Tectonic Impacts

This is the first topic of the Earth and Environmental Science HSC Course. Its very similar to the last topic you studied: Dynamic Earth however you will look at the tectonic theory in more detail.Topic 1- Tectonic ImpactsTopic OverviewThere are 6 parts to this Topic:Lithospheric PlatesVolcanic MountainsTectonic MountainsEvolution of Continents Earthquakes, Volcanoes and Natural HazardsClimate Change

Topic OutcomesDescribe the lithospheric plates and their motionExplain how the movement of plates results in mountain buildingExplain how continents grow and change shape as plate boundaries move and changeDescribe how natural disasters are often associated with tectonic activity like earthquakes and volcanoesExplain how environmental conditions caused by tectonic activity may contribute to the problems experienced by peopleExplain the link between plate tectonics and climate, both in the short term and throughout geological history.In this part you will be studying lithospheric plates in great detail. We will be reviewing a lot of content covered in the last Topic.Part 1-Lithospheric PlatesLesson 1 IntroductionIn the last topic you studied: Dynamic Earth, you studied plate tectonics and the forces which drive them. These plates are the top part of the lithosphere.

The LithosphereThe lithosphere is the rigid, brittle outer layer of the Earth. Lithos means rocks. Above the lithosphere is the hydrosphere and atmosphere.

AsthenosphereThis is the section below the lithosphere and consists of the upper part of the Mantle. Astheno means weak or without strength. Its made up of partially molten rock which scientists call its state to be plastic.

MesosphereEverything below the asthenosphere: the rest of the mantle, outer and inner core is called the mesosphere. Meso means middle. Earths InteriorHow do scientists know so much about Earths interior? After all humans have never even breached the crust..

Seismologists provide us with the information.

Earths InteriorSeismologists are scientists who study earthquakes. When an earthquake happens an extreme amount of energy is released. This energy travels in waves (seismic waves) through the Earth. By studying the behaviour of these seismic waves scientists are able to estimate the thickness and composition of the Earths interior.

Earths InteriorAs the seismic waves pass through layers of the Earth, the energy changes speed and direction. This is due to the materials having different densities.

Earths InteriorUnderstanding the Earths interior is vital to our understanding of crustal plates and their movements. So why do we care?Earths InteriorWe care because this information can save lives and property by helping to predict earthquakes and volcanoes. It also helps us to explain the current distribution of plants and animals and predict future outcomes.

Lesson-2Crustal Plate ReviewThe Theory of Plate TectonicsIf we look at a globe carefully, most of the continents seem to fit together like a puzzle. The West African coastline seems to fit nicely into the east coast of South America.

The fit is even more striking when the submerged continental shelves are compared rather than the coastlines.

The Theory of Continental DriftAlfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a singe protocontinent which he called Pangea (meaning All lands) and over time they have drifted apart.

The Theory of Continental DriftWegeners Hypothesis lacked a geological mechanism to explain how the continents could drift across the Earths surface as he had proposed. In 1929 Arthur Holmes elaborated one of Wegeners many hypothesis:

The Theory of Continental DriftArthur Holmes stated that the mantle undergoes thermal convection. And that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents.

The Theory of Continental DriftNot until the 1960s did Holmes idea receive any attention. Greater understanding of the ocean floor and the discoveries of features like mid-ocean ridges, geomagnetic anomalies parallel to the mid-ocean ridges, and the association of island arcs and oceanic trenches occurring together and near continental margins, suggested convection might indeed be at work.The Theory of Continental DriftTo understand this theory better we need to look at the geological processes taking place and the evidence which supports it.

Well start by looking at the crust.

Earths CrustThe crust covers the mantle and is the Earths hard outer shell, the surface we are living on .

Compared to the other layers the crust is much thinner.

Earths CrustThe crust is made up of solid material but this material is not the same everywhere. There is an oceanic crust and a continental crust.

Oceanic CrustThis crust is below the ocean and is 6-11 km thick. The rocks of the oceanic crust are very young compared with rocks of the continental crust; not older than 200 million years. The main rock type is basalt and the average density is 3g/cm^3

Continental CrustContinental crust is the part of Earths crust not covered by water. This is much thicker than oceanic crust with an average of about 30-40km and a maximum of 70km. Continental crust is much older, some rocks are 4.1 billion years old. Continental crust consists of igneous rocks such as Granite. The average density is 2.7g/cm^3

Plate BoundariesThe Theory of Plate Tectonics builds on Wegeners Theory of Continental Drift. This theory states that the Earths crust or Lithosphere is broken up into tectonic plates

Plate BoundariesIf we analyse the distribution of earthquakes around the world, we are able to identify the locations of the plate boundaries.

