Rock Basics: Background to Understand Earth’s Landscapes The Wave – Western Australia The Wave – Utah
Jan 17, 2016
Rock Basics: Background to Understand Earth’s Landscapes
The Wave –Western Australia
The Wave – Utah
Composition of the Continental Crust
• O- Oxygen 47%• Si- Silicon 28%
These together = 75% of continental crust • Al- Aluminum 8.1%• Fe- Iron 5.0%
• Ca- Calcium 3.6% - nutrient• Na- Sodium 2.8% - nutrient• K - Potassium 2.6% - nutrient• Mg- Magnesium 2.1% - nutrient
Examples
Silicates: have Si as base, Quartz (Si02)obvious 75%
Feldspars: have Si and Al, and when add (Ca, K, Na) = Feldspars
Clay minerals stick to your boot and have layered structures
Colorado & Minerals
Basic Rock TypesIGNEOUS SEDIMENTARY METAMORPHIC
Igneous (made by “fire”) - Solidified from molten rock (i.e., magma).
Sedimentary - Deposited (strata) and buried close to Earth’s surface.
Metamorphic (“changed form”) - Transformed from preexisting rocks under high pressure, temperature & fluids.
Rock TypesSedimentary
rocks are most abundant near Earth’s surface- poor preservation
Igneous and Metamorphic rocks make up most of the crustal volume
- limited exposure!
Sediments make up only 5% by
volume
Sediments make up
75% surface area
Entire classes on rock minerals…but for us…
• Minerals crystallize from melt, derived from deep within Earth’s crust or mantle
– High temperatures, 700° C & more
– Crystal size depends on cooling rate• Small = rapid; large = slow (millions of years)
• Intrusive rocks cool slowly within deep magma chambers:
– Coarse, interlocking crystals
• Extrusive rocks cool rapidly at (or near) the surface of the earth:
– Fine-grained, often “glassy”
Details on Igneous Rocks
Igneous Rocks
Common in volcanic areas & plate boundaries
Basalt
• Loose particles (sand, silt, marine shells) accumulate on shorelines, basins, rivers, etc.,
–Clastic Sediments (clast = “little”)
• Minerals precipitate from dissolved chemicals in water
–Chemical & Biochemical Sediments
• All are the products of Weathering—the breaking apart & decaying processes, and Erosion—the transportation processes from source to point of deposition
Details on Sedimentary Rocks (strata)
Lithification
Shale
Sandstone
Conglomerate
Breccia
Large to Small
• High temperatures and pressures at depth cause changes in mineralogy, texture, and composition (very complex…lots of ways to squish rocks)
– Changes take place in Solid State by recrystallization and chemical reactions
– Temperatures greater than 250° C, less than 700° C
• Regional Metamorphism - High pressures and temperatures derive from regional collision, deformation and mountain building (tectonics)
• Contact Metamorphism - Locally high temperatures, adjacent to intrusions
– Igneous body touching & “cooking” surrounding area
Details on Metamorphic Rocks
Mafic & felsic minerals align…
Metamorphic Rocks
Common at convergent
plate boundaries
Notice alignment of minerals = foliation
Metamorphic Rocks Fig 4.6
• Foliations - Planar fabric defined by
– Alignment of platy minerals (micas & clays)
– Alternating bands of mineral types
• Indicative of high pressures and deformation during formation
– Pressure-Temperature-time paths
• Not Foliated
• Distinct low- pressure minerals
Typical Rock Types Seen by Geologic Origin
Grain
Aspects
Clastic Solution (carbonate)
Foliated Non-Foliated
Intrusive Extrusive
Coarse Conglomerate
Breccia
Limestone & Dolomite
Gneiss Marble Granite
Gabbro
Diorite
Medium Sandstone
Siltsone
Limestone
& Dolomite
Schist
Phyllite
Quartzite
Fine Shale
(Mudstone)
Calcareous Mudstone
and Chert
Slate Amphibolite Basalt
Rhyolite
Obsidian
Sedimentary Types
Metamorphic Igneous Types
The Rock Cycle
-Melting & Intrusion-Solidification of melt-Mountain Building-Uplift & Exposure-Weathering-Erosion-Deposition & Burial-Metamorphism-Melting & Intrusion
Large blocks of time represented by changes in evolution
Geologic Time
Animation about Geologic Time
Geologic Time as Football Field: click here to see the animation
Plate Tectonics
Rearranging Earth’s Surface
Alfred Wegener and how a scientific paradigm operates
“Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence...It is only by combing the information furnished by all the earth sciences that we can hope to determine ‘truth’ here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw.Alfred Wegener. The Origins of Continents and Oceans (4th edition)
Field work: Atlantic once closed
Shape MatchRocks MatchFossils Match
Lifetime of work
Theory of Continental Drift
Meteorologist proposesto change geology
1st edition 1915
Until 1960s – Utter Rejection by Geologists
Dr. Rollin T. Chamberlin of the University of Chicago said, “Wegener's hypothesis in general is of the footloose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.”
