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Mountains to Bay: A travelogue by a sand grain. Karen Bemis MARE August 7, 2007. The life of a sand grain. Observations and Interpretations. New Jersey Beach Sand. What is it? Where is it from? How did it get here?. Sand from other beaches.
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Mountains to Bay: A travelogue by a sand grain
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The life of a sand grain
Observations and Interpretations
Sand from other beaches
Close-up of sand from a beach in Vancouver, showing a surface area of ~1-2 cm2. (Wikipedia)
Close up of black volcanic sand from Perissa, in Santorini, Greece. (Stan Zurek, Wikipedia)
Sand from Guam. (http://www.microscope-microscope.org/applications/sand/microscopic-sand.htm)
Coral sand. (Wikipedia)
Some questions to ask at a beach
- Overall composition
- Minerals? Shells? Other material?
- Color
- White? Yellow? Red? Black? Green?
- Shape
- Rounded? Angular?
- Long & skinny? Round?
- Size
- Mud? Normal sand size? Pebbles? Boulders?
How do these sands differ?Shape
Angular
Coral sand from Hauhini, Tahiti
Intermediate
Shell sand from St Maartin, Caribbean
Angular
Shell sand from Lake Powell, Utah
Round
Shell sand from North Shore, Oahu, HI
Rounding
Sediment grains get rounder and smoother as they bang together -- just like agates or stones put in a tumbler to polish!
High energy environments can round grains
Round
Shell sand from North Shore, Oahu, HI
Angular
Coral sand from Hauhini, Tahiti
Energetic beach
Calm beach
What can you infer about NJ sands?
Are they in a high energy environment? Today? In the past?
Do they come from far away? Or nearby?
What’s different about these beaches?
Pebbles at the beach, Bay of Fundy, Nova Scotia (T. Mcculloch)
Red sand beach, Santorini, Greece (kgb)
Mudflats in Brewster, Massachusetts. (Wikipedia)
Pebbles at the beach, Budleigh Salterton, England (I. West & D. Harvey)
What’s different about these beaches?Not sand!
Big angular grains
Big round grains
Red sand beach, Santorini, Greece (kgb)
Tiny grains
Mudflats in Brewster, Massachusetts. (Wikipedia)
Pebbles at the beach, Budleigh Salterton, England (I. West & D. Harvey)
Grain size related to the combination of creation (erosion) and transport
Smallest grains at final resting place
Median grains en route
Big grains still being broken down
Physical Weathering: Mechanically breaking rocks down into smaller chunks.
What can you infer about NJ sands?
Are they in a high energy environment? Today? In the past?
Do they come from far away? Or nearby?
Sand grains are parts of rocks or individual minerals.Two factors control exactly what rocks or minerals …
- Type of rock at source of sediment
- All grains of sand started with bigger pieces of rock somewhere else.
- Time
- Chemical weathering takes time.
Chemical Weathering: The effects of time
Relative Mineral Stability:
Olivine + Pyroxene: Least stable
Feldspar: Somewhat stable
Quartz: Stable
Chemical Weathering: The chemical reactions that destroy or alter rock at the Earth’s surface.
What can you infer about NJ sands?
What kind of source rocks do you expect based on the minerals or rock bits you see?
Are there locally derived materials like shells or shell bits?
The life of a sand grain
A plate tectonic interlude
Plate Tectonics: a framework for geologic processes
Midwest
East coast:
New Jersey!
West coast:
Oregon & Washington
Plate boundaries
Defined by earthquakes ( ), volcanoes ( ), and trenches ( )
Convergent Margins
Continental rift
Subduction Zone
Continental Collisions
West Coast
East Coast
Causes of mountain building…
Continental Rifting
- Crust thins
- Asthenosphere rises
- Melting, volcanism.
- New ocean floor?
This happened on the East coast in early Mesozoic time as the dinosaurs started to populate the landscape.
Differences
Active Margin
e.g., subduction zone
Passive Margin
Former continental rift
- Wide shallow shelf (continental crust thins and transforms to oceanic)
- Subsidence
- Sediment deposition
- Narrow steeply dipping shelf
- Earthquakes, tsunamis, volcanoes!
- Erosion of new mountains
New Jersey’s Newark Basin is a failed rift system related to the opening of the Atlantic ocean
Geology and geography of New York Bight beaches at http://www.geo.hunter.cuny.edu/bight/index.html
Geology and geography of New York Bight beaches at http://www.geo.hunter.cuny.edu/bight/index.html
Paleozoic (old) interbedded sandstones and limestones
Valley and Ridge:
Highlands:
Precambrian (even older) marbles, gneisses and granites
Peidmont (rifting):
Mesozoic (dinosaurs) conglomerate, sandstone and siltsones (lots of red rocks)
Coastal Plain:
Late Mesozoic to Cenozoic (recent) Sand and clay deposits – not quite rocks yet!
Some of the material on NJ beaches may have been used several times!
The life of a sand grain
Putting it into a geologic context
Pulling the trigger…factors causing slope failure…
• Shocks and vibrations
• Changes in slope angles, slope load, and slope strength
The importance of tectonic setting…
• Earthquakes and vibrations
• Faulting and uplift
• Volcanic eruptions
Fig 7.34
Beaches move in and out as sea level changes:regressions and transgressions
Sea level changes with climate, ice caps and plate tectonics
Longshore Currents
Geology and geography of New York Bight beaches at http://www.geo.hunter.cuny.edu/bight/index.html
Sandbar = A moving target!
Geology and geography of New York Bight beaches at http://www.geo.hunter.cuny.edu/bight/index.html
Barrier Island: offshore sandbar that grew and grew and grew
Geology and geography of New York Bight beaches at http://www.geo.hunter.cuny.edu/bight/index.html
Diagram showing the environments of a barrier island system
(from Blatt, et al, 1980, Petrology of Sedimentary Rocks).
Where mud ends up!
Fig 18.5
Summary
- Interactions of “rigid” lithospheric plates results in mountains
- Mountains erode to create sediment, which gets progressively smaller, rounder and mature as it travels towards the ocean
- The final resting place of sediment depends on its size and density
- Sand ends up at the coast – beaches!
- Mud in the deep ocean
- The composition of a beach depends on
- The sediment source in the nearest mountains
- Local production of shells and other biologically related material
- The distance to mountains
- The energy of the average waves that reach the beach
- Barrier islands
- Form on shallow gently dipping continental shelves (usually associated with passive margins, that is, former continental rifts)
- Move along the coast as controlled by wave directions
- Are really neat! (I hope you agree. )
