Week 2: Plate Tectonics and the Rock Cycle (PDF)

Week 2 | Plate Tectonics and the Rock Cycle | Igneous Rock Textures Overview Learning objectives - Describe the characteristics of a lithospheric plate - Describe how continental composition and density - Describe the three types of plate boundaries - Explain how crust and lithospheric m...

Week 2 | Plate Tectonics and the Rock Cycle | Igneous Rock Textures Overview Learning objectives - Describe the characteristics of a lithospheric plate - Describe how continental composition and density - Describe the three types of plate boundaries - Explain how crust and lithospheric mantle form along a mid-ocean ridge - Describe the major features of a convergent boundary - Describe the motion that takes place along a transform boundary - Describe the characteristics of continental rift - Describe the process of continental collision - Describe the major forces that move lithosphere plates - Describe what is rock - Describe the three rock groups - Describe the rock cycle Part 2 → Learning materials Important content 1. Divergent, convergent and transform plate boundaries 2. Seafloor spreading 3. Driving forces of plate motion 4. Rocks as recorders of plate tectonics Podcast: Reconstructing plate motions over a billion years of earth history The Basics of Plate Tectonics - Earth's surface is composed of ~12 rigid plates in constant motion. - Two plate types: Oceanic plates: Created at mid-ocean ridges, destroyed at subduction zones. Continental plates: Formed from mantle material. - Plate tectonics explains processes but not the historical evolution of plate motions. Reconstructing Plate Motions Over Time - Challenges: Heterogeneous and incomplete data, especially pre-Jurassic. Loss of ocean floor older than 170 million years (Ma). Difficulty reconstructing vanished mid-ocean ridges and subduction zones. - Data Sources: Magnetic stripes on ocean floors (like a "tape recorder"). Geological, paleomagnetic, paleontological, and structural data. Seismic tomography to locate subducted slabs. GPlates Software - Developed to handle complex plate tectonic reconstructions. - Functions: Enforces plate tectonic rules (e.g., no quadruple junctions, maintains global balance). Models "continuously closing plate boundaries" and reconstructs plate movements. Incorporates seismic tomography to track subduction zones. Key Historical Insights - Pangea and Earlier Supercontinents: Supercontinent assembly, breakup, and dispersal cycles affect plate motion history. Reconstruction of Rodinia (~1.1 billion years ago) is challenging due to sparse data. - Slab Superflux (Mid-Cretaceous): Increased subduction (~130 Ma) due to Pangea breakup and faster plate movements. Led to mantle return flow and the creation of superswells like the Darwin Rise. Geodynamics and Climate Impacts - Deep Earth Processes: Subduction affects mantle convection and core-mantle boundary heat flow. Mantle plumes formed from subducted slabs can trigger volcanic eruptions and mass extinctions (e.g., Siberian Traps). - Snowball Earth (750 Ma): Breakup of Rodinia accelerated weathering, reducing atmospheric CO2. Led to global ice coverage, eventually reversed by volcanic CO2 emissions. Biological and Geological Implications - Plate tectonics influence mountain building, volcanic activity, and ocean circulation, which affect: Climate patterns. Evolution and extinction events (e.g., “The Great Dying”). Carbon cycling and long-term environmental change. Significance of the Model - The billion-year plate model integrates over 1,000 geological studies. - Applications: Understanding tectonic-climate interactions. Linking surface processes with deep mantle and core dynamics. Providing insights into past and future Earth system changes. Lecture video: plate tectonics and the rock cycle The lithospheric plate - Forms the earth's relatively rid shell that bends or breaks when a force acts on it - Consists of the crust and the top (cooler) part of the upper mantle, called lithospheric mantle - Floats on a softer plastic player called asthenosphere composed of (>1300C) mantle that can flow slowly acted soon by forced Continental lithosphere vs oceanic lithosphere Continental is under the continents and oceanic is under the ocean Lithospheric plates and plate boundaries - Lithosphere contains 12 major tectonic plates and several microplates - Plate boundaries describe the relative movement between the two neighbouring plates along the boundary 3 types of plate boundaries based