Where is Weathered Igneous Rock found?
Igneous Rock (likely basalt or diabase, heavily weathered)
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Open the appWeathered Igneous Rock occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Weathered Igneous Rock
Deccan Traps, India
Extensive basaltic lava flows that have undergone significant weathering, forming thick lateritic soils and saprolite profiles.
Columbia River Basalt Group, USA
Large flood basalt province in the Pacific Northwest, where weathering has produced saprolitic zones, particularly in older flows.
Brazil and West Africa
Regions with extensive lateritic weathering profiles developed over mafic igneous and metamorphic rocks, often rich in iron and aluminum oxides.
Appalachian Mountains, USA
Diabase dikes and sills (e.g., Triassic-Jurassic) are often deeply weathered, forming saprolite that can be several tens of meters thick.
Collecting tips
- Look for Color Changes. Observe changes in rock color from dark gray/black (fresh) to reddish-brown/yellowish-brown (weathered) along road cuts, stream banks, or excavations.
- Test Hardness and Friability. Use a geological hammer or even your hand to test the rock's integrity. Weathered rock will be significantly softer and more friable than its fresh counterpart.
- Identify Relict Textures. Even in heavily weathered samples, relict textures like original grain boundaries, vesicles, or joint patterns might be preserved, providing clues to the parent rock type.
- Check for Clay Content. Rub a small amount of the material between your fingers; a smooth, plastic feel indicates a high clay content, characteristic of weathered mafic rocks.
- Observe Soil Profiles. Weathered igneous rocks often form the lower part of soil profiles (C-horizon or saprolite layer) above fresh bedrock. Look for transitions from soil to weathered rock to fresh rock.
Quick Check
- Color:
- Reddish-brown, yellowish-brown, mottled gray/brown
- Luster:
- Dull, earthy
- Streak:
- Variable, often brownish-red to yellowish-brown (due to iron oxides)
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