Dynamic Earth

Plate Tectonics-The Birth of a Theory

Topic 2-Lesson 1The 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

VideoPlate Tectonics Video

HomeworkRead pages 122-125 Prelim Spotlight TextUpdate vocab listCompete DOT Point 2.1 pg 89Evidence for Continental DriftTopic 2-Lesson 2Plate TectonicsThe 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 each of which are moving as a result of convection currents in the mantle.

Plate TectonicsThe plate tectonics theory became established during the 1960s and is now regarded as the unifying theory of geology. This means that all geological processes can be explained and understood in terms of this model.

Convection CurrentsConvection currents are in all unevenly heated fluids. If we look at the diagram below of a jug of water being heated by a flame, the water at the bottom of the beaker is heated the most. It expands, becomes less dense and rises to the surface.

Convection CurrentsWater from besides the point of heating replaces the rising water. As hot water moves to the surface, it looses heat and is pushed sideways by the rising water below. This cooler surface water has a higher density and so sinks to the bottom where the cycle starts again.

Convection CurrentsConvection currents are driven by the effects of gravity on different densities within a fluid.

Convection CurrentsThe mantle isnt as fluid as water, its only partially molten and able to flow slowly over time. Magma rises above the hottest parts of the mantle, strikes the overlying crust and them moves sideways. As it moves sideways it looses heat, becomes more dense and sinks back into the mantle

Convection CurrentsIts the sideways motion of the convection current in the mantle that is believed to be the driving force behind continental drift.

Evidence for Continental DriftDespite the fact that Wegener collected a variety of evidence to support this theory, it wasnt widely accepted until exploration of the ocean floor found a number of features that could only be explained by continental drift. The evidence includes:Continental ShapesCross-continental geological formationsGlacial depositsPalaeoclimatesApparent polar wanderingMagnetic ReversalsDirect Measurement

We are going to look at some of the evidence today and the rest next lesson.

Continental ShapesFor centuries scholars have commented on how perfectly South America fits with Africa. This jigsaw-type fit is not apparent with the shapes of other continents however the mapping of the sea floor tells another story. When using the entire continental shape rather than just the dry land component, the jigsaw fit occurs with many continents.

Cross-continental geological formations The position of some geological features on widely separated continents align perfectly when the continents are placed together. Various features such as folded mountains, sedimentary deposits and fossils act like parts of a picture printed onto blank jigsaw pieces.

Glacial DepositsWe know that during ice ages, ice and glaciers spread outward over large regions from the poles. These giant glaciers leave deep scratches in rocks that can today be easily identified and dated. During the Palaeozoic glaciation 300mya, rocks far away from the poles near the equator have such marks on them.

Palaeoclimates (ancient climates)Numerous types of rocks and minerals can only be created under specific climactic conditions. We know coal is formed from organic materials deposited in warm, swampy areas. Antarctica has coal deposits! What does this tell us about its location over time?

Homework Read pages 125-127 Prelim Spotlight TextComplete DOT Point 5.1 pg 103Update vocab list Evidence for Continental Drift

Topic 2-Lesson 2 continuedReviewIn the last lesson we looked at the following evidence which supports continential drift:Continental ShapesCross-continental geological formationsGlacial DepositsPalaeoclimates

Today we are going to look atApparent polar wondering Magnetic reversalsAge of ocean rocks and sedimentsDirect measurementApparent Polar WanderingWhen lava erupts onto the surface, the iron minerals within it, such as magnetite, align themselves with the Earths magnetic field in the same way as a compass.

Apparent Polar WanderingOnce the lava cools and solidifies, the minerals are locked into position. By analysing the alignment of magnetic minerals within lava, geologists are able to determine the directions of the North and South poles when the lava erupted.

Apparent Polar WanderingIn addition to the magnetic bearings, the distance at which the lava formed from either pole can also be estimated by the angle of tilt of these magnetic minerals.

But How???

Apparent Polar WanderingIf you hold a hand compass vertically to the ground, it tells us the angle of the Earths magnetic field.

Apparent Polar WanderingAt the equator, the compass needle will be parallel with the ground. This is how the minerals would align themselves.

Apparent Polar WanderingAt the magnetic South Pole the south end of the compass needle will point straight at the ground.

Apparent Polar WanderingMidway between the equator and the poles, the compass needle will point down at a 45-degree angle

Apparent Polar WanderingIf lava erupted today in Sydney, the south end of the magnetic minerals (compass needle) would point down at 34 degrees.

Apparent Polar WanderingBy measuring the angle of the iron minerals formed in rocks, scientists are able to determine both the direction of the Pole and the distance from the pole when the mineral solidified. Magnetic ReversalsFor reasons not fully understood, the polarity of Earths magnetic field reverses every few million years. In other words, the magnetic North Pole becomes the magnetic South Pole and vice versa.

Magnetic ReversalsSuch magnetic reversals are believed to be caused by the changes in the circulation patterns of molten iron in the Earths outer core.

Magnetic ReversalsThis flipping is rapid and the locations of the poles of the globe stay much the same. If this were to happen today, the north end of our compass would point towards Antarctica.

A series of magnetic reversals were first identified in lave fields on land. Using radiometric dating scientists were able to measure dates for the reversals.

But why does this matter?Magnetic ReversalsThe importance of magnetic reversals to continental drift can be seen when examining the polarity of rocks across mid ocean ridges.

