COMPOSITION OF THE EARTH
THE ROCK CYCLE
- Inner core is solid iron & some nickel
- Outer core is liquid iron
- A solid shell of ULTRAMAFIC materials surrounding the core
- THICKNESS – 2900km
- TEMP – 2800 – 1500 degrees C
- Thin outer layer composed of mafic (Mg, Fe) and felsic (Ca, Al, Si) rocks – mostly igneous
- THICKNESS – 8-50km
- TEMP – 0-1000degrees
- Boundary between crust and mantle is sharp; known as the Mohorovicic Discontinuity (MOHO)
- ~35 km thick
- 2 layers – upper felsic and lower mafic
- ~7km thick
- Composed of mafic rocks
THE CRUST & ITS COMPOSITION
- Crust: thin outer layer of the earth
- 8.5km thick
- All the relief on the Earth’s surface (from high mountains to low ocean floors) is contained in the
top ~10km (0.2% of total thickness)
MATERIALS AT THE EARTH’S SURFACE
- Rock: a solid, cohesive aggregate of grains composed of one or more minerals. Grains are
essentially all inorganic. - Sediment: Generally unconsolidated inorganic materials originally derived from weathering and
erosion (fractionation) of rocks. Deposited after transport by water, ice, air, or gravity.
- Organic matter: Deposits composed of mainly dead and decaying matter from organisms (plans
& animals). Includes soil (litter), peat, shell beds.
***The circulation of rocks = a slow process powered by the heat of earth’s interior
- Rocks constitute earth’s crust
- Formed by crystallization of (cooling) of molten material (magma)
o INTRUSIVE (plutonic)
Magma cools slowly below the surface
Larger crystals formed
o EXTRUSIVE (volcanic)
Magma cools rapidly at the surface
Small crystals formed
Can also be classified as mafic
- Intrusive rocks – granite – felsic
- Extrusive – basalt columns – mafic
ALTERATIN OF IGNEOUS ROCKS
- Chemical weathering can lead to formation of clay minerals (secondary minerals = sediments)
- Oxidation: addition of oxygen to minerals (rust)
- Hydrolosis: addition of water to minerals
Physical weathering breaks off individual mineral grains (fragmentation, erosion = sediments),
facilitates chemical weathering
- Deposition compaction and cementation (lithification) of eroded rock/mineral particles
- Derived from sediment and chemical precipitates that result from weathering and erosion
processes. Sediment and chemicals precipitates accumulate in sedimentary basins (lakes &
oceans) and are converted to sedimentary rock over time
- Salt flats form when lakes evaporate
HOW ARE THEY FOUND?
- Weathering of rocks > small mineral particles
- Particle transported by air, water or gravity > sediment - Sediment deposited in various environments > strata
- Over geological time, sedimentary strata become buried
- Physical and chemical changes > strata hardened, compacted and cemented to form
sedimentary rocks > lithifaction
- Radioactive decay, the sun (affecting the hydrological cycle) drive the rock cycle
- Sediments are small particles that haven’t been changed into a rock (lithification)
- Metamorphic: physical or chemical change of existing igneous or sedimentary rock by
application of heat and/or pressure
o Limestone > marble
- Igneous or sedimentary rock altered by intense heat and/or pressure (plate tectonics)
- Can classify metamorphic rocks by degree of metamorphism (grade):
o (LOW) Basalt > Greenstone > Amphibolite (HIGH)
Where does the heat come from?
Radioactive decay in the earth’s core
- Uranium (U) and Thorium (Th)
o Other elements + lead + radiation (about 90%)
- Potassium ( K)
o Calcium-40 (or Argon- 40) + radiation (10%)
Where does the pressure come from?
- Air pressure = 1 atmosphere (Atm) or 1 bar
- Pressure beneath 10 meters of water
o = 1 Atm. = bar
o = 3.5 m of overlying rock
- Pressure in the deepest part of the ocean
= 1000 bars
= 3 km of rock
- Pressure under 1 km of rock = 294 bars
The Tectonic Cycle
Upwelling, sea-floor spreading, subduction, crust formation
- There are 2 major layers in the outer part of earth
o Lithosphere and asthenosphere (not the same as the crust/mantle boundary) Lithosphere
- Geological term describing the outer, rigid rocky shell of the crust and uppermost mantle (60-
- Includes felsic, mafic, and ultramafic rocks
(2.7kg/m^3, 3.3 kg/m^3, >3.3kg/m^3)
- Heating of rocks below lithosphere results in “plastic” or soft rocks.
