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Lecture 1

GEOL 1121 Lecture 1: Geology Test 1
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Department
Geology
Course
GEOL 1121
Professor
Rice
Semester
Fall

Description
Geology Test 1 August 19, 2015 Earth in Space • Population density- populations tend to form along coast lines because easy to make a living and transport goods, etc. • Lights at night show that there are differences in comparing populations and energy usage because although in US and Europe the populations are less dense, however we use more energy and density reflects energy and affluence, not just population. • Earth is one the 4 relatively small planets. All planets travel in the same direction and most rotate in the same direction. They orbit close to the ecliptic, meaning they rotate close to the sun. • Dust-rich disks are common in the universe, which provides evidence for a model. (Andromeda galaxy) • The “terrestrial” planets are the four inner planets o Mercury: Density of 5.4 gm/ cm -3. Doesn’t have an atmosphere. Surface is densely cratered because of what is thought to be done by impacting bodies. Such surfaces represent the oldest surfaces in the solar system. We don’t have any samples from mercury so estimated age. o Venus: 96% Co2 atmosphere. You can’t see any features on the surface with regular camera/light because of clouds. If you have the photographic machine that can penetrate the surface using radiation you can then see the surface. Not a ton on craters, which makes us think that Venus isn’t super old. Using Magellan measurements to create topographic map of Venus. Similar in size and density to earth. Surface atmosphere P = 100 x that of earth. Surface T= 475 C and water boils at 100 C o Mars: 95% CO2 in the atmosphere. Surface pressure= .006 x Earth. Average surface temperate = -60 C and water freezes at 0 C. Surface of Mars varies from relatively young to relatively old because some places with nothing and some places with many craters. Clear evidence for the existence of water because flood paths. And there are layered sedimentary rocks that look like those on earth created by wind and water. Mars has 2 rovers working on it now. Curiosity is currently working there after landing August 2012. It landed on the equator because weather is milder and the rover is solar powered and there was the most sun there. o Our Moon: No atmosphere. Surface has a lot of impact craters including very large craters. Filled with lava flows. In the center of the largest impacts, a mountain range forms. o Craters on Earth: Present but not common. Only about 200 known craters on Earth. Many of the craters have eroded and because of the active surface (plate tectonics) they have been depleted. o Earth: 78% N2, 21% O2, <4% H2O, and .035% CO2. Oxygen is produced from living things and is not a constant. Note that CO2 is a trace constituent in spite of its important effect on climate. Coal, fossils, and the ocean have a lot of CO2 saturated in them. Surface T= 20 C. Atmosphere is O2 rich and CO2 poor because earth has a mechanism to remove CO2. The atmosphere reflects Plant production of oxygen August 21, 2015 Previous review points: • Relentless population growth  contributing cause of environmental problems • Solar system: Planets orbit sun in same direction + about the same plane consistent with nebula model of rotating cloud of dust  planets around proto sun • Terrestrial (rocky planets): Mercury, Venus, Earth (moon), Mars • Mercury, Moon: old, densely cratered surface, no atmosphere • Venus: Young surface, surface temperature is about 475 C • Mars: Young to old surface, evidence for water (floods, rivers), Surface temperature is about -60 C • Earth get oxygen from photosynthesis: 6 CO2 + 6H2O + Light  Sugar + 6 CO2 • C-Banks: Carbonate rocks, coal, petroleum, natural gas, plants Pressure CO2 N2 O2 SO2 H2O Ar Venus 100x Earth 96% 4 - .015 - - Earth .035 78 21 Traces <4 Trace Mars .006 x Earth 95 3 .13 ? - 2 What current additions to the atmosphere exist? • Main addition are volcanic gases with the two common being water and CO2. • Volcanoes are critical evidence that the interior of the earth is quite hot. • Hot springs also reflect internal heat of the earth • Earth scientists have the “Geothermal gradient” which is the change in temperature with depth. • Temperature increases all the way to the center of the earth • Surface temperature of the sun is about 5500 C • Average depth of the oceans: 2.3 miles • Average height of the continents: 800 ft • The continental shelf is considered to be part of the continent. Therefore, the actual edge of a continent is called the continental rise. • Mid-ocean ridges (ocean mountain ranges) are one of the more distinct topographic features. • Terrestrial ranges tend to grow long and narrow. • A trench is on the convex (outside of the curve) side of the arc in a volcanic island arc and deep-sea trench relationship. Earth’s Topography • Continent/ocean dichotomy, i.e. bimodal distribution of elevations • Mid-Ocean ridge system: 80,000 km long, 1500-2,500 km wide, 2-3 km above abyssal plain • Long, narrow continental mountain ranges • Deep-sea trenches