on the relative motions of the plates on either side of the boundary a) Divergent b) Convergent c) Transform Divergent plate boundaries and seafloor spreading - At divergent boundaries, 2 oceanic plates move apart and no open space develops, rather new oceanic lithosphere forms - New sea floor forms only along the axis (centreline) of the ridge - As soon as the new fools forms, it moves laterally away from the ridge, and the youngest sea floors occurs on either side of the ridge axis, and sea floor becomes progressively older away from the ridge How do oceanic crust and lithospheric mantle form at a mid-ocean ridge? - Mid ocean ridge axis lies at water depth of 2-2.5km - The sea floor slopes away from the ridge axis. Reaching the depth of 4-5km at the abyssal plain, at a distance of 500-800km from the ridge axis - Hot asthenosphere rises beneath the ridge and begins to melt, producing magma - Magma has a lower density than solid rock, so it rises, accumulating in the crust blew the ride axis and filling a region called magma chamber - Magma solidification forms the new oceanic crust. Some magma solidifies along the side of the chamber to make gabbro (coarse-grained, mafic intrusive rock) - The rest rises higher to fill vertical cracks and form sheet like dykes of basalt (fine-grained mafic extrusive) or rise all the way to the sea floor forming small submarine volcanoes - Lithospheric mantle effectively does not exist at the ridge axis The 1300C isotherm (the boundary between lithospheric and asthenospheric mantle) coincides with the base of the oceanic crust - The oceanic crust and underlying uppermost mantle move away from the ridge and gradually cool, as soon as the uppermost mantle cools below 1300 C it comes becomes part of the lithospheric mantle and the oceanic lithosphere grown thicker Convergent plate boundaries and subduction - At convergent boundaries, one oceanic plate bends and sinks (i.e subducts) into the asthenosphere beneath another oceanic or continental plate - Oceanic lithosphere over 1- million years old, is denser than asthenosphere and can sink through it - When the convergent plate bends and slips into the mantle, it begins to sink like an anchor - Continental crust cannot be subducted because it is too buoyant; if continental crust moves into a convergent margin, subduction eventually stops - Earthquakes occur along the downing plate down to depths of 660 km. This band of earthquakes, known as the Wadati-Benioff zone, defines the position of the subducting plate. Geologic features of convergent boundaries Trench - A long narrow steep-sided depression in the ocean bottom which orms where the oceanic plate bends and subducts under the overlying plate Accretionary prism - A wedge-shaped material consisting of sediments scrapped off the downing plate - The overriding plate works like a bulldozer Volcanic arc - A chain of volcanoes that develops behind the accretionary prism in the overriding plate - Continental volcanic arc is the overriding plate is continental Volcanic island arc if it is oceanic Back-arc basin - Is small ocean basin that forms between the volcanic arc and the continent Transform plate boundaries - Tuzo wilson suggested that mid-ocean ridges consist of separate segments linked by fracture zones - Ridge axes and fracture zones form at the same time Fracture zones - Narrow belts of broken sea floor oriented at right angles to the ridge segments, intersecting the ends of the segments and extending beyond the ends of segments Transform boundary - The actively slipping segment of a fracture zone between two ridge segments - At a transform boundary, one plate slides sideways past another - Transform faults may cut across continental lithosphere - San andreas fault is the plate boundary between the north american plate and the pacific plate Formation and demise of plate boundaries - continental rifting - The lithosphere stretches horizontally and thins vertically - Near the surface, stretching causes rock to break and faults develop - A low area develops and gradually fills up with sediment - Deeper in the crust and mantle, stretching takes place in a plastic mammer, because rock is warmer and softer - As continental lithosphere thins, hot asthenosphere rises beneath the rift and starts to melt - Magma erupts at volcanoes along the rift - If rifting continues for a long time, the continent breaks into two, a mid-ocean ridge forms, and sea-floor spreading begins - If rigting stops early, it becomes a fault bounded