FROST SHATTERING & WEDGING
Just about the commonest breakage shape for rocks which have natural cleavage planes is triangular, closely followed by trapezoidal and diamond shapes.
Frequently people remark on the resemblance to an arrowhead or spearhead shape of some piece of rock they have found – normally some kind of rock that is quartz-rich, and often it’s a piece of quartzite or slate. They also comment on sharp edges, a highly pointed tip, symmetry that can’t possibly be natural and so on.
Unfortunately, all of those characteristics are typical of rocks that have simply broken through natural causes and the commonest natural cause is frost shattering. Here’s a typical example of what can happen on an exposed rock face when mother nature gets going:
[Picture by LarsBr on panoramio.com]
This kind of breakage happens when water finds its way into the bedding planes of sedimentary (and to a lesser extent, metamorphic) rocks, or rocks which have a macro-crystalline structure. The planes and faces in such rocks are less weakly bonded such that when the water freezes during cold weather, the expansion fractures the rock. When water turns to ice it expands by about 10% in volume and that’s what creates the pressure which causes the fracturing. Symmetry is not at all uncommon since the nature of macro-crystallisation in rocks is that it usually follows a mathematical pattern with very precise angles. The angular pieces fall away and collect in a scree at the base of rock faces and may be carried away by water during periods of heavy rain or flood.
Even igneous rocks with a macro-crystalline structure are prone to this kind of shattering. In these cases, the breakage may follow the crystal planes within the rock where the bonding is less strong. Here’s some frost-shattered granite:
[pic by EA Fitz Patrick]
The forces created by ice can be extremely powerful, splitting even very large rocks by a principle known as “wedging”:
Glaciation produces even more extreme shattering and wider distribution of the fragments from either glacial movement or the sheer volume of melt-water after retreat of the glacier. Smoothing of the angular edges during water travel and tumbling may increase the resemblance to artefacts and create the impression of antiquity.
It’s not always necessary for water to be involved in these processes. In desert environments when there is an extreme difference between hot daytime temperatures and cold frosty nights, the expansion and contraction of the rock itself can create internal pressures from thermal stress that the rock may not withstand. Here’s a shattered rock from the Mojave Desert in California which has experienced this:
Just about the commonest breakage shape for rocks which have natural cleavage planes is triangular, closely followed by trapezoidal and diamond shapes.
Frequently people remark on the resemblance to an arrowhead or spearhead shape of some piece of rock they have found – normally some kind of rock that is quartz-rich, and often it’s a piece of quartzite or slate. They also comment on sharp edges, a highly pointed tip, symmetry that can’t possibly be natural and so on.
Unfortunately, all of those characteristics are typical of rocks that have simply broken through natural causes and the commonest natural cause is frost shattering. Here’s a typical example of what can happen on an exposed rock face when mother nature gets going:
[Picture by LarsBr on panoramio.com]
This kind of breakage happens when water finds its way into the bedding planes of sedimentary (and to a lesser extent, metamorphic) rocks, or rocks which have a macro-crystalline structure. The planes and faces in such rocks are less weakly bonded such that when the water freezes during cold weather, the expansion fractures the rock. When water turns to ice it expands by about 10% in volume and that’s what creates the pressure which causes the fracturing. Symmetry is not at all uncommon since the nature of macro-crystallisation in rocks is that it usually follows a mathematical pattern with very precise angles. The angular pieces fall away and collect in a scree at the base of rock faces and may be carried away by water during periods of heavy rain or flood.
Even igneous rocks with a macro-crystalline structure are prone to this kind of shattering. In these cases, the breakage may follow the crystal planes within the rock where the bonding is less strong. Here’s some frost-shattered granite:
[pic by EA Fitz Patrick]
The forces created by ice can be extremely powerful, splitting even very large rocks by a principle known as “wedging”:
Glaciation produces even more extreme shattering and wider distribution of the fragments from either glacial movement or the sheer volume of melt-water after retreat of the glacier. Smoothing of the angular edges during water travel and tumbling may increase the resemblance to artefacts and create the impression of antiquity.
It’s not always necessary for water to be involved in these processes. In desert environments when there is an extreme difference between hot daytime temperatures and cold frosty nights, the expansion and contraction of the rock itself can create internal pressures from thermal stress that the rock may not withstand. Here’s a shattered rock from the Mojave Desert in California which has experienced this:
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