Transcript Document
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