The effect of intrusions

Today we processed a lot of core, to a depth of 858 ft.  Last night, the drillers were able to advance downward a significant amount because the rock was so loose that they could push right down through it without the bit having to cut much.  You may recall that they encountered conditions like this before, when we drilled through what we believe was the cinder of a cinder cone.  However, this time the loose rock was created by a different process.

The photo above (co-worker's hand for scale) shows the bottom contact of an intrusion that measures ~16 ft thick.  The rock on the upper side of the photo has been welded together, but the rest of that unit is intensely broken (as unconsolidated as the cinder we encountered earlier in the hole).  Similarly, the rock above the intrusion is also broken to a cindery mush.  The intrusion itself is in great shape, as you can see by the unbroken nature of the lower part of the core shown in the photo.

I think that this intrusion caused the breakup of the rock above and below it, both fracturing it and baking it (there is abundant, black baked olivine in the rock above and below the intrusion, not to mention the intense baking of the loose rock fragments themselves to a bright orange color).  In addition, there may have been groundwater in the vesicular rock that was invaded, which was turned to steam by the intrusion's heat.  The expansion of this steam probably contributed significantly to baking and breaking apart the surrounding rock, particularly above the intrusion where the most extensive baking is found. 

This intrusion is different from the one described yesterday in that it is at least five times thicker.  Yesterday's intrusion didn't significantly affect rock above and below it, because it was thinner and cooled more quickly.  Both intrusions support the idea that we are drilling in an area not far from the eruptive source of the lavas that the drilling is sampling.