Half and Half

To me, every rock tells a story, and I find this one particularly interesting. It is clearly a piece of what geologists call a "contact," a point where two different rock types abut one another. This can be two layers of sedimentary rock, reflecting a change in the depositional environment (e.g. a layer of mud, followed by a layer of silt) or...

(NOTE: if these were sedimentary layers, they would have been laid down horizontally, not vertically as displayed..)

...or they could represent a molten rock intruding into a crack in an existing rock. (in which the orientation could be in any direction) For example, the black rock may be part of a basalt dike that intruded into whatever the gray-green stuff is.

Or maybe there is something else going on. But whatever the story, it was a striking find on the beach this morning...

Structure

Complex cobble, Lincoln Park

Geology was my first major at the University of Washington, and although I eventually got bogged down in the chemistry and calculus, I loved the field trips. We rafted down NW rivers, hiked in the mountains and even chartered a plane for an educational - and spectacular - flight around Mt. Rainier.

My favorite course was one called Structural Geology, in which we used the patterns of rock to understand their history and formation. From a hand sample, we learned to use the patterns of intrusions, folds and colors to trace its story.  It was challenging, but fun.

Since then, I have been fascinated by rocks with complex stories to tell. It may be hard to see in this picture, but the middle of this rock (found this morning in Seattle) is a contact between two different kinds of rock, both with strong, but perpendicular linear elements. One half  is dark with layering along the rock's long axis - the other half is much lighter in color, and with layers at a 90 degree angle. Weird, huh?

Weird...and beautiful.  I don't know what to call it, or how it formed; I'll know more after I've polished it and can study it a bit more.

Complex Stories

Banded Agate/Jasper, Olympic Peninsula

When I studied geology at the University of Washington, my favorite class was one called "Structure" in which we tried to piece together the sequence of events that have altered rocks and landscapes by reading the stories within them. If one continuous layer in a sedimentary rock is offset along a plane, for example, you could infer that a fault had shifted AFTER the deposition of the layers. (see the faulted jasper in my August 25 post). Sometimes, however, multiple events can overlap, creating a very confusing picture.

That's what's happening with this rock, which I found among beach cobbles on the Pacific Coast of the Olympic Peninsula. I'll call it an agate/jasper (or "jaspagate" as I've seen it written elsewhere) which is simply another name for a rock that has features of both: the transparency of agate and the opacity of jasper.  Whatever you want to call it, this small pebble seems to tell the story of fracture and fill: an existing red jasper was fractured, possibly several times, and injected with quartz solution - both clear and vivid red. How did this happen?  I have no idea, but whatever the story, its this complexity that helps create this handsome visual pattern, one with tantalizing hints of the paintings of Jackson Pollock.