“Utter, damned rot!”W.B. Scott, President of the American Philosophical Society
American Association of Petroleum Geologists organized a symposium specifically in opposition to the continental drift hypothesis.
Today: We Map Tectonic Plates
See animation
Today: We accept that most Earthquakes and Volcanoes occur along boundaries of
Tectonic PlatesSee animation
Today: We Accept Plate Movement
Today: We Accept Pangaeaand its breakup
Why the Opposition & Shift?
• Opposition:– An example of how a discipline can get “locked into” a
pathway of mental thinking for decades (paradigm)– An example of how a discipline protects its “turf” –
rejecting the thoughts of a meteorologist• Shift:
– Enough time for the opponents to die, and a bunch of new geologists in the 1960s to challenge “the establishment”
– Reminder throughout this class that ideas shift and much depends on the mental framework of the scientist
Structure of Earth allows continents to move
Wegener could not answer a fundamental question of HOW continents could move.
But geology advanced to the point to begin to understand Earth’s internal structure & how it allows continental drift...
Earth’s Structure
Brittle Crust Floats on Flowing, Plastic-like Asthenosphere
Different Crusts: 5/6th submerged(isostatic balance)
Ocean Crust Continental CrustMore Dense (sima) Less Dense (sial)
Isostacy crust adjusts to create a balance
(equilibrium)
Peep the ani...
Two continental plates under Tibet
To reach isostatic balance, Tibet Plateaurises
Too thick, so get uplift
Reason for Movement(we think…)
Excess heat from radioactive decay creates liquid outer core (and magnetic field), and forces mantle to have convection
TYPES OF PLATE INTERACTION
Stream Systems onDynamic Earth
Type of Plate Interactions:Divergent Boundaries
Mid-oceanridges areplaces of
plate divergence
Mid-Atlantic Ridge is the Icon
EastPacific
Rise
MidAtlantic
Ridge
Mid Indian
Ridge
Sea Floor Spreading:Key to Rejecting the Old Ways
• “Plate Tectonics” = continental drift + sea-floor spreading
• Rename the theory with new evidence:1. Potassium-Argon dating2. Paleomagnetism
Age of the Ocean Crust
Animation showing ages of ocean crust getting older away from spreading
Always New Details Being Added
Submarine Volcano erupting!
Paleomagnetism
Animations – Click on the PicturesVisualizing reversals of magnetic field and how this creates a mirror image on the two sides of a spreading center
Lava has minerals that “record” the magnetic field
Paleomagnetism in Symmetry
Conclusionwas thatsea floorsspreadingapart
Don’t Worry!
Hollywood movie “The Core” – silly
It will take thousands of years to shift that extreme
Source for Latest Info:
Website
Obvious question:With making new sea floor, why isn’t
Earth expanding?
Old ocean plates undergo “subduction”
Type of Plate Interactions:Convergent Boundaries
Oceanic-Continental Convergence
Denser Oceanic Crust SubductsTrench Forms
Volcanoes FormLithosphere Subducts Into AsthenosphereEarthquakes Common All Along Boundary
Andes
Nazca plate collide with South American Plate
Cartoon
Latest Research from Nature
Ocean – ContinentConvergencein PacificNorthwest
Oc-Co Convergence...
Island Arcs from Ocean – Ocean Convergence
Oc-Oc Converg ani...
Continent – Continent Collision
Example of Himalaya
Himalaya Formation
Formation of Appalachian Mountains resulted from the collision of the African and European Plates with North American Plate prior to 300 million years ago.
Ural Mountains
Formation of the Ural Mountains related to collision tectonics prior to 300 Ma ago.
Convergence & Tsunami
Effects Of Plate Locking
EQ (or landslide or volcanic eruption) Movement Generates the Wave that travels about 500
mph
AnimationsShows how Tsunami can be made from subduction
Shows time sequence in Tsunami destroying coastal city in Alaska
Mega-Quake set off 2004
Tsunami & will happen again
Energy ripple
Sumatra UpliftSri Lanka
Type of Plate Interactions:Transform Boundaries
Side-by-side motionalong San Andreas zone
Transform motion of the San Andreas Fault can be seen by offset of streams that cross the fault zone.
California Won’t Fall into the Ocean
Continent is 5/6ths submerged, so it is anchored very tight
Other Transform Boundaries
Hot Spots
Often in the middle of a plate
Focus here on Hawaii & Yellowstone
Hawaiian Hotspot
Animation of Hawaii as moves over Hot Spot
Trail of the Hot Spot
Note change in direction 43 myr agoChange in direction 43 myr ago
Alternative Hypothesis
Yellowstone Hot Spot