Magnetic ReversalsThe pattern of magnetic reversals on one side of oceanic ridge is a mirror image of that on the other. This can only be explained if the crust is being pulled apart at these ridges.

Magnetic ReversalsAs the crust breaks open in the central valley of an oceanic ridge, lava erupts, forming a long and narrow plug. Further spreading at the ridge will cause the old plug to tear down the middle and fresh lava to create a new plug.

If Earths magnetic field reversed between these eruptions, the different lava flows can be identified.Magnetic ReversalsThink about it:If new crust is being created at mid-ocean ridges and the planet isnt growing in size. Whats happening to the old crust??

Age of Ocean Rocks and SedimentsIf new crust is continuously formed at oceanic ridges and is then pulled apart sideways, rocks should get older the further they are from the ridge.

Age of Ocean Rocks and SedimentsRadioisotope dating has shown this to be the case at all oceanic ridges. Ocean basins formed at ridges are only 200 million years old.

Age of Ocean Rocks and SedimentsOrganic debris and other sediments rain down onto the deep ocean floor at a steady rate. The fact that little or no sediment covers rocks at the ridge, while sediment layers get thicker as you move away from it. This is further evidence that ocean rocks increase with age away from the ridge.

Direct MeasurementWith today's technologies such as computers, laser measurement and satellite remote sensing, the slow movement of the Earths plates can be measured directly. By calculating the distance between two points on ether side of a plate boundary, the rate and direction of relative movement can be determined. Evidence of Continental DriftThe scientific community finally accepted Wegners theory because of all the evidence. It was unfortunate that this didnt happen until 30 years after his death.

HomeworkRead pages 127-131 Prelim Spotlight TextUpdate vocab listComplete To Think About pg 135 Prelim Spotlight TextComplete DOT Points 3.1-3.5 pg 93-94The break up of PangaeaTopic 2-Lesson 3The break-up of PangaeaActivityRead pages 131-134 togetherStart To Think About pg 135HomeworkUpdate electronic vocabularyFinish To Think About pg 135Characteristics and features of mid-ocean ridgesTopic 2-Lesson 4Plate 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. Subductinos 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.

Mid-Ocean RidgesSea floor spreading is happening at this very moment. The mid-ocean ridge is a continuous chain of volcanoes on the ocean floor where lava erupts and the crust of the Earth is created.

Mid-Ocean RidgesBasalt and Gabbro are two common rocks which form at mid-ocean ridges.

Mid-Ocean RidgesRocks which form along mid-ocean ridges are composed of mostly mafic minerals such as olivine, pyroxene and amphibole.

Mafic: is used to describe silicate minerals, magmas and rocks which are relatively high in heavier elements.

Mid-Ocean RidgesThe following geological features can be found at mid-ocean ridges:

Fissures (under(water of course)Black smokers Pillow lava (basalt)

Mid-Ocean RidgesHere the Earths crust is spreading, creating new ocean floor and literally renewing the surface of our planet. Older crust is recycled back into the mantle elsewhere on the globe, typically where plates collide. Using today's technology we can measure this plate movementDirect MeasurementWith technologies like computers, laser measurement and satellite remote sensing, the slow movement of the Earths plates can be measured directly. By calculating the distancebetween two points on either side of a plateboundary, the rate and direction of relative movement can be determined

Direct MeasurementThe plates move in different directions which means they collide, separate and rub past one another. Individual plates have one general direction of movement but the rates at which different parts of the plate move can vary.

Direct MovementThese variations in movement cause the tearing or folding of plates where the pressures are greatest or rocks are weakest. These forces cause fault lines.

The Break-up of PangaeaFrom the geological evidence available, scientists have been able to piece together the previous positions of the continents for the last 200 million years.

The Break-up of PangaeaAt the beginning of this period (200mya), the Earths continental crust was joined into one supercontinent that stretched from pole to pole on one side of the planet.Pangaea means all lands The massive ocean on the other side is called Panthalassa meaning all seas

The Break-up of Pangaea180 mya Pangaea had begun to break up into two main slabs.The northern slab contained what was to become North America, Greenland, Europe and Asia.The southern slab, Gondwana contained land that would form South America, Africa, Antarctica, India and Australia

The Break-up of PangaeaBy 135 MYA South America had begun to separate from Africa; North America and Greenland from Europe; and Australia and Antarctica from the rest of Gondwana.

The explanation as to why previously joined continents begin to break up and travel in different directions lies in the convection currents that drive them.Australias Wandering WaysAustralia has the longest geological record of any landmass: over 90% of Earths complete time span is recorded in our rocks.

Australias Wandering WaysBy analysing many rocks of various ages and plotting the latitude at which they formed against time, continental drift is apparent.

The oldest Australian rocks have drifted at least 100,000 kilometres (twice around the world).

Australias Wandering WaysAustralias journey included two trips to each of the polar regions and crossed the equator a number of times.

A movement of only a few centimetres per year over the huge amount of geological time amounts to great distances

Future ContinentsIt has taken 200 million years for Pangaea to break up. This is less than 5% of the Earths life span!! Equal to about 8 months of your life span.

Given the rate and direction of movement of the continents, another supercontinent is likely to form in around 250 million years

Homework DOT Points 2.2, 3.6-3.7, 5.2Read pages 131-134 Prelim Spotlight TextUpdate vocab list