GEOLOGIC PROCESSES ON EARTH OUTLINE 1. Elements of Geology The Original Planet Earth Plate Tectonics Eras in Geologic History 2. Processes on Earth Glaciation Periods Earth’s Internal Heat Engine Tectonic Plates Continental and Oceanic Crusts Volcanoes and Earthquakes 3. The Rock Cycle Types of Rocks Mountains Layering Features Weathering and Erosion The Geology of Fossil Fuels 1 – Elements of Geology The Original Planet Earth - Earth is 4.6 billion years old - Only known habitable planet - Right temperature, atmosphere, necessary gases for life, and mantle magnetic shield outer core - Heat trapped inside Earth - Heavier elements (iron) in core, crust inner core lighter elements (rocks) closer to the surface 35 700 2885 5155 6371 Depth in km - Thin outer crust (silicates) Figure 6-1: Schematics showing the Earth’s solid inner core, liquid outer core, mantle and curst. The crust consists of continental and oceanic crusts. Early Earth - Early Earth was full of toxic gases. Volcanoes released sulfur. - Water vapor trapped in magma got released to form oceans. - Original atmosphere was saturated with carbon dioxide. - Primitive algae absorbed carbon dioxide and water and produced oxygen and carbohydrates. - Favorable conditions for basic life to start in oceans 2 to 3 billion years ago. - Earth hosted animal life form for 600 million years. - Humans have been around for less than 1 million years. Earth’s Heat Engine - Early Earth has an internal heat engine (original heat plus radioactive decay) - Planets Mars and Mercury lack such heat engine. - Heat convection inside Earth drives volcanoes, earthquakes and plate movement that formed the oceans and continents. - Plate tectonics and volcanoes form mountains. - Igneous rocks form when lava cools down. - Weathering and erosion grind mountains down and transport sediments to the ocean forming sedimentary rocks. - Tectonic plates move (spread apart, collide and slide under each other). - The continental crust is composed of granitic rocks. The oceanic crust is composed of basaltic rocks. Plate Tectonics - Outline of the coasts of south America and West Africa are similar. Laurasia equator - Continents drift over time. - Pangea supercontinent before 180 Gondwana million years ago. Broke up 180 million years ago giving 5 continents. Figure 6-2: Representation of the two original supercontinents (Gondwana and Laurasia) that were part of Pangea some 200 million years ago (during the Triassic period). Plate Tectonics - Continents drifted apart as well as into each other over time. - World War II brought sonar technology. - In the 1950s, ocean floor got mapped out. - Mid-ocean ridge was discovered. - Sea floor spreading at this ridge - Evidence of magnetic field reversal. - Oceanic crust is less than 200 million years old. Figure 6-3: Some 130 million years ago, the - Continental crust is older than west coast of the US was located in Idaho 700 million years. Eras in Geologic History Figure 6-4: The geologic time scale is divided into eons, eras, periods and epochs. Early Earth History - Radiodating over geologic time. Uranium-235 decays with half-life of 700 million years. Uranium-234 has half-life of 233,000 years. Carbon-14 has half life of 5730 years. - It took 700 million years for heavy bombardment to stop and for Earth to cool down enough. - It took 375 million years for liquid water to form oceans. - During the Archean eon, the atmosphere was rich in carbon dioxide. The Proterozoic eon saw an early form of life (cyanobacteria) which absorbed carbon dioxide and produced oxygen. - Photosynthesis started some 3.5 billion years ago. - Basic life form started in the ocean. Multicellular organisms appeared. Then plants and animals. Early Earth History - Life expanded tremendously during the Paleozoic era. Outburst of life during the Cambrian period. Appearance of the vertebrates land animals some 380 million years ago. - Oxygen levels increased due to photosynthesis. Ecosystem started diversifying. Appearance of flowering plants, fruit trees, birds and mammals. - Mesosoic era corresponds to the age of the reptiles including the dinosaurs. Jurassic and Cretaceous periods. Dinosaurs disappeared at the end of the Cretaceous period (some 65 million years ago). - Dinosaurs got extinct due to a supervolcano eruption or due to the impact of a huge meteorite. - The Cenozoic era corresponds to the age of the mammals. Humans appeared during the last million years. 