Geologic Time Scale
Eras, periods and epochs
Superposition: youngest rocks superimposed on older rocks
“Relative time”
Dating by radioactive isotopes
Half-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
The Structure of the Earth’s Interior
Heaviest elements gravitated to centre
Lighter elements concentrated in the crust
How do we know? Behaviour of seismic waves
1. Earth’s Core
Dense (1/3 of mass, 1/6 of volume)
Inner core
Remains solid, despite heat, due to pressure
Mainly iron, possibly some silicon, oxygen and sulphur
Outer core
Molten iron, lighter density than inner core
Earth’s magnetism likely due to circulation patterns in outer core,
which generate electrical currents
Gutenberg discontinuity
Transition zone between outer core and mantle
Bumpy, uneven, ragged peak and valley formations
Mantle
80% of Earth’s volume
Rich in oxides and silicates of iron and magnesium
Gradual temperature and density increase with depth
Lower mantle: solid despite high temperatures due to pressure
Upper mantle:
Asthenosphere is plastic
pockets of increased heat from radioactive decay
10% 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, the
denser 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 chemical
formula and possessing a crystalline structure. Examples include
silicates (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 ocean
Sites. Laid down in horizontally-layered beds.
Cementation, compaction and hardening follow (lithification)
Sedimentary rocks include the following:
1. Sandstone – sand cemented together
2. Shale – mud compacted into rock
3. Limestone – calcium carbonate, bones and shells cemented or
precipitated in ocean waters
4. 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 altered
metamorphic rocks
Generally harder and more resistant to weathering than the
original sedimentary and igneous rocks
Occurs when subsurface rock is subjected to strong compressional
stresses and high temperatures over millions of years
Igneous rocks can be compressed when plates collide or
rocks can be crushed under a great weight when they are thrust
beneath 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
Shale
Granite, slate, shale
Basalt, shale, peridotite
Limestone, dolomite
Sandstone
Metamorphic equivalent
Slate
Gneiss
Schist
Marble (non-foliated)
Quartzite (non-foliated)
Plate Tectonics
The continents fit like a jigsaw puzzle
Why ?
Continents are adrift due to convection currents
in the asthenosphere, so part of the mantle is literally
dragging 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 magma
extrudes 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 crustal
rock, inland. This creates the “ring of fire”
Plate Boundaries (Fig 8-15(e), 8-16)
1.
Divergent Boundaries - Constructional
Zones of tension - Crustal plates are spread apart
Characteristic of sea-floor spreading centres
Upwelling material from mantle creates new sea floor
2.
Convergent Boundaries - Destructional
Collision zones between continental and oceanic plates
Zones of compression and crustal loss
Ocean 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 spreading
centre - transform faults occur in small sections perpendicular
to divergent boundaries where they are disjointed, causing
plates to slide past one another in opposite directions
URL: http://pubs.usgs.gov/publications/text/Vigil.html
Plate boundaries are the location of most
earthquake and volcano activity (next lecture)
A “ring of fire” surrounds the Pacific Ocean
Subducting edge of Pacific Plate is thrust deep
into the crust and mantle, creating molten material
that often makes its way back up to the surface
in 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