Lecture 4-Earth Continental Drift Plate Tectonics PDF

The Earth’s Continental Drift, Plate Tectonics and Rock Cycle Global Environmental Processes (GEOG1020 E1) Dr Lawal Billa  The Earth’s Layers o The Earth is made of many different and distinct layers. The deeper layers are composed of heavi...

The Earth’s Continental Drift, Plate Tectonics and Rock Cycle Global Environmental Processes (GEOG1020 E1) Dr Lawal Billa  The Earth’s Layers o The Earth is made of many different and distinct layers. The deeper layers are composed of heavier materials; they are hotter, denser and under much greater pressure than the outer layers. o Natural forces interact with and affect the earth’s crust, creating the landforms, or natural features, found on the surface of the earth.  The earth’s crust o The crust is the rigid, rocky outer surface of the Earth, o It is composed of two rocks. The continental crust is mostly granite. The oceanic crust is basalt. Basalt is much denser than the granite. Because of this the less dense continents ride on the denser oceanic plates. o The crust is only about 3-5 miles (8 kilometers) thick under the oceans (oceanic crust) and about 25 miles (32 kilometers) thick under the continents (continental crust). The Lithospheric Plates The crust of the Earth is broken into many pieces called plates. The plates "float" on the soft, semi-rigid asthenosphere. The crust and the upper layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere. o The MANTLE is the largest layer of the Earth is composed of silicon, oxygen, magnesium, iron, aluminum, and calcium. o The middle mantle is composed of very hot dense rock that flows like asphalt under a heavy weight. The movement of the middle mantle (asthenosphere) is the reason that the crustal plates of the Earth move. o The middle mantle "flows" because of Convection Currents caused by very hot material at the deepest part of the mantle rising, then cooling and sinking again, this cycle is repeating over and over again o The CORE of the Earth is like a ball of very hot metals composed of two layers o The outer core is so hot that the metals in it are all in the liquid state. The outer core is composed of the melted metals of nickel and iron. o The inner core of the Earth has temperatures and pressures so great that the metals are squeezed together and are not able to move but are forced to vibrate in place like a solid.  Continental Drift Theory German scientist (Alfred Wegener) proposed the continental drift theory in the 1900s He hypothesized that the earth was once a single super continent (Pangaea) He believed Pangaea floated on the oceanic crust (like an iceberg ) Most scientist at the time, rejected his theory due to lack of evidence. Evidence of continental drift: Continents fit together like a puzzle Fossils match across oceans Rock types and mountain ranges match across oceans Climate evidence found in glacial deposits Ocean floor spread 1. Continental puzzle pieces Continents look like they could be part of a giant jigsaw puzzle Wegener thought the continents had been joined from the almost perfect fit of the shorelines of Africa and South America 2. Distribution of fossils o Plant and animal fossils found on the coastlines of different continents o The Mesosaurus, an aquatic reptile has fossils found only in eastern South America and southern Africa, if it were able to swim well enough to make it across the Atlantic Ocean, the fossils would be more widespread 3. Sequence and age of rocks o If the continents existed together in Pangaea, they should have matching rock types and features should match closely in age and type o Same rock patterns found in South America, India, Africa, Antarctica and Australia o Rock evidence for continental drift exists in the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean o The Appalachian Mountains on the Eastern side of North America have similar ages to mountains in the British Isles and Scandinavia  4. Climate evidence o Tropical plant remains (coal deposits) found in Antarctica o Glacial deposits in Africa, South America, India, and Australia during the same time. Evidence of massive glaciers all over Africa and South America that matched each other in age and position o The problem was figuring out how ancient glaciers were on these now tropical regions, unless the continents moved to their current positions  5. Sea-floor spreading The process that continually adds new material to the ocean floor while pushing older rocks away from the ridge (Harry Hess 1960’s) New ocean