Built by a geologist

Rock ID in the Field

Putting it all together - practical strategies for identifying rocks in the field when perfect samples and laboratory equipment aren't available. Systematic Field Approach Step 1: Classify Rock Type Ask yourself: - Are there visible crystals of similar size? → Likely igneous intrusive or metamorphic - Is it glassy or has tiny crystals? → Likely igneous extrusive - Are there layers, fossils, or sedimentary structures? → Likely sedimentary - Is there foliation or banding? → Likely metamorphic Step 2: Apply Type-Specific Criteria For Igneous: 1. Estimate texture phaneritic, aphanitic, porphyritic 2. Estimate color index % dark minerals 3. Identify key minerals if visible 4. Match to classification grid For Sedimentary: 1. Is it clastic, chemical, or organic? 2. For clastic: estimate grain size 3. Look for fossils and structures 4. Check for HCl reaction carbonates For Metamorphic: 1. Is it foliated or non-foliated? 2. For foliated: assess grade slate→phyllite→schist→gneiss 3. Identify index minerals if visible 4. Consider protolith Essential Field Kit - Hand lens 10x : See small crystals and textures - Pocket knife : Hardness testing - Dilute HCl 10% : Carbonate identification - Magnet : Magnetite identification - Streak plate : Mineral identification - GPS : Record locations - Field notebook : Document observations - Sample bags : Collect specimens Common Mistakes to Avoid 1. Weathered surfaces : Always examine fresh breaks 2. Wet samples : Water darkens rocks - let them dry 3. Single diagnostic feature : Use multiple properties 4. Confirmation bias : Consider alternatives 5. Poor documentation : Note location, orientation, context Documentation Best Practices For each sample, record: - Location : GPS coordinates - Context : Outcrop description, structural position - Fresh color and texture - Grain size and mineralogy - Structures : Foliation, bedding, veins - Photo : With scale bar - Preliminary ID : With confidence level