GEOL 101 1nd Edition Lecture 32 Outline of Last Lecture I. Plate BoundariesII. ConvergentIII. DivergentIV. TransformOutline of Current Lecture I. Evidence of Plate TectonicsII. Force Driving Plate TectonicsCurrent Lecture- Evidences that support plate tectonics:- Fit of the continents, fossil correlation, etc-Other evidences include observations from:- Ocean drilling- Hotspots- Paleomagnetism - Satellite data- Ocean drilling- Between 1968 to 1983 the National Science Foundation sponsored several drilling projects that extracted drill cores from the seafloor- The goal of these projects was to establish ages of the rocks on the seafloor- It was observed that the youngest rocks were at the sea floor spreading ridges, and the rocks got older as you move away in either directiono The oldest rocks on the seafloor have been radiometrically dated at 180 million yearso Contrast to crustal rocks, some with radiometric dates of 4 billion years- HotspotsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Mapping of the Pacific seafloor revealed several linear chains of volcanic seamounts (submarine volcanoes)- Over 100 seamounts stretch a distance of 6000 km to make the Hawaiian Island-Emperor Seamount chaino Hawaii is the only island or seamount in this chain that is currently volcanically activeo The ages of the seamounts get progressively older the farther they are away from Hawaii- The most widely accepted theory for the formation for the Hawaiian Islands and Emperor Seamounts is that there is a hotspot, currently active beneath Hawaii- For reasons that are still widely debated, hot mantle material rises up in the form of a mantle plume.o The mantle plume initiates volcanism- The Pacific plate moves over top of the stationary hotspot, eventually cutting off one volcano’s magma supply, and forming a new volcano- Paleomagnetism- As lava is cooling, iron-rich minerals will align themselves with the earth’s magnetic field,like tiny compass needleso When the lava completely cools, the iron-rich minerals are frozen in the rock, preserving the paleo-magnetic field- It has been observed in various igneous rocks throughout the world that the earth’s magnetic field has gone through polar reversals o The iron-rich minerals are oppositely aligned so that south is north, and north is south- As oceanographers were mapping the ocean bottoms, they noticed magnetic variations within the basalts on the seaflooro When mapped out, these variations exhibited a definite pattern, especially at themid-ocean ridges, known as magnetic stripping- There are equally thick normal and reverse polarization patterns on either side of spreading centerso There are also equal dates for these rocks on other side of the ridge- Satellite and GPS data- Using satellites and global positioning system (GPS) technology, we are able to track the motion of the plates- Receivers placed around the world can track both the direction and rates of plate movements over timeo Plates move between about 2-8 cm per yearDriving Force- Slab pull – as a subducting oceanic plate sinks down into the mantle, it “pulls” the rest ofthe plate alongo Providing main driving force for plate tectonic o Plates with more subducting lithosphere tend to be faster moving plates- Ridge push – as more seafloor is generated at the ridges, material gets “pushed” away bygravity-driven mechanisms- Subducting suction – mantle circulation tends to drive the two plates together, creating asuction zone at the trencho Similar to pulling the plug in a bathtub- Ultimately, the main driving force for plate tectonics comes from convection in the mantle.- Lower mantle heated by core- This heated material is more buoyant and so it rises- When it gets closer to the surface it cools off and gets denser - The cooler denser material sinks back down towards the core- 660 model- This model proposes that there is convection occurring in the lower mantle and separateconvection in the upper mantle- The boundary between these convection cells is the 660 discontinuity- Geophysics indicates that subducting slabs end up at the 660; many have nicknamed thisboundary the subducted plate “graveyard”- Whole-mantle convection model- In this model, the subducting slabs sink past the 660 discontinuity into the lower mantleo Supported by some geochemical data- Mantle plumes originate near the core/mantle boundaryo Not supported by new geophysical data- Deep-layer model- This model features a deep mantle layer that is chemically different from the upper layer- The two layers move like the liquids in a lava lamp, allowing deep mantle material to riseup as hotspot plumes- There is little geophysical evidence to support this