floor forms along cracks in the ocean crust as molten material erupts from the mantle spreading out and pushing older rocks to the sides of the crack. Ocean floor moves like a conveyor belt carrying continents with it as new ocean floor is continually added by the process of sea-floor spreading. Evidence of Sea-Floor Spreading 1. Evidence from Molten Material Rocks shaped like pillows (rock pillows) show that molten material has erupted again and again from cracks along the mid- ocean ridge and cooled quickly 2. Mid-Ocean (Atlantic) Ridge o The mid-ocean ridge system is the most extensive chain of mountains on earth, but more than 90% of this mountain range lies in the deep and extends around the globe for more than 65,000 km o It occur along plate boundary where new ocean floor is created as the plates spread apart. "divergent plate boundary." o The plates spread apart at rates of 1 cm to 20 cm per year. As the plates move apart, rock melts and swells up from tens of kilometers deep. 3. Evidence from Magnetic Stripes Rocks that make up the ocean floor lie in a pattern of magnetized stripes which hold a record of the reversals in Earth’s magnetic field 4. Evidence from Drilling Samples Core samples from the ocean floor show that older rocks are found farther from the ridge; youngest rocks are in the center of the ridge  Plate Tectonic Theory The idea that the earth’s landmasses have broken apart, rejoined, and moved to other parts of the globe forms part of the plate tectonic theory. These pieces of Earth’s top layer are called tectonic plates. and moving very slowly unnoticeable to the human eye, but constantly Along the mid-ocean ridge the seafloor is pulling apart and the two parts are moving in opposite directions, carrying along the continents and oceans that rest on top of them Currently Earth’s surface layers are divided into nine very large plates and several smaller ones. These plates are not anchored in place but slide over a hot and bendable layer of the mantle.  Plate Boundaries Three types: – pulling away from each other (Divergent) – crashing head-on (Convergent) – or sliding past each other. (Transform)  Mechanisms of Divergent Boundaries Boundary between two plates that are moving apart or rifting  Rifting causes Seafloor Spreading Features of divergent boundaries: Mid-ocean ridges Rift valleys Fissure volcanoes  Pulling Apart o When plates pull away from one another they form a diverging plate boundary, or Spreading zone. o Iceland has a divergent plate boundary running through its middle Thingvellir, the spreading zone in Iceland between the North American (left side) and Eurasian (right side) tectonic plates. January 2003. Mechanisms of Convergent Boundaries o Boundaries between two plates that are colliding   o There are 3 types: Ocean to continent collision Ocean to ocean collision Continent to continent collision Ocean to continent collision: o Ocean plate colliding with a less dense continental plate o Subduction Zone: where the more dense oceanic plate slides under the less dense continental plate, it heats and dehydrates as it subsides o Volcanoes: occur at subduction zones as the melt rises forming volcanism  Continental/Oceanic Crush Subduction – Process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle; allows part of the ocean floor to sink back into the mantle Deep-Ocean Trench – Occurs at subduction zones. Deep underwater canyons form where oceanic crust bends downward Andes Mountains, South America Ocean to ocean collision: o Ocean plate colliding with another ocean plate causing it to sink into the mantle o The more dense plate slides under the less dense plate creating a Subduction Zone o The subducting plate is bent downward to form a very deep depression in the ocean floor called a Trench. o The worlds deepest parts of the ocean are found along trenches (e.g. The Mariana Trench is 11 km deep) Both oceanic plates crash (converge) o When both are oceanic plates, one slides under the other. Often an island group forms at this boundary. The Mariana Trench Aleutian Islands, Alaska Continent to continent collision: o A continental plate colliding with another continental plate o Collision Zones: The plates push against each other, creating mountain ranges e.g. European Alps and Himalayas.  Both continental plates crash (converge) o Earth’s highest mountain range, the Himalayas, was formed millions of years ago when the Indo-Australian Plate crashed into the Eurasian Plate. Even today, the Indo-Australian Plate continues to push against the Eurasian Plate at a rate of about 5 cm a year!  