ERB222 Chap.9 Mapping and Sampling Igneous Rocks
Mapping and Sampling Igneous Rocks
Observation comes first
Week 7 prepares you for the Noosa field trip by treating earth science as an observation-based discipline. Mapping the extent of each rock type constrains where and when it formed and how it relates to its neighbours, so the trip combines outcrop descriptions with an introductory mapping exercise on the headland.
The workshop refreshes compass use and ends with an individual campus competency test that aims for full marks.
Measuring and recording orientations
A geological compass reads reversed on its face and carries a clinometer; the Krantz model used at QUT returns dip azimuth and dip in one position once the declination of +11 degrees east for Brisbane is set.
The unit records planes either as dip azimuth and dip or as strike, dip and a dip-direction quadrant, converting between them by 90 degrees, and draws them with the standard strike and dip symbol.
From maps to a marked report
The Field Project is worth 15% and has two parts.
The group submits a field fact map, a complete geologic map and a key sample description; each student writes a field report that keeps description, interpretation and rock names apart and uses normalised whole-rock data for classification, petrogenetic relationships and tectonic discrimination.
What this chapter covers
- 01
Geological compass and magnetic declination
- 02
Strike, dip and dip direction notation
- 03
Converting strike to dip azimuth
- 04
Lineation trend and plunge
- 05
Field fact map versus geologic map
- 06
Field report structure and criteria
- 07
Whole-rock data treatment
Worked example · free
Convert a strike and dip reading to dip azimuth notation
- 1The dip direction is NE, so add 90 to the strike: 310 + 90 = 400.
- 1Remove a full circle: 400 - 360 = 040, which sits in the NE quadrant and agrees with the logged dip direction.
- 1Record 040/35 with three figures for azimuth, two for dip and no degree sign; 130/35 NE is the same plane, because strike runs both ways.
Key terms
- Field Fact Map
- A map completed in the field that shows only observations, such as stops, outcrop areas, measurements, traced contacts, structures and rock types.
- Dip Azimuth
- The compass bearing of the direction in which a plane dips most steeply, always at 90 degrees to the strike.
- Magnetic Declination
- The angle between magnetic north and true north, which varies with place and time and must be set on the compass in each new area.
- Loss on Ignition
- The percentage weight lost when a powdered rock is heated to over 800 to 1000 degrees Celsius, mainly H2O and CO2, used as a guide to alteration.
Mapping and Sampling Igneous Rocks FAQ
What is the difference between a field fact map and a geologic map?
The fact map is made in the field and records only what was observed: numbered stops, outcrop areas, strike and dip readings, traced contacts, structures and rock types. The geologic map is the interpretation built on it, with the whole area filled by units, boundaries drawn solid or dashed by certainty, and a legend in relative stratigraphic order.
How do you convert strike and dip to dip azimuth?
Add or subtract 90 degrees from the strike, choosing whichever result falls in the recorded dip-direction quadrant. A strike of 070 dipping SE becomes a dip azimuth of 160, written 160/25 for a 25 degree dip. Use three figures for azimuth and two for dip, and leave out the degree sign.
Why do you need to set magnetic declination on a geological compass?
The needle points to magnetic north, but most maps use true north, so uncorrected readings plot rotated on the map. The compass guide gives Brisbane as +11 degrees east, measured in 2018, and expects you to check and reset the value whenever you move to a new mapping area.
Do trace elements need to be normalised for a field report?
No. Only the major elements are normalised to 100 wt% on a volatile-free basis, after deciding how to report iron and excluding loss on ignition from the sum. Trace elements, reported in ppm, are used as supplied. Any recalculation of iron between ferric and ferrous forms has to be stated in the report.
Exam move
Practise converting ten invented readings each way between dip azimuth and strike notation until the quadrant check is automatic, then sketch each as a map symbol. Rehearse the compass workflow, including the declination setting, before any field day.
For the report, build a template with separate Description, Interpretation and Rock Name headings, normalise one analysis by hand, and tick your draft against every criterion on the report and map sheets.
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