2 – Processes on Earth Glaciation Periods - 5 glaciation periods are known. - 700 million years ago. Snowball Earth. ice sheet - Last glaciation period over past 2.6 million years. - Glaciations are caused by astronomical North America cycles, geologic activity, change in the levels of green house gases. - The last ice age ended some 10,000 year ago. Figure 7-1: Ice sheet covering part of North America during a glaciation period. Earth’s Heat Engine Parts Temperatures - If Earth were of the size of a basketball, o crust Ambient to 600 C Its crust would be a thin sheet of paper. upper mantle600 oC to 1600 oC o o lower mantle 1600 C to 4000 C - Continental crust is 10 – 70 km thick. Granitic in composition. - Oceanic crust is 2 – 10 km thick. o o outer core 4000 C to 6000 C Basaltic in composition. inner core 6000 oC to 6650 oC Figure 7-2: Temperature increases with depth inside Earth. Earth’s Heat Engine seafloor spreading trench continental crust subduction zone oceanic crust volcano - Lithosphere is 100 km thick. Asthenosphere is 250 km thick. - Oceanic crust subducts under lithosphere the continental crust. Heats up when asthenosphere it dives into the asthenosphere. - Magma is produced. magma chamber Figure 7-3: Schematics of the seafloor spreading, subduction and the formation of magma. Earth’s Heat Engine - Mantle is very hot due to continental crust seafloor spreading volcano radioactive decay of uranium, oceanic crust subduction zone thorium and potassium and crust due to original heat. lithosphere - Rocks melt into lava. magma convection currents - Molten lava escapes at mid- asthenosphere ocean ridges and from volcanoes. mantle - Geothermal convection drives magma the tectonic plates motion. Figure 7-4: Tectonically active mantle and crust, showing two plates moving apart away from the seafloor spreading ridge. The lithosphere consists of the crust and the top part of the upper mantle. Convection heat currents keep rocks in the upper mantle moving. When the lithosphere dives deeper into the mantle below a subduction zone, rocks melt and change into magma. Tectonic Plates - Plates pushing into each other Europe form mountains. Asia North America Asia Atlantic Pacific Ocean - Plate diving under another Africa creates uplift. Oceania South America - Plate collision zone is prone to Indian Ocean earthquakes. Figure 7-5: Boundaries of tectonic plates are shown as dashed lines Tectonic Plates - Pacific plate collides against North American plate. - San Andreas fault in California. - Seafloor spreading forms new oceanic crust. Figure 7-6: Overhead photo of the San Andreas Fault. Tectonic Plates Pacific Ocean Atlantic Ocean South America Africa moving plate moving plate moving plate magma mid-ocean ridge subduction zone mantle Figure 7-7: Schematics of the tectonic plates process Tectonic Plates - Iceland is the site of a “mid-ocean ridge” on land. - Iceland is famous for its geysers. - Lots of geothermal energy. Figure 7-8: Geysers in Iceland Continental and Oceanic Crusts 30 below sea level oceanic 25 - Continental crust is 4 km high. crust continental crust 20 - Mountains are higher. Mount Everest is 8.8 km high. 15 - Part of the continental crust is 10 Relative Fraction (%) Relative Fraction under sea level. over land 5 0 -10 -8 -6 -4 -2 0 +2 +4 Vertical Dimension (km) Figure 7-9: Distribution of the continental and oceanic crusts with respect to the sea level Continental and Oceanic Crusts - The continental crust drops slowly at first. Then drops fast continental shelf mid-oceanic ridge mountains to 2-3 km at the continental slope. mountains abyssal plain sea level - The abyssal plains are flat and 5 km deep. continental slope trench seamount - The mid-ocean trench goes down to 8 km depth. - Deepest part of the ocean is the Figure 7-10: Schematics of the continental Mariana trench at 10 km depth. and oceanic crusts profiles Continental and Oceanic Crusts - Magnetic field lines from rocks changes at the plate boundary. - Conveyor belt from mid-ocean ridge. - Continental crust is much older than oceanic crust. Figure 7-11: Aerial survey of the magnetic field taken over the west coast produces a field orientation map. Volcanoes and Earthquakes lava flow - Magma accumulates in the upper mantle due to high temperatures and to convection currents in the asthenosphere. crust magma chamber - Magma rises up since it has lower density. upper mantle - Pressure builds up in the magma chamber till the volcano erupts. lower mantle Figure 7-12: Pressure build up in the magma chamber leads to volcanic eruption. Volcanoes - There are 500 active volcanoes on continents. Thousands in oceans. - Volcanoes tend to be located at plate boundaries. - Volcano eruptions can be predicted. Figure 7-13: Volcano spewing lava that flows down the flanks Earthquakes - Earthquakes happen at plate boundaries. A B - Earth shakes for thousands of kilometers. - Epicenters can be km to tens of km deep. epicenter C Figure 7-14: Recording from various seismographs point to the epicenter location by triangulation Earthquakes - The USGS operates a network of seismometers. - Richter scale to classify earthquakes. - Earthquakes cannot be forecast. Figure 7-15: Seismometer Earthquakes - At the San Andreas fault, a warning is sent out when there is at least 30 % chance that a magnitude 6 earthquake may happen over the next 3 days. Figure 7-16: Seismograph showing earth shaking waves Earthquakes earthquake - 1976 earthquake in China caused 240,000 deaths - 2004 earthquake in Indonesia caused 228,000 deaths. s-waves - 2010 earthquake in Haiti caused 316,000 deaths.