Mechanisms of Transform Boundaries o Boundary between two plates that are sliding past each other o Earthquakes along faults This causes: Faults Rift valleys Earthquakes Volcanic activities Transform Boundary Eg. The San Andreas Fault lies on the boundary between two tectonic plates, the north American Plate and the Pacific Plate. The two plates are sliding past each other at a rate of 5 to 6 centimeters each year. This fault frequently plagues California with earthquakes. These areas are likely to have a rift valley, earthquake, and volcanic action. San Andreas Fault, California Consequences of Plate Tectonics 1. Pasific Ring of Fire Shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism focused along the plate boundaries Volcanism is mostly focused at plate margins: known as the “Pacific Ring of Fire”. 2. Hotspot Volcanoes o Hotspot Volcano is hot mantle plumes breaching the surface in the middle of a tectonic plate o A mantle plume may rise at any location in the mantle, thus hotspot volcanoes are independent from tectonic plate boundaries. o A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time. o The source of the volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface o The Hawaiian island chain are an example of hotspot volcanoes. 3. Global Distribution of Earthquakes o Earthquakes are not randomly distributed over the globe o At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. o Dots show earthquake activity globally, unlike volcanoes, the earthquakes are not randomly distributed around the globe, o Earthquakes occur in linear patterns associated with plate boundaries (Map shows earthquake distribution closely resembles the Pacific Ring of Fire volcanism). Global distribution of earthquakes 4.Tsunami Formation o Huge mass of water with tremendous momentum o Shallow focus earthquake o Large volume of oceanic crust displaced o The movement of the crust also displaces a large volume of water o Tsunami reach greater heights when they enter harbor or narrow space - 8 m wave on open coastline - 30 m wave in narrow harbor Tsunami, Japan, 2011  The Rock Cycle The continuous cyclical changes of chemical, organic, cementation, pressure, compaction, heating, cooling, transformation, melting, weathering and decomposition from rock through climatic, environmental and tectonic processes into soil sediments and back again into rock. Simply, the process of rocks changing from one type of rock to another The Rock Cycle is a group of changes in which: o Igneous rock can change into sedimentary rock or into metamorphic rock o Sedimentary rock can change into metamorphic rock or into igneous rock. o Metamorphic rock can change into igneous or sedimentary rock.  Types of Rock Three types :  Igneous  Sedimentary  Metamorphic Igneous Rock Igneous rocks are formed by magma and lava as it cools. Igneous rocks form in two different ways. o Rock that comes from lava cools very fast and can either be very light and airy or crystal (glassy). o Rocks that come from magma are formed under ground. Mostly very hard and have more colors.  Sedimentary Rock Sedimentary rock is made when sediments (sand, gravel, and dirt) are pressed together over time and become a rock Sedimentary rock is formed in layers near the earth's surface at relatively low temperatures and pressures primarily by: deposition by water, wind or ice The can form by precipitation solution, biological media and organic processes (e.g., carbonate reefs) Metamorphic Rock Metamorphic rocks are made from other rocks when their rocky material experiences intense heat and pressure in the crust of the earth. Heat and pressure help change an igneous or sedimentary rock into a new kind of rock The heat and pressure do not change the chemical makeup of the parent rocks but they do change the mineral structure and physical properties of those rocks. Rocks have many different Classification Volcanic Rocks, are extrusive rocks or lava rocks, crystallize when the magma reaches the earth’s surface cooling quickly. Plutonic Rocks or intrusive rocks crystallize within the crust of the earth, and as a result plutonic rocks cool at a much slower pace then volcanic rocks Rock Parts: Rocks can be sorted into groups by looking at their parts. Some rock parts are: o Crystals o Grains  Crystals are different atoms that are formed in a pattern. They can be big or small, fat or thin. Grains are smaller pieces of rock or sediment. Grain size is something that is used a lot to help decide the rock type. Thank You