trough with volcanic rocks and a thick layer of sediments Continental rifting - when a continent splits and separates in 2 continents Formation and demise of plate boundaries - collision - When two continental plates collide, the attached oceanic plate breaks off and sinks into the deep mantle - The rocks and sediments once between the two plates are squeeze and forms mountain belts - Collision leads to rise of the earth's surface and thickening of the crust The crust beneath a mountain formed by collision can be up to 60-70 km Collision - Is the process when two buoyant pieces of lithosphere converge and squeeze together What drives plate motion? - Early models depicted mantle convection as the driver of plate motion. It is impossible to draw a global arrangement of convection cells that can explain the complex geometry of plate boundaries - Ridge push force develops because the lithosphere of mid-ocean ridges lies at higher elevation than that of the adjacent abyssal plains - Gravity causes the elevated lithosphere along the ridge axis to push on the lithosphere that lies farther from the axis, making it move away, while new asthenosphere rises to fill the gap - The local upward movement of asthenosphere beneath a mid-ocean ridge is a consequence of sea-floor spreading, no the cause - Slab-pull force is the force applied by the downing plate on the oceanic lithosphere at a convergent margin, because the oceanic lithosphere older than 10 million years old is denser than the asthenosphere, and can sink into it Rocks - The Recorders Of Plate Tectonics! - Rocks provide record of plate tectonic processes and give insight into interactions among components of the Earth System. What is rock? Rock - A coherent, naturally occurring solid, consisting of minerals or less common, a mass of glass Coherent - A rock holds together, and thus must be broken to be separated into pieces. Naturally occurring - Only naturally occurring materials are considered to be rocks, so manufactured materials, such as concrete and brick, do not qualify. An aggregate of minerals or a mass of glass - The majority of rocks consist of an aggregate of many mineral grains and/or crystals, stuck of grown together. Some rocks consist of only one kind of mineral, whereas others of several different kinds. Some rock types that form at volcanoes consist of glass The basis of rock classification - rock groups - Inherited by the work of James Hutton, the “father of modern geology”, rocks are classified on the basis of how they formed - genetic scheme. Igneous rocks - Form by the solidification of molten rock Sedimentary rocks - Form either by the cementing together of fragments broken off bt preexisting rocks or by the precipitation of mineral crystals out of water solutions at or near the earth’s surface Metamorphic rocks - Form when preexisting rocks change character in response to a change in pressure and temperature conditions and/or as a result of squashing, stretching or shear. Metamorphic change occurs in the solid state so does not require melting Physical Characteristics Used to Distinguish Rocks 1. Grain Size - Refers to the dimensions of individual grains in a rock. - Can be fine or coarse. - Grains may be: Equant: Same dimensions in all directions. Inequant: Different dimensions in various directions. - Rocks may have grains of uniform size or a range of sizes. 2. Composition - Refers to the proportions of different chemicals (and thus minerals) that make up the rock. 3. Texture - Describes the arrangement of grains in the rock. - Includes: How grains connect to one another Whether inequant grains are aligned parallel to each other in space. 4. Layering - Defined by: Bands of different compositions or textures. Alignment of inequant grains. - Types of layering: In sedimentary rocks, layering is called bedding. In metamorphic rocks, layering is called metamorphic foliation. The Rock Cycle Definition: The rock cycle describes the transformation of Earth materials from one rock type to another. - Key Features: It is a mass-transfer cycle, involving the movement of materials between different parts of the Earth system. There are multiple pathways through the cycle, reflecting the diverse processes of rock formation and transformation. - Possible Pathways: Igneous → Sedimentary → Metamorphic → Igneous Igneous → Sedimentary → Metamorphic → Sedimentary Igneous → Metamorphic → Sedimentary - These pathways demonstrate the continuous and dynamic processes shaping Earth's crust. Rock-Forming