Recap: Earth is Structured A) Metal Core and Silicate mantle and Crust B) Deduced by: -Observation of crust and mantle derived rocks -Seismic structure (P and S waves) -Knowledge of Earth mass deduced from its gravitational attraction and moment of inertia. Recap: Element Segregation Volatile-low vapor pressure: H 2 O, H 2, CH 4, O 2, CO 2 Refractory-high vapor pressure: Ca, Al, Ti, Mg, Fe (Ni) Lithophile elements, occur with oxides; e.g. Rb, K, Ba, Mg Chalcophile elements, concentrate in sulfides; e.g. Cu, Pb, Zn, Sn, and Ag; Siderophile elements, tend to be metallic, e.g. Fe, Ni, As, Pt, Ir and Au 1
Selective Condensation of different elements from high-t, low- p 2
How was structure/layering created A) Primary feature of accretion or result of differentiation? B) Two end-member models of planetary accretion and layeringi.heterogeneous accretion (pre-accretion element segregation) ii. homogeneous accretion (accreted uniformly and subsequently differentiated) C) Both processes seem to have been operative i. Increase in mean density with size for the planets Mars, Venus and Earth (homogeneous accretion) ii. Mercury doesn t follow trend (heterogeneous accretion) iii. Earth, Moon, Mars can be approximated by CI (homogeneous accretion) iv. Late-veneer of highly siderophile elements in Chrondritic relative abundances (heterogeneous accretion) v. Variations between terrestrial inner planets compared to outer planets (heterogeneous accretion) 3
4
5
MGG Lecture 3: Geological Time Scale and Interpretting the Rock Record Geological Time Scale and the Earth s History in the Rock Record A) Concept of time is central to Geology B) Geological processes occur over many time scales i. Earth Quakes (seconds to minutes; 10-8 to 10-6 years) ii. Volcanic Eruptions (hours to years; 10-4 to 10 0 years) iii. Orogeny, process of building of Mtn chains (millions of years; 10 6 yrs ) C) Therefore as geologists we need to assess processes over large range of time scales. i. Erosion (observable over human time scales; i.e historical record) ii. Spreading of the Atlantic (ca 1-2 cm/yr not observable in human terms but can now be measured with GPS; longer time-scale measurements are more precise) iii. Most Geological Processes involving the evolution of the Earth are extremely slow in human terms (requires the rock record) 6
Estimating (Slow) Rates of Earth processes? A) Simple Rough, Back of the Envelope Calculations: i. Spreading Rate of an Ocean Basin: The oldest rocks found are ca 200 million years (Ma) taken as upper limit on age; width of basin is ca. 10,000 kms. So spreading rate is 10,000kms/ 200 Ma--> 5 cm/yr. ii. Vertical uplift associated with Orogeny: Mtns with marine fossil units at 3000 meters altitude dated at 15 Ma. This gives a time-averaged uplift rate of 0.2 mm/yr iii. Viscosity of the mantle: Fennoscandia (Norway, Sweden and Finland) covered by thick ice sheet (2-3 kms) during ice age @ 40 Ka. This depressed lithosphere, when warming occurred ice melted and this rebounded by ca. 500 m, as evidenced by ancient beach rocks at that elevation. Postglacial uplift was ca. 1 cm/y. 7
The Rock Record A) Outcrops- Canyons, Mtn. sides, road cuts expose sequence of rock which we interpret B) General Rock type classification: i.) Igneous Rocks- Volcanic and intrusive rocks formed by solidification of magma (melt) ii.)sedimentary Rocks: formed by accumulation and cementation of mineral grains transported by wind, ice, water to side of deposition or precipitation (sandstones, limestones) iii.) Metamorphic Rock- original mineralogy, texture and/or composition changed by effects temperature, pressure or the gain or loss of chemical constituents The Rock Record (contd) C. Sedimentary Record -layering or stratification -bedding (cross bedded sands, graded beds, conglomerates, limestones) -stratigraphic time scale (Nicholas Steno, 1669) i. principal of original horizontality ii. principal of superposition -unconformities an interval of time that separates two strata -principal of uniformitarianism (Charles Lyell) geological processes that shape the Earth throughout geological time are uniform/constant. 8
The Rock Record (contd) D) Fossil Record (powerful tool for establishing time sequence of series of layers in sedimentary rocks) i. Occur in limestones, shales, sandstones ii. Paleontology- study of ancient life (paleobotany, paleobiology) iii. Faunal Succession 9
10
Relative Time Dating Using Field Relationships i. Different geological regimes ii. Unconformities (time gaps) iii. Cross-cutting relationships 11
Geological Time Scale -Eons -Eras -Periods -Epochs 12
13
14
Absolute Time Scale A) Biblical- Archbishop Ussherof Ireland (1664) Earth created at 9:00am, Oct 26, 4004 B.C. B) Comte de Buffon of France (1687) Analyzed melting and cooling rate of fe (iron) balls; Earths core solid Fe-> 75ka C) Herman von Helmolz, Lord Kelvin accounted for gravitational contraction of the Sun- calculated age of system 20-40 Ma D) Radioactive decay Ernest Rutherford (1905) Earth was billions of years (Ga) Radioactive Dating of Rocks Rate of decay of an unstable parent is proportional to the number of atoms remaining at time (t); i.e. dn/dt proportional to N Derivation of General Age Equation Linear Form- isochrons (internal and external) When does the Clock Start- phase changes (e.g. condensation, melting, crystallization) Several different isotope systems with different half-lifes enable many time scales to be addressed 15
Recap: Radioactivity & Radioactive Decay α Emission of a helium nucleus which is a particularly stable entity (2 protons + 2 neutrons) β Transformation in the nucleus: either 1 a neutron transforms into a proton plus an electron (β- emission) 2 a proton transforms into a neutron plus a positron (β+ emission) γ Relaxation of an excited nucleus with the emission of electromagnetic radiation. 16
Radioactive Decay Law λ is the decay constant t 1/2 is the half life Typical values of half life, t 1/2 Isotope Half Life Decay Polonium 214 84 P 1.64x10-4 s α, γ Krypton 89 36Kr 3.16 minutes β, γ Strontium 90 38Sr 28.5 years β Radium 226 88Ra 1600 years α, γ Carbon 14 6C 5730 years β Uranium 238 92 U 4.5x109 years α,γ 17
18
19
20