adjacent to arcs of volcanic islands. Earth’s cross-section • Is like an avocado • Divided into crust, mantle, and core (inner and outer core) • The densest material is at the center • The mantle is softer and flows more than the core (like the avocado vs. the seed) • Earth’s core is thought to be similar to the material that makes up iron meteorites: 90% metallic iron (Fe) and 10% metallic nickel (Ni) • The densest crust underlies the ocean basins August 24, 2015 Review: • Current addition to atmosphere: Volcanic gases H2O and CO2 • Earth’s interior is hot as indicated by volcanoes and hot springs • Geothermal gradient is about 20-25 C/km • Center of Earth at about 5500 C (Same as sun’s surface) • Earth’s key topographic features are: o Continents: average elevation of 500 m o Ocean Basins: Average depth of 4.5 km o Mid ocean ridges: 2-3 km above the ocean floor, circles the globe o Volcanic arcs: the deep sea trench is always on the outside o Deep sea trenches ^ o Continental mountain ranges are long and narrow • Bimodal distribution of elevations is unique to Earth Chapter 2: Mineralogy and Petrology (Study of rocks) Element vs. mineral vs. rock • Element: found on the periodic table (number= atomic number = number of protons) o Proton, neutron, and electron o Most elements do not occur as a pure substance in nature o All atoms of material have the same number of protons • Mineral: Can be identified on the basis of crystal shape, density, chemical composition, atomic structure, hardness, and many other physical properties. To be a mineral, it has to be natural, solid, and crystalline (Atoms are arranged in a regular, repeating order). In most minerals the atoms are bonded ionically, meaning by attraction between + and – charges. Positive ions are cations and negatives are anions. Most minerals are made up of some combination of elements in rows 2, 3, and 4 of the periodic table. The overall structure of the mineral is electrically neutral. o *Know the elements Main mineral groups: Oxides, Sulfides, Halides, Carbonates, Hydroxides, Native Elements (Gold, silver, graphite, diamond), and Sulphates o Minerals to know! ▪ Quartz: SiO2- A silicate, very common, rock-forming mineral. Persists in the surface environment. ▪ Feldspar: (Na, K) AlS3O 8- Pink to white grains in rock in picture. A Silicate, most abundant mineral in Earth crust. Weathers away rapidly to form soils and clays. ▪ Fe-MG silicates: (Mg,Fe) 2SiO4-Dark-colored minerals of common rocks, weather rapidly ▪ Pyrite: FeS 2– A sulfide, various sulfides are common ore minerals, commonly known as fools gold, weathers readily, the culprit in acid rain and acid mine drainage, produced sulfuric acid when weathered or burned. ▪ Calcite and aragonite: CaCO 3– carbonates, with same chemical formula there is a different atom arrangement. Will slowly dissolve in fresh water but precipitates from shallow, warm seas (Bahamas) and shells & corals are made of these carbonates- are carbon sinks or “banks.” ▪ Table salt or Halite: NaCl- Generally forms from evaporation of sea water. ▪ Magnetite: Fe 3O 4– Oxide, naturally magnetic, crucial to plate tectonic theory August 26, 2015 Review: • Earth segregated by density: Layer Density Core Fe-Ni Metal 12 gm/cm^3 Mantle Fe-Mg Silicates 4 gm/cm^3 Ocean Crust Fe-Mg Silicates > feldspar 3 gm/cm^3 Continental Crust Feldspar + quartz > Fe-Mg 2.7 gm/cm^3 Silicates • Earth materials: o Minerals: natural, solid, crystalline (repeating, 3 dimensional atomic structure) o Cations: Positive charges ions o Anions: Negative charged ions o Both ^ Free ions common in fluids, e.g. ocean water. They balance charges in minerals- electrically neutral • Minerals classified by anions: −4 Silicates SiO 4 Most common minerals Carbonates CO3 -2 Contain C -2 Sulfides S Common metal ores Halides Cl or F - Salt (halite) is one -2 Oxides O Rust is an example Hydroxides OH - Contain water -2 Sulfates SO 4 Gypsum is one Native elements Pure Au, Pt, diamond, graphite elements • Quartz- is stable, persists in the Earth’s environment • Feldspar- Most abundant in crust, unstable on Earth’s surface (reacts with weather) • Fe-Mg Silicates: Unstable, black • Calcite: One of the big carbon banks! • Magnetite: Magnetic • Halite: Salt • Pyrite: Sulfide, Unstable, when it weathers it gets oxidized and end up with sulfuric acid. Mineral Stabilities • Where is ice stable, or where does it form in terms of pressure and temperature? Does ice form at the same T in Athens and Denver? • As pressure increases, water will freeze at a slightly higher temperate (aka 33 instead of 32). As Pressure increases, water will boil at a slightly lower temperature (for example, 98 degrees vs. 100). • Diamond is not stable on Earth’s surface- yet we still have it and they persist of many life times. They are stable however in the earth interior- still doesn’t answer how we can have them on the exterior • Pressure in the earth increases at a constant rate around the globe with dept. P=Density x gravity x depth • Deepest mine on earth: 3.5 km and T of 55 C and P of 920 atmospheres. Temperature is over 100 degrees, which limits a workers time to about 10 minutes in the cave. • Measurement in