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01003110 Jia Chengling

01003110 Jia Chengling. Columnar Joints. Columnar joints are prominent in basic lava flows and shallow intrusions.

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01003110 Jia Chengling

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  1. 01003110 Jia Chengling Columnar Joints

  2. Columnar joints are prominent in basic lava flows and shallow intrusions

  3. Such shallow and quickly cooled igneous bodies are typically fractured into long polygonal columns oriented approximately perpendicular to the outer surface of the lava

  4. WHY? The fractures are a result of differential contraction during the cooling of the lava.

  5. （1） inhomogeneous cooling of the lava (2)fixed boundaries of the country rock with shrinking lava. two mechanisms

  6. both the top and the base of the flow are placed inhorizontal tension the country rock just below the lava is placed inhorizontal compression

  7. Once the lava is too cool to flow,the stress is relieved by tensile fractures perpendicular to the outer surface of the lava.

  8. Tensile fractures will propagate from the top of the flow downward and from the base of the flow upward, but it will not propagate any significant distance into the country rock because it is in compression.

  9. Spacing of columnar joints wider in the lower colonnade than in the upper of a lava flow reflects the higher thermal gradients in the top of the flow during much of the cooling

  10. two joints intersecting, one abutting the other in a right-angle,T-intersection (2) three joints joining in a Y-intersection Joint intersections

  11. T-intersections If one joint is growing toward another preexisting joint, it must come in at a perpendicular because the preexisting joint, being open, is a free surface; therefore, the principal stresses are parallel and perpendicular to the surface.

  12. Y-intersection As a crack starts to grow out from a flaw in the material, it accelerates. When the velocity of propagationreaches about 0.6 times the shear-wave velocity, the stresses near the tip of the crack are sufficiently reoriented through dynamic effects thatthe crack bifurcates into two more slowly propagating cracksrunning off at 120 degree.

  13. Giant's Causeway Y-intersection T-intersection

  14. THE END

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