- Rocks are dense (ultramafic)
- SCALE NOTE: Together, the lithosphere and asthenosphere form a thin (300km) skin over the
earth (about 5% of its total thickness).
- Temperatures in the asthenosphere is higher (uniform temp)
- Fit like jigsaw puzzle pieces (or armadillo armor)
- Produces large earthquakes, volcanism, mountain range
- The movement, collision, bending and breaking of lithospheric plates due to internal earth
- Rigid lithospheric plates are “floating” on the denser, plastic asthenosphere
- The lithosphere is broken into large plates (tectonic plates
o Oceanic plates: consist of relatively thin, dense oceanic lithosphere (~60km, 3.3 kg/m^3)
o Continental plates: made of thick, light continental lithosphere (~150km, 2.7 kg/m^3)
- Plates are both flexible and brittle > fractures are known as FAULTS.
What drives this movement? >>> CONVECTION
PLATE TECTONICS AND CONTINENTAL DRIFT
- In 1915, Alfred Wegener proposed that landmasses were once united as a single great
continent, PANGAEA, surrounded by a single great ocean, PANTHALASSA
o PANGAEA broke apart over hundreds of millions of years, first splitting into LAURASIA
and GONDAWANALAND, to form the present-day continents, islands, and oceans.
- Wegener’s continental drift hypothesis:
o Fit of South America and Africa
o Fossils, rock types, and ancient climates match across continents - Main objection to Wegener’s proposal was its inability to provide a mechanism.
- Inclination of the earth’s magnetic field varies over latitude. It is parallel to the surface over the
equator and vertical over the magnetic poles.
DISCOVERY OF SEA-FLOOR SPREADING
THE RISE OF PLATE TECTONICS
- Invention of SONAR revealed ocean-floor topography, with deep trenches on some continental
margins and linear submarine mountain ranges along mid-ocean ridges.
1. Converging boundary
a. Collision of plates
b. ‘Subduction’: one plate diving beneath another
2. Diverging (spreading) boundary
a. Plates moving away from each other, new (oceanic > mafic/ultramafic) crust formed
3. Transform boundary
a. Plates sliding past each other
- Two basic types of tectonic activity at the earth surface:
1. Compression causes folding at converging boundaries (alpine mountain chain)
2. Extension causes faults by rifting > occurs where plates are diverging.
Convergent boundary: collision of an oceanic plate with continental plate
- Subduction of oceanic plate beneath continental plate > intense tectonic and volcanic activity.
- Oceanic plate sinks because it is thin, dense, and cooler than asthenosphere below
- Deep oceanic trench forms where plates meet.
- Accretionary prism of metamorphic rock forms along edge of continental plate.
- Oceans and continental sediments carried into zone of heat/pressure as oceanic plate sinks
- Descending oceanic plate melts at depth
- Rising magma can form a chain of volcanoes.
Convergent boundary: collision of two continental plates
- Subduction may not occur
- Rocks are uplifted, folded, and faulted into mountains.
- Ultimately, continents may be joined together by a deformed, metamorphic rock mass >
‘continental suture.’ Convergent Boundaries
- Arc – continent collisions
- Island arcs may collide with a large continental plate and become welded or joined.
- Western North America: at least 50 accreted terranes recognized as products of ancient
- Much of British Columbia
- Continental plate ruptures and new ocean basins form
o Continental crust is uplifted and cracked by convective movements in asthenosphere.
o Long narrow rift valley forms, widens as the two new plates begin to move apart at the
new spreading boundary
o Blocks of crust slide up or down
o Ocean invades the new valley
o New oceanic crust formed along the mid ocean rift, and ocean basin widens.
East African rift valley system and the Red Sea
- The East African rift system is a divergent plate boundary at an early stage of its development.
- It has reached locally the stage of continental rift, with thinning of the continental crust and
- The Red Sea is a fully developed divergent plate boundary that has reached the stage of
Ex. Iceland (Mid-Atlantic Ridge)
(California/San Andreas Fault Zone)
Example: The San Andreas Fault is a right-lateral strike-slip transform fault – the surface expression of
the Pacific plate and North American plate moving by one another.
Plate tectonic setting of western N. America
- The Juan de Fuca plate is currently being subducted beneath the N. American continent
- Cascadia subduction zone
- The San Andreas and Queen Charlotte faults lies adjacent to the coastline
- Seismic activity along this fault ACTIVE REGIONS
- Orogeny (mountain building) is ongoing
- Formed by volcanism or tectonic processes
- ‘Young’ landscapes
- Creates mountain bel