample of basalt that you will obtain from at or near the quarry. It is best to grab a handful of basalt chips rather than sand or cobbles.

The specific heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1g of a substance by 1^oC. You can find this out for basalt if you can put a known amount of heat into a known mass of basalt and can measure the temperature change.

The formula for heat capacity (C) is:

C = Q / T x m

Where:

C = specific heat capacity. Unit: "joules per gram degree Celsius" or J/g^oC

Q = change in amount of heat energy. Unit: joules or J

m = mass. Unit: grams or g

T = change in temperature. Unit: ^oC

Difficulties arise in putting a measured amount of heat into the rock and sticking a thermometer onto the side of some basalt gravel won't exactly give you an accurate reading.

Here is what you will do to overcome that: We know C for water, can measure its T with a thermometer and find its mass, m easily with scales. We can rearrange the above formula to make Q the subject.

Q = m x C x T

So, that is enough to find the amount of heat energy, Q put into some water.

So, weigh some water (m_water), chuck it a cup. Leave it for twenty minutes. Take it's temperature (T_waterstart). Take your stones, weigh them (m_stones), chuck them in a pot of water, let them boil for ten minutes, no lid. As water boils at 100^oC (T_waterend) then the temperature of the stones (T_stonestart) is also 100. Say something while you wait like "bubble, bubble boil and trouble, fire burn and cauldron bubble" - hey, doesn't that feature in another Dunedin cache? Chuck the stones in the cup. Put a cardboard lid on (Why? You want the heat to go into the water not the air). Push the thermometer through it and watch the thermometer closely. Record the maximum temperature the water gets to (T_waterend) this is also the end temperature of the stones (T_stoneend). The specific heat capacity for water is 4.18 J/g^oC (C_water).

Plug your measured numbers into the following formula to find out how much heat went into the water and therefore was in the stones. Becareful of your mathematical order of operations!!

Q = m_water x C_water x (T_waterend - T_waterstart)

= ________ x 4.18 x _________

= ________J

Take the calculated heat energy and plug that into the equation below along with your other measured numbers to find the specific heat capacity of the basalt.

C_basalt = Q / ((T_stonestart - T_stoneend) x m_stones) !!do the bits in the brackets first!!

= _________ J/g^oC.

Whew! Well done!

I followed this method to get a result within 0.06 J/g^oC of the assumed specific heat capacity.

Logging Requirements

0. Preface any email with GC28QY0 Basalt: A Hot Rock! in the subject line and title and clearly note the question number and answer. You don't need to wait for a confirmation email to log but you might be asked for more info if your answers are slim.

1. Email me your calculated Q and C_basalt.

2. Upload a photo of either the rock sample being taken or the tests being done.

2010 Stegan’s EarthCache Masters Programme

Since it is not very feasible for many NZers to easily get more than two states for the earthcache.org masters programme and therefore not get past Bronze I’m running my own one on a number of Earthcaches that I plan to release this year (all things being equal) with the following points system:

5 points: First to find

3 points: 2nd to find

1 point: Logging a find

1-5 points: Special bonus questions.

Shared points for joint finds.

At the end of 2010, I’ll issue certificates for 1st, 2nd and 3rd and the first place getter will be awarded a geocoin. If I feel richer and/or it looks like this is popular I’ll upgrade the prizes.

Updated information and Leaderboard at: Stegan’s Earthcache Programme


CO is open to constructive criticism on this cache (via geocaching.com messaging). Cheers!