Day 3: Santa Rosa to Weed

 This is the first day we are beyond the holiday weekend range.   Things look different.   This is also when we begin to truly go inland.

So much of what geology texts teach intro classes is a simplified model. The real world is so much more complex.   A lot of what we saw today was not photographed (i may check the dashcam) but it was a major piece of a former active margin that even now at the Weed American Bistro I am struggling to make sense of (Also when the heck are we going to get our Pizza?)

From Santa Rosa to somewhere past Lower Lake you have 2 stories:  the younger story is an oblique stretching of the crust as the cold oceanic lithosphere that was once below is replaced by warmer asthenosphere.   The decompression melting gives you features like Hood Mountain, Mt St Helena and Mt koncoti (still active).  The weakening of thr crust results in deep valleys like the Napa and clear lake basins . . . abrupt transitions between harrowing steep descents and the flat valley floors.  Picture here is Hood mountain above the Sonoma valley

The rocks these events cross cut reflect even greater complexity . . . roughly from west to east you have an ophiolite complex transitioning to a forearc basin.   Lots of serpentinite, some pillow basalts,  likely other elements i was unable to spot.  Very few places to pull over here and take it all in, just dollar generals, gas stations, wineries and defunct resorts.   The roadside was more prosperous decades ago in a time when people had a different sense of what travel meant something different.  

A lot of this has burned in recent years.   I would love to do a whole bit about warming and drier climates in this region, for it is all real . .  just so much.  You might see it in some of our pictures.

Then we turned east.  CA 20 tours some impressive exposures as we descend the leeward face of the coast ranges.  In contrast to points south the ridges are straighter,  the sediments are less chopped up by subsequent actions.  Some of the Coast Range get quite large here . . . relics from more recent subduction and compression I would imagine.

My favorite stretch of this road was between CA 16 and Williams, where we turned north and essentially followed the strike of one of the bedding planes, presumably of some sediments of the great valley sequence that were less resistant to erosion,  Then we tuned east crossing the eastward dipping beds of the Great Valley sequence going forward in time.  

you can catch the video here 

The Great Valley represents a good bit of the weathering products eroded from the Ancestral Sierra.  When one looks at the impressive granites of the Sierra Nevada one sees the deep roots of what was once a chain of volcanoes dwarfing the modern day cascades (where I type this now).  In order for those deeply formed rocks to come to the surface, a lot of erosion needed to happen.  Those erosional products had to go somewhere.  A lot of them piled up in the forearc basin in places this sequence of rocks is up to 40,000 feet thick. read a bit more about it here https://en.wikipedia.org/wiki/Great_Valley_Sequence 

Essentially we are making our way through an active margin - the place where continents grow.

Interesting Earth Science in the Sacramento Valley

One of the first things we see in the Valley is Sutter Buttes.  These igneous rocks represent a volcanic system that formed in the last few million years.  Sometimes affectionately referred to as the "World's smallest Mountain Range"  their highest summit top out at ~ 2100 feet.   I don't know if they are a relic of magma left over from the Cascade subduction zone or part of that "Slab window" volcanism taking place further west.  Generally thick sediment packages tend to suppress volcanism, as the formation of partial melts favors decompression . . . for now they are a mystery.

Some of the more interesting Sacramento Valley crops include Rice, indeed the valley is reputed to produce a decent chunk of rice exported to the rest of the world including some of the most premium sought-after varieties.  We saw some of this along the 5 freeway. We also saw olive trees.

And then the Sierras.  The valley narrows noticibly as one goes north.  By Red bluff you have left the flatlands and begun a mild climb.  Somewhere just south of red bluff was our first sighting of Shasta.

It almost doesn't look real.  Its scale and distance done quite make sense pyschologically to where if you look at directly you may not recognize it.  Your brain will find other patterns to match it.  Also challenging our sighting are haze and clouds.  the mountain makes its own weather and it can be sunny in the valley and very different on the higher flanks.

Redding and the Sundial bridge

We stopped for some forgotten provisions at the Redding Target.   We had heard of a giant sundial not far away.  It seemed worth time to check it out.  

It's hard to capture the scale of this, except to say you can see its shadow move in real time.  Shadows move opposite the sun.  We've already covered more than 7.5 degrees of latitude and we are starting to notice our shadows at solar noon grow longer, and the days ending later.  some of this is also Longitude.  San Diego is about the same longitude as the Oregon Idaho border.  But now up here In Weed I am noticing the sun is also rising earlier.

  We are about 1/2 way to our northernmost point and actually still going slightly backward with respect to East west.  This will change starting on Wednesday.

This is also a great spot to view Lassen the southernmost Active Cascade volcano.  You can see it peeking over the cutbank on this meander in the Sacramento River

Entering the Continental arc (Cascadia)

Past Redding the Road Climbs in earnest.  Soon you pass 1000 feet and you're crossing one of California's Great lakes:  Shasta

This year it is as dry as anyone can remember.   The exposed shoreline reveals long hidden features and colors that stand out brilliantly - the kind of beauty that is associated with an approaching cataclysm.      

The Cascades are America's textbook subduction zone.  Cold oceanic lithosphere slides under continental lithsphere.  When the top of that seafloor reaches mantle at a critical temperature ~ 100 km down or so, a reaction forms Magma, that then migrates upward, full of trapped gasses.  The result is some of the most dangerous mountains in the lower 48, including Mount Saint Helens, which erupted in 1980 in a violent show damaging lands for miles in all directions.  

While the stratovolcanoes dominate the high points of the Cascades, the long history of uplift and erosion also exposes older magama chambers formed earlier at greater depths.  We see this in the Castle Craigs Just west of I-5 - their own testimony to erosive forces and rapid uplift.

We have spent a good chunk of our day in the Sacramento river basin, with the approach of Black butte on the horizon we are now soon to leave this and join the Klamath basin.  It is still California, but culturally, geologically, and botanically we see more and more of Oregon and Washington.  

I leave you all with one last shot of Mt Shasta from our Motel room in Weed.  More to come on Day 4