deep mines and drill holes indicate that T typically increases at a rate of 25 C/km. This is called the geotherm. • If pressure and temperature drop rapidly, a “metastable” mineral can be preserved because at low temperature, atoms have trouble rearranging themselves. So if volcanism brings diamonds rapidly to the surface and cooling is quick, diamonds are preserved. Even when we think it’s hot, to an atom it’s typically cold and therefore doesn’t feel like moving around and changing it’s arrangements. What is a rock? • Composed of numerous grains of one or more minerals cemented together or having an interlocking texture like a jigsaw puzzle. • Igneous rocks preserve minerals and textures formed at high T, and sometimes P August 28, 2015 Review: • Stable: Mineral will remain as in indefinitely (quartz is the only common mineral that approaches this state) • Metastable: Mineral will react with water, O2.. to form new minerals but this process may take more than 1,000,000,000 years. An example is a diamond • Temperature: Increases at a rate of 20-25 C/km in most areas of continents (Athens is about 18 C/km • Pressure: increases at a rate of .1 Gigapascals/km • Hurricanes rotate counterclockwise Rocks come in 3 flavors • Igneous: crystallized or frozen from a very high temperature viscous liquid (“magma”) o Two kinds- Volcanic crystalize on the Earth’s surface and Plutonic freeze in the Earth’s surface o If freezing occurs very quickly, a black glassy matrix forms o Volcanic rocks give us a “snapshot” of magmas which consists of a viscous liquid (melt) + crystals + gas (which may or may not be dissolved in the melt). o The freezing process is analogous to the freezing of water or the growth of a snowflake ▪ Except the number of crystalizing forces is 3 or 4 vs. 1 in water. • Sedimentary: formed at or near the earth’s surface by accumulation of sediments or chemical precipitates o Come in 3 flavors- ▪ Clastic sediments: form by the accumulation of the sediment (sand, mud, gravel) transported by water, wind, and ice. Example is sandstone formed by the accumulation and lithification (cementation) of the sand grains. The grains are held together by cement, generally calcite (CaCO3) or silica (SiO2). The cement is precipitated by groundwater after the sediments are buried by younger sediment. Recognizable features such as ripple marks and layering. Mars has layered marks but we do not know what from ▪ Biochemical sediments: include the peat (organic matter) that accumulates in swamps and ultimately is changes to coal. Limestone, made of CaCO3, can be biogenic if made up of shell fragments. ▪ Evaporates or chemical precipitates: salts. Form from water evaporation. Include limestones but also halite (salts) and gypsum (CaSO4) • Metamorphic: Formed at higher temperate and pressure in Earth’s crust with no magma involved (no melting) o Banded gneiss: Squeezed at high pressure and temperature within the Earth’s interior (looks like how fudge is “layered” into ice cream). Glacial ice is a good low temperature material analogous to metamorphic rocks. The snowflakes are the base but then as more builds up so does pressure then the flakes change to more of a grain shape that becomes more solid of a state (this change is metamorphism) • Igneous, metamorphic, and sedimentary rocks form under distinct P and T conditions o Lithification and diagenesis= Cementation of sedimentary rocks o Low, medium, and high grades refer to relative conditions or metamorphism o Igneous processes require a high temperature, viscous liquid - “melt” August 31, 2015 Review • Rocks: Composed of numerous grains of one or more minerals cemented together or held together by interlocking texture • Igneous: preserve textures formed at high T and P in presence of melted crystallized (frozen) from a high T (700-1000 C) riscous melt, over several hundred degrees, to a mix of solids. o Volcanic: formed on surface, plutonic, formed with Earth o Grain size depends on the rate of cooling (fine/glassy=fast, coarse= slow) o Gas (H2o, CO2, minor SO2,H2S) present as bubbles or dissolved in melt o Magmas: mix of melt, solids, and gas • Sedimentary: Form at surface by accumulation of sediments/chemical precipitates o Clastic: formed by accumulation of sand, gravel, silt; transported by wind, water, or ice; held together by cement (CaCO3 or SiO2) precipitated by ground water; commonly formed in layers o Biochemical: mediated or created by organisms (peat, coal, some limestone); o Evaporate: chemical precipitates; commonly from seawater (ex: salt, gypsum) • Metamorphic: recrystallized at high P and T; no melting; preserve metastable minerals (how did they get to surface?); banded or foliated; • Low Tanalogue: Snow  glacial ice • Campus bedrock: Athens Gneiss Geologic Time: • Absolute key constraint for earth evolution • Relative time: rock A is older than Rock B • Absolute time via geochronology o Are very needed o Based on natural radiological activity o Superposition: In a set of layers, the lowermost is the older. Layers build up over time; superb exposure of layered sedimentary rocks in Grand Canyon and other parks. • Correlation of unique fossils between locations constrain