Geologic Time Scale

Eras, periods and epochsSuperposition: youngest rocks superimposed on older rocks“Relative time”

Dating by radioactive isotopesHalf-life: time for ½ of unstable isotopes to decay “Absolute time”

Uniformitarianism: Hutton (1795), Lyell (1830)“The same physical processes active in the environment today have been operating throughout geologic time”

See: Fig. 8-1

Source: University of Calgary

The Structure of the Earth’s InteriorHeaviest elements gravitated to centreLighter elements concentrated in the crustHow do we know? Behaviour of seismic waves

1. Earth’s CoreDense (1/3 of mass, 1/6 of volume)

Inner core Remains solid, despite heat, due to pressureMainly iron, possibly some silicon, oxygen and sulphur

Outer coreMolten iron, lighter density than inner coreEarth’s magnetism likely due to circulation patterns in outer core,which generate electrical currents

Gutenberg discontinuityTransition zone between outer core and mantleBumpy, uneven, ragged peak and valley formations

Mantle80% of Earth’s volumeRich in oxides and silicates of iron and magnesiumGradual temperature and density increase with depth

Lower mantle: solid despite high temperatures due to pressure

Upper mantle: Asthenosphere is plasticpockets of increased heat from radioactive decay10% molten asymmetrical patterns (hot spots)Hot spots create tectonic activity

Uppermost mantle is rigid – crust + uppermost mantle = lithosphere

Earth’s crust

0.01% of Earth’s mass, but extremely important for life

Solid zone of lower density and variable depth (5km below oceans, 30km below continental land masses and 50-60km below mountain ranges)

Oceanic crust is denser than continental crust – in collisions, thedenser oceanic crust plunges below the buoyant continental crust

Continental crust is mainly granite, whereas oceanic crust is basalt

What is meant by the term isostasy ?

http://mediatheek.thinkquest.nl/~ll125/en/fullstruct.htm

The Rock Cycle

A rock is an assemblage of minerals bound together

Mineral: A natural, inorganic compound having a specific chemicalformula and possessing a crystalline structure. Examples includesilicates (quartz, feldspar, clay minerals), oxides (eg., hematite) and carbonates (eg., calcite)

Rocks are identified by the three processes that formed them:

1. Igneous (solidify and crystallize from molten magma)2. Sedimentary (settling)3. Metamorphic (altered under pressure)

See Fig. 8-6

Igneous Processes

Igneous rocks are those that solidify and crystallize from a molten state. They form from magma (molten rock beneath the surface).Magma either intrudes into crustal rocks, cools and hardens, or extrudes onto the surface as lava.

Intrusive igneous rock that cools slowly in the crust forms a pluton

• Batholith – irregular-shaped, large mass of intrusive igneous rock • Sill – parallel to layers of sedimentary rock• Dike – crosses layers • Laccolith – lens-shaped deposit of intrusive igneous rock bulging between rock strata

See Fig. 8-7

Sedimentary Processes

Existing rock is digested by weathering, picked up and moved by erosion and transportation, and deposited at river, beach and oceanSites. Laid down in horizontally-layered beds.

Cementation, compaction and hardening follow (lithification)

Sedimentary rocks include the following:

1. Sandstone – sand cemented together2. Shale – mud compacted into rock3. Limestone – calcium carbonate, bones and shells cemented or

precipitated in ocean waters4. Coal – ancient plant remains compacted into rock

Clastic sedimentary rocks

Derived from weathered or fragmented rocks (clasts)In order of decreasing grain size, resultant rocks include conglomerate, sandstone, siltstone and shale

Chemical sedimentary rocks

Formed from dissolved minerals, transported in solution and precipitated from that solution. The most common example is limestone (lithified calcium carbonate), which is easily weathered.

See Fig. 8-9

Metamorphic Processes

Igneous or sedimentary rock can be transformed, under pressure and increased temperature, into physically and chemically alteredmetamorphic rocks

Generally harder and more resistant to weathering than the original sedimentary and igneous rocks

Occurs when subsurface rock is subjected to strong compressionalstresses and high temperatures over millions of years

Igneous rocks can be compressed when plates collide orrocks can be crushed under a great weight when they are thrustbeneath another crust

Collection of sediment may also create enough pressure with their own weight, transforming the sediments into metamorphic rock

Foliated vs. non-foliated metamorphic rock: Parent rock with more homogeneous (evenly-mixed) make-up leads to non-foliated metamorphic rock

Original rock Metamorphic equivalentShale SlateGranite, slate, shale GneissBasalt, shale, peridotite SchistLimestone, dolomite Marble (non-foliated)Sandstone Quartzite (non-foliated)

Plate Tectonics

The continents fit like a jigsaw puzzle

Why ?

Continents are adrift due to convection currentsin the asthenosphere, so part of the mantle is literallydragging around the continents

225 million years BP: Pangaea

See Fig. 8-15

The proof for continental drift

Mid-ocean ridges (huge undersea mountain ranges) result from upwelling magma flows form the mantle. The magmaextrudes to form new sea floor (Fig. 8-13)

The youngest crust exists at the sea floor centre, based on analysis of magnetic orientation of sea floor rock (Fig. 8-14)

Subduction zones exist at the edges of the oceans, as the denser ocean crust slides beneath the continental crust.Deep ocean trenches may be found in these regions

Subducted crust is dragged into the mantle, where it melts.Magma also rises through deep fissures and cracks in crustalrock, inland. This creates the “ring of fire”

Plate Boundaries (Fig 8-15(e), 8-16)

1. Divergent Boundaries - ConstructionalZones of tension - Crustal plates are spread apartCharacteristic of sea-floor spreading centresUpwelling material from mantle creates new sea floor

2. Convergent Boundaries - DestructionalCollision zones between continental and oceanic platesZones of compression and crustal lossOcean plates are subducted below continental plates, leading to mountain chains and related volcanoes

3. Transform Fault Boundaries (no construction/destruction)Plates slide laterally past one another at sea floor spreadingcentre - transform faults occur in small sections perpendicular to divergent boundaries where they are disjointed, causingplates to slide past one another in opposite directions

URL: http://pubs.usgs.gov/publications/text/Vigil.html

Plate boundaries are the location of mostearthquake and volcano activity (next lecture)

A “ring of fire” surrounds the Pacific Ocean

Subducting edge of Pacific Plate is thrust deepinto the crust and mantle, creating molten materialthat often makes its way back up to the surfacein volcanoes

The Ring of Fire

Hot Spots

See Figure 8-19

•50 – 100 worldwide

•Deep-rooted upwelling plumes

•Remain fixed beneath migrating plates

•Last hundreds of thousands or millions of years

Source: USGS