Why would we associate earthquakes with plate boundaries?

Plate BoundariesIf we imagine the tectonic plates like blocks of wood, there are three ways they can interact.

Plate BoundariesRemember what Holmes said about convection currents? He stated that the mantle undergoes thermal convection. And that this thermal convection was like a conveyor belt and that the upwelling pressure could break apart a continent and then force the broken continent in opposite directions carried by the convection currents. What type of plate boundary would assimilate with Holmes idea?

Divergent Plate BoundaryDivergent plate boundaries are located where plates are moving away from one another. This occurs above rising convection currents as Holmes suggested. Mid-ocean ridges are found at this boundary.

Convergent Plate BoundaryConvergent plate boundaries are locations where plates are moving towards one another. The plate collisions that occur in these areas can produce earthquakes, volcanoes and crustal deformation. Subduction zones are found at this boundary.

There are three different types of convergent boundarieswhich we will discussmore later.

Transform Plate BoundaryTransform plate boundaries are locations where two plates slide past one another. The fracture zone that forms a transform plate boundary is known as a transform fault. Most transform faults are found in the ocean basin and connect offsets in the mid-ocean ridges.

Homework Complete DOT Point 1.1, 1.3

Read pages 1-5 HCS Spotlight Text

Start electronic vocab list of all bold terms pg1-5 Spotlight TextEg:

Tectonic Impacts Vocab ListPlate: Large pieces of the Earths crustPlate Interactions

Lesson-3

Divergent Plate BoundaryThis is where two plates separate. These spreading zones are where new crust is being generated. Mid-ocean ridges mark divergent plate boundaries. Divergent Plate BoundaryThese boundaries are some of the most active on Earth. Volcanoes at these boundaries produce basalts low in silica. This means the lava has a low viscosity and releases gasses easy. Eruptions are therefore less explosive.

Divergent Plate BoundaryLava usually erupts in long cracks in the Earths surface called fissures rather than mountains. This is because the crust is being pulled apart rather than forced together.

Divergent Plate BoundaryRocks at divergent boundaries do not usually undergo metamorphism. Earthquakes are shallow as the forces cause rocks to crack and sink along fault lines.

Divergent Plate BoundaryGeneral Composition of Igneous Rocks:Oceanic DivergenceBasalticLow silica contentHigh concentration of dark coloured mafic minerals

Continental DivergenceRhyoliticRich in silica and felsic mineralsLight in colour

Convergent Plate BoundaryThis is where two plates come together collide. There are three types of convergent zones: oceanic-oceanic oceanic-continentalContinental-continental

Oceanic-Oceanic ConvergenceThis type of boundary occurs between two pieces of oceanic crust. Because no buoyant continental crust is involved we do not get large mountain ranges.

Oceanic-Oceanic ConvergenceAs one plate is forced beneath the other, it is melted and a line of volcanoes form in the same way as described in a oceanic-continental convergent boundary.

Oceanic-Oceanic ConvergenceThe volcanoes that are created at this boundary form a curved line out of the sea and scientists call these island arcs.

Oceanic-Oceanic ConvergenceExamples of island arcs include the Philippine Islands, Japan and Indonesian Islands.

Oceanic-Oceanic ConvergenceMetamorphism occurs at these boundaries and the trenches formed are the deepest on the planet. The Mariana Trench is 11 kilometres deep. These boundaries also have deep and shallow earthquakes.

Ocean-ocean Convergence Plate BoundaryGeneral Composition of Igneous Rocks:Mainly BasaltsIncrease in Andesites as island arcs matureHigh silica granite and less silica gabbro intrusions can be found within the crustOceanic-Continental ConvergenceBecause oceanic crust is more dense than continental crust, when these two types of crust collide the oceanic crust is always forced into the mantle (subducted).

Oceanic-Continental ConvergenceWhere the oceanic crust is bent downward, trenches form. These are the deepest parts of the ocean and are used to help indicate the edge of a plate.

