Geology Notes
Origin of Planet Earth and Solar System: * Earth as a unique planet 1. Presence of a “hydrosphere”/ there’s liquid water/ makes life possible 2. Presence of a life supporting atmosphere/ there’s oxygen: not too much, not too little (too much oxygen results in fires) 3. Distance from the sun/ controls overall temperature of planet, Goldilocks Zone: not too hot, not too cold/ also controls composition of planet * Terrestrial Planets- Mercury, Venus, Earth, Mars, Asteroid Belt (all relatively small and rocky) * Jovian Planets- Jupiter, Saturn, Uranus, Neptune (huge and gaseous) 4. Size- Dictates gravitational pull (larger body= more gravity)/ gravity holds in atmosphere/ 2 most abundant gasses are hydrogen and helium/ cooling rate of planet- over time interior of planets cool down (smaller planet cools quicker, larger planet cools slower) 5. Presence of an internal heat engine- breakdown of radioactive substances gives off heat, causing interior of Earth to be hot/ due to internal heat engine, we have: earthquakes and volcanoes (most abundant material ejected form volcanoes is water * Basic observations of our solar system 1. Jovian vs. Terrestrial planets 2. All planets revolve around sun in same direction 3. All planets and our sun are coplanar/ lie along same imaginary surface in space 4. All planets and our sun have a common age of formation, 4.6 billion years ago
The Nebular Hypothesis: Formation of solar system * Originally proposed in 1700 * Immanuel Kant and Pierre La Place * Begins with a nebula (cloud of gases and metallic dust) in space * Remains of an exploded star * Stars are nuclear reactors that generate elements * Nebula will expand for a while, but will contract as gravity kicks in * Most of the material in the nebula, because of gravity, will be pulled towards the center * Under gravitational force fusion begins/ our star (sun) begins to burn * When stars are first born they go through “T-Tauri Stage of Evolution” * Characterized by intense solar wind/ radiation from the sun * Acts like a normal wind * Early in solar system’s history the solar wind “blew” lighter gases to outer portions of solar system/ as result inner planets are small and rocky, outer planets gaseous * Accretion- Growth by gravitational attraction * Craters * 4.6 billion years ago Earth impacted by a Mars sized asteroid * Formed our moon
The Interior of Planet Earth * Look at Earth in its modern configuration * 2 types of crust * Oceanic crust- underlies all of the ocean basins * Made of rock, basalt, rich in iron and magnesium/ has relatively high density * 5 to 10 kilometers thick * Continental crust- makes up the continents * Made of granite, rich in silicon, aluminum, sodium, and potassium/ low density * Averages 35 kilometers thick, maximum thickness is 100 kilometers * Mantle * Solid, made of peridotite * Plastic zone- characteristics of a solid and a liquid (Asthenosphere) * Lithosphere- Crust and solid mantle above asthenosphere * Upper mantle- Base of crust to base of asthenosphere * Lower mantle- Base of asthenosphere to top of outer core, special layer near mantle outer boundary * Core * 2 part core- Outer (liquid) and inner core (solid) * Made up of iron with lesser amounts of nickel and sulfur * Circulation of iron in outer core generates Earth’s magnetic field * As temperature of Earth’s interior fluctuates, the circulation in the outer core also fluctuates, causing magnetic field to fluctuate * Every 750,000 years Earth’s magnetic field flips 4 How do we know? * Rocks- we live on the crust * For the mantle we have xenoliths (pieces of Earth’s mantle that have been brought to surface in volcanic eruptions) * The core is interpreted to be the same composition as iron meteorites * Deep Mine/ Drilling- the deepest mine on planet Earth are 1.5 miles * Deepest drill holes are only 7 miles, too hot to go deeper, gives great information on temperatures of Earth’s interior * Temperature increase 25 Celsius every kilometer * Geotherma Gradient- Increase of temperature with depth within the Earth’s interior * Seismic Waves from earthquakes- tells us everything else that we know about interior of Earth * 2 basic forms of seismic waves, body and surface waves * Body waves- Travel through body (interior of planet Earth), 2 types of body waves in nature/ primary (P) waves are fastest, travel through all substances, travel by alternating by alternating compression expansion and secondary (S) waves are 2nd fastest, only travel through solids, travel with lateral shearing motion * Surface waves- Travel along Earth’s surface * Use a seismograph to detect and record * S-wave shadow zone- no direct s-waves recorded in region (core) * As seismic waves travel through Earth’s interior they can reflect off a boundary, refract through a boundary, velocity may change depending on the material they travel through * Aftershock- small earthquake that follows a large one * Late 1800s Swiss Geologist Edward Suess * Recognized that fossil species on continents currently separated by wide ocean basins were identical evidence for Gondwanaland * Early 1900s German Meteorologist Alfred Wegener * Theorized that 200 million years ago, all of the present day land masses were joined together as a single “supercontinent” Pangea * Over past 200 million years continents have slowly drifted to present positions * Pangaea breaks into 2 pieces, northern (Laurasia) and southern (Gondwanaland) * Wegner had many lines of evidence to support his idea * Jigsaw puzzle fit of continents * Fossil evidence * Rock types match up * Climatic evidence from glaciers * Evidence from mountain belts * 1928 Wegner presents his theory to world… Rejected * Wegner had no mechanism for how continents drifted * Late 1930s to 1940s World War II, submarine warfare important * Development of “echo sounder (sonar)” * Used this technology to map ocean floor * Important Discoveries * Existence of the mid-ocean ridge system, 10,000 foot high continuous mountain chain that runs through the middle of Earth’s oceans * Entire ocean floor made of basalt * Ocean floor is oddly young, oldest oceanic crust is 250 million years old compared to continents over 4 billion years old * Deep sea trenches * Age progression on sea floor, youngest at mid ocean ridges and oldest at edges adjacent to continents * Late 1960s Princeton Geologist Harry Hess “Seafloor Spreading” * The Earth’s ocean floor is opening up at the mid ocean ridge and new ocean floor is being generated there * Lithosphere bends and cracks, asthenosphere partially melts as it rises, magma cools and solidifies as it reaches the surface * Convergent Plate Boundary- Area where 2 pieces of Lithosphere are pushed toward one another, oceanic crust pushed beneath continental crust because it is more dense * Melting beings 100 km below surface, magma rises to surface resulting in volcanoes Earth History: * Instability of igneous rocks at surface * Igneous rocks form at very high temperatures (600 to 1200 degrees Celsius)- temperature rocks/minerals are stable at * Not stable at Earth’s surface * Bowens reaction series * Olivine, pyroxene, amphibole, micas, potassium feldspar, quartz * High temperature (form) Low temperature * (least stable) (most stable) * Only 3 minerals that are truly stable at Earth’s surface- quartz, clay, and hematite (rust) * Everything else on planet breaks down to some combination of these * Materials at surface may break down (weather) by either physical or chemical methods * Physical Weathering- Breaking material down by mechanical methods * Frost wedging- Small amounts of water enter cracks in rock, water freezes and expands making crack bigger * Exfoliation- Expansion and cracking of rock due to decompression * Chemical weathering- Breaking down of rock by chemical means * Rain is most common type of chemical that breaks down rocks at surface (CO2 in atmosphere mixes with rain, carbonic acid), breaks bonds in minerals “dissolving bonds” * Physical and chemical work together to form soil, once soil is formed it may then be eroded * Erosion= Transport * Most common method of erosion is running water * All rivers begin in mountains, water velocity is high * Water velocity controls two very important factors- competence and capacity * Competence- Largest particle size running water can carry * Capacity- Total amount of material water can carry * Higher water velocity the higher the competence and capacity * Sorting- Separating particles according to size, gravel, sand, clay, dissolved ions * Once these particles (sediment) are deposited, they then may be lithified (turned into sedimentary rock) * Conglomerate- Sedimentary rock composed of gravel sized fragments * Sand Stone- Sedimentary rock composed of sand * Shale- “ “ composed of clay * Limestone- “ “ composed of dissolved ions Sedimentary Rocks: * Record entire history of Earth’s surface * History of life * History of climate * Contain all of our fossil fuels and groundwater (largest supply of available fresh water on planet) * Measuring time, two ways to measure * Radiometric Age Dating/Absolute Age Dating/Numeric Age Dating- 1896 Henri Becquerec discovered radioactivity * Bertram Boltwood developed technique to determine the age of rocks using radioactivity * Radioactive substances over time break down (decay) * Carbon 14 decays to Nitrogen 14, rate at which radioactive (parent) isotope decays into a new stable (daughter) isotope is called the half life * Half life- Time required for ½ of a radioactive (parent) isotope to decay into a stable (daughter) isotope * If we are determining the age of an igneous rock, we are determining the age of crystallization of that lava or magma * If we determine the age of metamorphic rocks, we determine the age of the change * We cannot determine age of sedimentary rock because sedimentary rocks can be formed from multiple sources * Use relative age dating to determine age of sedimentary rock * Set of common principles (Steno’s Laws) proposed by Nicholas Steno in 1600’s * Principle of original horizontality- Sedimentary rock layers deposited horizontally * Principle of superposition- Sedimentary layers in a sequence are oldest at bottom, youngest on top * Principle of unconformities- Gaps in geologic record exist due to erosional events * Angular- Layers are at an angle, new layers form on top * Disconformities- Layers missing, fossils missing * Nonconformity- Sedimentary rock forms on igneous rock (crystalline) because of erosion * Principle of cross cutting relations- If feature cuts across rock units that feature must be younger * Principle of lateral continuity- Sedimentary rock layers at time of deposition end by thinning, butting up against a barrier, or by gradually changing into a new rock type * Changes in sea level, rise (transgression) and drop (digression) evident by Grand Canyon * Transgression may be caused by… * Melting of polar ice caps * Change in Earth orbit around sun * Change in Earth axis * Cause oceans to displace water on continents (ocean floor rise) * Increase heat on ocean floor * William Smith 1800 (English engineer/Surveyor) * Different rock layers contain different fossils * Principle of faunal succession- Fossil species succeed one another in a regular/recognizable pattern * Invertebrates, fish, amphibians, reptiles, dinosaurs/birds, mammals * Built time scale for sedimentary rocks