Environments and the Rock Cycle - Igneous Rocks: Develop where melt rises from depth and cools. Intrusive igneous rocks: Form when magma cools underground. Extrusive igneous rocks: Form when lava and ash erupt at the surface. - Sedimentary Rocks: Formation: Weathering and erosion break up existing rocks, producing sediment. Settings: Different sedimentary rocks form in various environments, depending on the composition of the source and the sediment accumulation setting. Examples include: - Alluvial fans - Desert dunes - River channels - Deltas - Coral reefs - Coastlines - Continental shelf - Deep sea - Metamorphic Rocks: Form when preexisting rocks undergo change in the solid state, a process known as metamorphism. Contact metamorphism: Occurs near magma intrusions. Regional metamorphism: Happens where tectonic processes bury rocks deeply Reading: chapter 2, plate tectonics Alfred Wegener’s Continental Drift Hypothesis Concept: - Wegener proposed that continents were once part of a supercontinent called Pangaea, which began breaking apart approximately 200 million years ago. - Evidence includes: Jigsaw fit: The coastlines of continents, such as South America and Africa, fit together like puzzle pieces. Fossil correlation: Identical fossils (e.g., Mesosaurus, Glossopteris) are found on continents now separated by oceans, suggesting these lands were once connected. Rock and mountain correlations: Mountain ranges and rock types on different continents (e.g., the Appalachians in North America and the Caledonian mountains in Europe) match. Paleoclimatic evidence: Evidence of glaciation in now tropical regions (e.g., India, South Africa) and coal deposits in currently cold regions indicates continental drift. - Criticism: Wegener lacked a mechanism to explain how continents moved, which led to skepticism until the mid-20th century. Earth’s Layers and Plate Movement - Structure of the Earth: Crust: The thin, outermost layer; includes continental (thicker and less dense) and oceanic (thinner and denser) crust. Mantle: Beneath the crust, consisting of semi-solid rock capable of slow flow. Core: Composed of a solid inner core and liquid outer core, driving Earth’s magnetic field. - Lithosphere vs. Asthenosphere: Lithosphere: The rigid layer made up of the crust and uppermost mantle; broken into tectonic plates. Asthenosphere: A ductile, partially molten layer beneath the lithosphere. The movement within the asthenosphere (driven by convection currents) enables lithospheric plate motion. - Convection Currents: Hot, less dense material rises, while cooler, denser material sinks in the mantle. These currents are the primary driver of plate motion. Types of Plate Boundaries and Features 1. Divergent Boundaries: - Plates move apart, creating new lithosphere. - Common settings: Mid-ocean ridges: Underwater mountain chains where magma rises, solidifies, and forms new oceanic crust (e.g., the Mid-Atlantic Ridge). Rift valleys: Found on continents when plates diverge (e.g., East African Rift). - Features: Volcanic activity due to magma upwelling. Shallow earthquakes. 2. Convergent Boundaries: - Plates move toward each other, leading to: Subduction zones: Denser oceanic plates sink beneath lighter continental or oceanic plates, forming deep trenches (e.g., Mariana Trench). - Causes volcanic arcs on overriding plates (e.g., Andes Mountains). Continental collisions: Two continental plates collide, forming large mountain ranges (e.g., Himalayas). - Features: Intense earthquakes. Volcanic activity (except in continental collisions). 3. Transform Boundaries: - Plates slide past each other horizontally. - Features: Major fault lines (e.g., San Andreas Fault in California). Shallow but powerful earthquakes. - No significant volcanic activity. Hotspots and Plate Motion - Hotspots: Areas where magma rises from deep within the mantle (not associated with plate boundaries). Create volcanic island chains as plates move over stationary hotspots (e.g., Hawaii). - Tracking Plate Movement: The age and position of volcanic islands indicate the direction and speed of plate motion over geological time. The Wilson Cycle - Concept: Describes the lifecycle of ocean basins: 1. Continental Rifting: Plates diverge, forming rift valleys (e.g., East African Rift). 2. Seafloor Spreading: New oceanic crust forms as rift valleys widen into ocean basins (e.g., Atlantic Ocean). 3. Subduction: Oceanic crust begins to sink at convergent boundaries, leading to shrinking oceans. 