Teacher Key Page
Keys to Stop Questions and Analysis:
STOP 1: Look at the pictures from STOP 1.
- What type of
rocks are in these pictures? bedded cherts
- Where and how does this rock
normally form? deep ocean environments, from the tests of
microscopic organisms called radiolarians
- Why is the rock all
twisted and folded? the rocks were compressed in a convergent
boundary/ subduction zone
- When was the rock twisted and folded- during
deposition or after? rocks were folded post deposition
- What is the dark sheen on the hand sample
in the last picture? patina from black smokers
- Where might that have formed? near hydrothermal vents at a
spreading ridge
STOP 2: Look at the pictures from STOP 2.
- What type of
rocks do you see in these pictures (2 types)?
greenstone/greenschist on the left and sandstones on the
right
- Where and how do
each of them form? greenstones are altered basalts formed originally at
spreading ridges from pillow basalts, the sandstones are from
turbidites from landslides near coastlines underwater
- What type of boundary is the contact of the two
rocks (where the people are)? a fault
STOP 3: Look at the pictures from STOP 3.
- What type of
rock is the main lighter colored rock in these pictures? pillow
basalts altered to greenstone/greenschist
- Where
does it normally form? at spreading ridges
- What type of rock is the darker rock, and
where does it normally form? deep marine shales
- What process probably caused this
finger of darker rock into the lighter colored rock? could have
formed from compression, and the shale got pinched up into the pillow
basalts as the pillow basalts formed, based on deformation of both
along contacts of the finger of shale.
STOP 4: Look at the pictures from STOP 4.
- What type of
rock is this in these pictures? (look closely at the arch pictures,
these give the best glimpse of the freshly eroded rocks) pillow
basalts. the arch shows fresh pillow surfaces, the lighter colored
rocks are the weathered and altered pillow basalts- altered to
greenstone/greenschist.
- Where do these rocks
normally form and how? form at spreading ridges by magma welling
up and cooling rapidly in the seawater at the ocean floor
- How might these rocks have gotten to where
they are now, from where they form? being scraped off of a
subducting sea floor, and then accreted onto the continent
STOP 5: Look at the pictures from STOP 5.
- What does it look like happened to form this outcrop? looks
like the basalts were shoved up under the sandstones, probably via
scraping off a sea floor subducting plate.
Analysis:
How might all of these rocks gotten from where they were deposited or
formed, to where they are now? prior to the San Andreas system,
the Pacific boundary was a subduction zone, with the oceanic plate
being subducted under the North American continent. As the sea
floor subducted, surface material was scraped off and accreted on to
the continent- including small volcanic sea mounts and guyots, pillow
basalts, and sea floor sediments.
What type of tectonics do you think were prevalent on the Pacific coast
around 175 my to about 65 mya, prior to the San Andreas Fault taking
over as plate boundary? subduction of a ocean plate under a
continental plate.
Does this type of tectonics explain the features seen in the Marin
Headlands today? yes. subduction zones are known to scrape
material off of the subducting plate and accrete this material onto the
continent. the scraping action of subduction adds "exotic
terranes" to the continent, such as the pillow basalts, sheeted dikes,
deep marine sediments and more. All of these get deformed and
folded as they are accreted (hence the folding of the cherts), and
faulted by the compressional action of accretion.
All STOP location rock and other interpretations and analysis are from
notes taken during the DiLeonardo
2011 Field Trip taken by the author on March 10, 2011