time o If you find the same kind of animal fossil in two different locations you can assume that the layers are the same age. Think of dinosaurs- they were only around for a short time and therefore, and layers with the fossils are the same age. By building up the context we built a geological time scale. o Correlations in US Southwest: Same age, age sequences extended based on correlations. o Why do layers of same age end up “tilted (at different heights)” ▪ Deposition of tilted layers as originally horizontal layers ▪ Tilting of layers ▪ Deposition of upper, horizontal layers ▪ Had to occur during the intercal between deposition of the uppermost tilter layer, and the lowermost horizontal layer. Video: • James Hutton, father of modern geology, Scotland. Spent years trying to understand how the rocks on earth were made. Sicker Point was the place that he discovered the earth was incredible old (the church said that it was only 6,000 years old). • Thought that the earth was formed from Meteorites colliding and the entire earth was molten lava. Kelvin believed that the Earth is cooling down and used thermodynamics to guestimate a new age for the planet. Wrong, but held the keys to unlocking the true age of the Earth. • 1911, Arthur Holmes, used radiation to date how old the earth is. Radio active uranium turns to lead. Collecting sample and data allowed him to accurately date the earth. 4.5 Billion years (still today the accepted time), Known as Deep Time Glacial Lake Sediments • One can count the dark and light layers (=1 year) and absolute constrain time. • The same thing can be done with snow layers in the polar ice caps • Problem: record only goes back a few 100,000 years. Radioactive Clocks • Isotopes: same element but different numbers of neutrons, and thus a different atomic mass; Uranium 238 vs. Uranium 235. Some isotopes undergo spontaneous decay to a new element. Decay involves ejection of particle from nucleus or capture of electron by nucleus. Also change in number of protons and in some cases atomic mass • Radioactive Decay generates heat and is part of the reason why Earths inner parts are hot and why we have volcanoes. • 4 radioactive elements present in significant amounts: U238, U235, Th232, K40 • Is analogous to an hour glass Sand empties from top to bottom- it takes a fixed amount of time. Each decayed parent (Top) becomes a daughter (bottom). It takes the same amount of time each time for the sand to empty. This is the same for radioactive elements: they each have a distinct half-life. Half-life is not changeable, each element has a different half-life. Half-life= the time it takes for ½ the parent atoms to decay to the daughter. o Rule of thumb: after 7 half-lives, radioactive clock no longer works- effectively no parent left. ▪ 14C has a half-life of 5,730 so why is it still around? • It’s currently being generated. Cosmic rays from the sun come into the top of our atmosphere and mixes with 14N, which causes 14C and 1H to form. • Our atmosphere is well mixed so the 14C is evenly distrusted close to equal in the air. • Living things have 14C in them while alive, including humans. However, after death the 14C begins to decay. It decays at a fixed rate and time since death and can be calculated from 14C/14Co= e^-(Lambda)t where Lambda is the decay constant for 14C; 14 Co is the standard o K40 decays to Ar40 ▪ K is 93% K39, .012 K40, and 6.7% K41 ▪ Half-life is 1.25 billion years, which is helpful because useful for a long period of time ▪ Daughter is a gas, Ar40 ▪ Particularly used to measure argon (gas) because it only accumulates after magma crystalized and cooled. Therefore, the older the rock, the more Ar40 it has • Therefore, No Ar40 has been gained or lost, which means that no K40 has been gained or lost (like the sand timer thing, you can’t take any sand out, it’s all still there) = Closed system. How old is the Earth? • As you go to the coast of continents the age of rocks gets younger. They grow outward • Where does the 4.55 Billion years of age come from? o We look out to our nearest neighbor, the moon. So how old are the oldest moon rocks? About 4.5 billion years, and are from the highlands (aka the light colored areas od the moon) o Another way is to look at meteorites. They peak in the meteorite ages at 4.5 billion years. It’s not derived from the materials on the Earth’s surface but rather from meteorites. (and the moon backs this up) • Model for the origin of solar system: all of the planets, moons, the sun and asteroids formed at the same time from a dust cloud around the “proto sun.” In some cases later events disturbed the geochronology- so the oldest ages date the age of the solar system. Thought that the Earth formed at the same time as the oldest rocks in the solar system. September 2, 2015 Review • Time: Relative ages o Lowest layer is the oldest o Correlation: key fossils can indicate 2 rocks are the same age o Cross cutting: igneous rocks that “intrude” (=cross cut) are younger than host rocks. • Siccar Point: o Inclined layers tilted o Incline layers deposited o Vertical layers tilted o Vertical layers deposited (oldes
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