Oceanic-Continental ConvergenceAs the subducting oceanic plate is forced beneath the overriding continental plate, the continental plate is lifted and folded upwards producing mountains.

Oceanic-Continental ConvergenceAn example of a oceanic-continental convergent plate boundary is along the west coast of South America. Here we find the Andes Mountains.

Oceanic-Continental ConvergenceThe Oceanic Nazac plate is slamming into the Continental South American plate

Oceanic-Continental ConvergenceIf we visit this plate boundary we can see the evidence which supports this theory.

Oceanic-Continental ConvergenceSomething else happens at this boundary which produces mountains. As the oceanic crust is forced deeper into the Earth, intense heat and pressure cause it to melt. And what happens to materials when we heat them?

Oceanic-Continental ConvergenceThey rise! The oceanic plate turns into magma and rises into the overlying continental crust. This is called a magma plume. If this plume reaches the surface we get volcanoes!

Oceanic-Continental ConvergenceA key feature to this magma is that it is rich in silica which comes from the sediments in the oceanic crust or the melted part of the continental crust.

Oceanic-Continental ConvergenceThis high silica magma makes it more viscous (thicker) which allows it to trap gasses within it. Volcanic eruptions at such boundaries tend to be very explosive because of the intense heat and pressure build up.

Oceanic-Continental ConvergenceThis type of silica rich and explosive eruption is called andesitic volcanism (after the Andes Mountains).

Oceanic-Continental ConvergenceBut what happens if the magma never makes it to the surface? Thats a great question! The magma that has collected within the continental crust can slowly cool to form granite or similar intrusive igneous rocks.

Oceanic-Continental ConvergenceEventually the surrounding material is eroded away and we can see these frozen granite plumes today on the surface of the earth.

Oceanic-Continental ConvergenceStone Mountain in Georgia (USA) is an example of just how big these frozen granite magma plumes can get.

Oceanic-Continental ConvergenceSo what happens to the rocks on the continental crust that come into contact with these magma plumes?

Oceanic-Continental ConvergenceThey are changed metamorphosed into a different rock. This is how we get metamorphic rocks.

Oceanic-Continental ConvergenceWhen these rocks are uncovered due to weathering and erosion, we can see where this contact metamorphism has happened.

Oceanic-Continental ConvergenceOceanic-continental convergence boundaries also experience a lot of seismic activity. Earthquakes are quite common as the two plates are crushing into each other.

Oceanic-Continental ConvergenceAn earthquakes focus is the exact point where the rocks in the crust break or move. At a subduction zone, scientist can measure both deep and shallow earthquakes. This provides further evidence that the oceanic plate is being forced into the mantle.

Ocean-continental Convergence Plate BoundaryGeneral Composition of Igneous Rocks:Andesites are most commonRhyolites and basalts can be foundHigh silica contentIntrusions of high silica granite and low silica gabbro can be found within the crustContinental-Continental ConvergenceThis type of boundary is where two continental pieces of crust slam into each other. This boundary is different than the last because there is not complete subduction.

Continental-Continental ConvergenceThis is because the continental crust is so buoyant. Instead the plates become intensely folded and uplifted. An example of this is the Himalayas.

Continental-Continental ConvergenceEarthquake foci are shallow at this boundary and there is no contact metamorphism. Instead there is regional metamorphism. Low heat, high pressure.

Continent-continent Convergence Plate BoundaryGeneral Composition of Igneous Rocks:No volcanic rocksGranite intrusions common (high silica content)Remnants of oceanic crust (basalts) found hereHomeworkComplete DOT Point 1.2Forces that drive plate movementLesson 4Two TheoriesThere are two hypothesis to explain the driving force behind plate tectonics.

Plates are carried and moved on top of the mantle by convection currents within this plastic molten material.

Plates are carried and moved by a combination of forces; ridge-push and slab-pull acting under the force of gravity.

VideoConvection CurrentsConvection currents within the mantle generated by radioactive decay in the core, cause molten material to rise towards the crust.

Convection CurrentsAs this molten material reaches the crust it cools and moves horizontally dragging the crust with it.

Convection CurrentsAs the molten material drags the lithosphere horizontally and cools, it begins to sink back into the mantle. The lithospheric plate is carried with it.

Convection CurrentsThe plate is recycled into the molten mantle and the convection cycle starts again.

HomeworkComplete DOT Point 1.4-1.5