\nSpreading zones are what produce oceanic crust, so by definition they are where the newest oceanic crust is; you will see the oldest oceanic crust farthest away from the rift.<\/p>\n\n\n\n
But you’ve got the causality reversed. The rift comes first, and then the ocean. That is, a rift opens somewhere and erupts, and if it continues it widens and lengthens and eventually opens all the way to an existing ocean, at which point it beomes an ocean. Over 10’s of millions of years it produces oceanic crust, and pushes apart the land mass where it formed.<\/p>\n\n\n\n
The foregoing assumes that the rift is forming on a continent — it is also the case that rifts can form in the ocean; and sometimes an existing oceanic rift will ‘jump’ to another location on the ocean floor.<\/p>\n\n\n\n
And, as hinted at above, sometimes rifts start to open and then ‘fail.’ This is the case in North America. About 1.1 billion years ago a rift opened in Laurentia (the archaic landmass that includes most of what is now North America), and gradually widened and lengthened until it was almost two thousand miles long. Had it continued, it would eventually have split Laurentia apart and resulted in an intervening ocean. But after 100 million years, it failed — I believe the leading theory is that a landmass collided with the eastern coast of Laurentia\/NA, and shoved the rift closed. Lake Superior is, in part, a surface remnant of this rift (although it is a bit more complicated than that). Look up “Mid Continent Rift” if you want to know more on this….<\/p>\n<\/blockquote>\n\n\n\n
#<\/p>\n\n\n\n
<\/span>Weathering (of Granite)<\/span><\/h2>\n\n\n\n\nNot at the exact same rate, but your flat granite barren will likely have lichen mechanically breaking down the rock, and pools of water that will accelerate alteration of the plagioclase into clay \u2014 and there will be freeze~thaw as well. Maybe a granite outcrop in a desert would weather more slowly (but there will still be wind and thermal contraction\/expansion). And, by definition, granite formed deep below the surface under great pressure so by the time it has reached the surface (or rather the surface has reached it) it will be unstable (as geologists reckon it) due to decompression.<\/p>\n<\/blockquote>\n\n\n\n
#<\/p>\n\n\n\n
<\/span>Weathering of Basalt and Gabbro<\/span><\/h2>\n\n\n\n\nCould be weathered\/altered basalt or gabbro. That would fit with the density\/location. If basalt the pits are gas bubbles, if gabbro the pits are left by grains of a softer mineral that has weather out. Cutting it open would tell you. Plenty of tectonic activity in California \u2014 I would guess Mt San Jacinto is the remnant of a volcano or pluton (solidified magma), but don\u2019t know So Cal geology.<\/p>\n\n\n\n
EDIT: didn\u2019t notice the last image \u2014 that makes me lean towards an altered gabbro or some other plutonic rock.<\/p>\n<\/blockquote>\n\n\n\n
#<\/p>\n\n\n\n
<\/span>Accretionary Wedges and Subduction<\/span><\/h2>\n\n\n\n\nThe phrase you may be referring to is \u201caccretionary wedge\u201d which refers to the sediment scraped off a plate that is subducting. California is very complex with multiple subductions and arc islands getting sutured onto the ever growing western edge. \u201cAssembling California\u201d by John McPhee is a very readable book on the tectonics.<\/p>\n<\/blockquote>\n\n\n\n
#<\/p>\n\n\n\n
<\/span>Weathering into Core Stones<\/span><\/h2>\n\n\n\n\nIt is most likely the boulders did not come from somewhere else.<\/p>\n\n\n\n
The idea is that wherever that mass of rock is, some it weathered away. In any mass of stone, there are likely to be joints and faults, and these erode away leaving chunks of stone. Just as ice cubes become more rounded as they melt, so do the chunks of stone weather more at the ‘corners’ or angles, and gradually become weathered.<\/p>\n\n\n\n
Look up spheroidal weathering in Wikipedia for a better explanation.<\/p>\n<\/blockquote>\n\n\n\n
#<\/p>\n\n\n\n
<\/span>Mafic\/Felsic Magmas\/Lavas<\/span><\/h2>\n\n\n\n\nLava\/magma ranges in composition from mafic (which contains lots of iron and magnesium and is dark and dense) to felsic (which contains more silicon and is less dense). Mafic lavas, which you find in places like Hawaii and Iceland and erupting in the mid-ocean ridges, are relatively thin and flow like liquids). Although these lavas have pretty similar compositions, they can have a variety of forms depending on the conditions under which they are cooling. Pahoehoe forms beautiful ropey wave patterns; a’a’ is very jagged; if the lave erupts under water it forms pillow basalt; and if you have a larger body of lava exposed to a cooling surface you get the six-sided joints forming perpendicular to the surface. All of these forms have the same composition, though over time they will oxidize and undergo other chemical alteration.<\/p>\n\n\n\n
Felsic lavas are very viscous (they have usually melted their way up through continental crust and have incorporated more silicon as a result, though its often more complicated than that) and when they reach the surface the explode out of the vents as ash and tephra and cascade down the volcano in what are called pyroclastic flows, or fall out of the air in ashfalls, leaving layered deposits. If you look closely at rhyolite, it may have a grainy composition with small bits of rock or crystals mixed into it.<\/p>\n\n\n\n
Of course, these are different ends of what is a continuum.<\/p>\n\n\n\n
# <\/p>\n<\/blockquote>\n\n\n\n
#<\/p>\n\n\n\n