4. Closure: Continents collide, forming mountain ranges and closing ocean basins (e.g., Himalayas after the closure of the Tethys Ocean). Supporting Evidence for Plate Tectonics 1. Seafloor Spreading: - Discovered through mapping mid-ocean ridges and studying magnetic patterns in oceanic crust. - Magnetic reversals recorded in the seafloor mirror each other on either side of ridges. 2. Earthquake Distribution: - Earthquakes occur along plate boundaries, with patterns corresponding to different boundary types. 3. Volcanism: - Active volcanism is concentrated at divergent and convergent boundaries, as well as hotspots. 4. Satellite Measurements: - GPS and other satellite technologies directly measure plate motion rates and directions. Pre prac Overview - Igneous Rocks form from the cooling and solidification of magma or molten rock. - Types of Igneous Rocks: 1. Extrusive (Volcanic): Form at the surface with fast cooling rates, resulting in fine-grained textures. 2. Intrusive (Plutonic): Form deep underground with slow cooling rates, leading to coarse-grained textures. - The practical focuses on identifying the textures and mineral assemblages of these rock types. Extrusive Igneous Rocks - Cooling Environment: Surface cooling; rapid. - Textures Reflect Cooling: Fine-grained or glassy. - Types: Lavas: Examples include rhyolites, andesites, and basalts. Pyroclastic Rocks: Explosive; examples include volcanic ash, tuff, and volcanic agglomerate. Intrusive Igneous Rocks - Cooling Environment: Beneath the surface; slow. - Textures Reflect Cooling: Coarse-grained; fully crystalline. - Examples: Granite: Visible mineral crystals; classic example. Diorite: Different mineral assemblage than granite; visible crystals with distinct colors. Gabbro: Dark-colored; coarse-grained. - Identification: Mineral crystals visible with the naked eye or hand lens. Classification of Igneous Rocks 1. Composition (Minerals): - Reflects chemical composition of magma. - Identifiable minerals studied in Practical 1 (aggregated form). 2. Texture: - Crystal Size and Shape: Indicates cooling environment (intrusive vs. extrusive). - Distribution of Mineral Grains: Interlocking grains (e.g., granite). Fine-grained or suspended grains (e.g., basalt). Igneous Rock Textures - Reflect formation environment and cooling history. - Examples: Granite (Intrusive): Large, interlocking crystals. Basalt (Extrusive): Fine-grained or glassy texture. Key Terms 1. Porphyritic: - Large crystals (phenocrysts) embedded in fine-grained groundmass. - Example: Rocks with visible feldspar crystals in finer groundmass. 2. Phenocrysts: - Large crystals within finer groundmass. - Example: Feldspar crystals in porphyritic rocks. 3. Holocrystalline: - Entirely composed of crystals. - Example: Granite or fine-grained basalt. 4. Hypocrystalline: - Mix of crystals and glass. - Example: Rocks with shiny patches (glass) due to rapid cooling. 5. Holohyaline: - Entirely glass (no crystals). - Example: Obsidian (volcanic glass). 6. Aphanitic: - Crystals not visible to the unaided eye. - Example: Basalt with fine-grained groundmass. 7. Phaneritic: - Coarse-grained; crystals visible with the naked eye. - Example: Granite. 8. Vesicle: - Gas cavity within the rock. - Example: Vesicular basalt with "frozen" gas bubbles. Crystal Shape Description - Attributes to Describe: Color: Visual observation. Size: Measurement or estimation. Shape: Degree of crystal formation: 1. Euhedral: Well-formed crystals. 2. Subhedral: Partially formed crystals. 3. Anhedral: Poorly formed crystals. Summary of Key Features - Extrusive Rocks: Rapid cooling, fine-grained, may include vesicles or glass. - Intrusive Rocks: Slow cooling, coarse-grained, fully crystalline. - Textures and Composition: Provide clues to formation and environment. - Key Terms and Features: Aid in describing and classifying rock samples effectively. Key Learning Ensure that you can answer the following questions after working on material throughout the week. These are the key points you should focus on from this week's content. What are the characteristics of a lithospheric plate? How does continental lithosphere differ from oceanic lithosphere? What are the three types of plate boundaries? What are the major features of a convergent boundary? Can you draw a transform boundary and explain the movement that takes place along the transform fault? What are the stages of continental rift? What are the major forces that move lithosphere plates? What are the three rock groups and how they form?