Topic 1- Tectonic Impacts This is the first topic of the Earth and Environmental Science HSC Course. It’s very similar to the last topic you studied: Dynamic Earth however you will look at the tectonic theory in more detail.
Topic Overview There are 6 parts to this Topic: • Lithospheric Plates • Volcanic Mountains • Tectonic Mountains • Evolution of Continents • Earthquakes, Volcanoes and Natural Hazards • Climate Change
Topic Outcomes • Describe the lithospheric plates and their motion • Explain how the movement of plates results in mountain building • Explain how continents grow and change shape as plate boundaries move and change • Describe how natural disasters are often associated with tectonic activity like earthquakes and volcanoes • Explain how environmental conditions caused by tectonic activity may contribute to the problems experienced by people • Explain the link between plate tectonics and climate, both in the short term and throughout geological history.
Part 1-Lithospheric PlatesLesson 1 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.
Introduction In 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 Lithosphere The lithosphere is the rigid, brittle outer layer of the Earth. Lithos means rocks. Above the lithosphere is the hydrosphere and atmosphere.
Asthenosphere This is the section below the lithosphere and consists of the upper part of the Mantle. Asthenomeans weak or without strength. It’s made up of partially molten rock which scientists call it’s state to be “plastic”.
Mesosphere Everything below the asthenosphere: the rest of the mantle, outer and inner core is called the mesosphere. Meso means middle.
Earth’s Interior How do scientists know so much about Earth’s interior? After all humans have never even breached the crust….. Seismologists provide us with the information.
Earth’s Interior Seismologists 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 Earth’s interior.
Earth’s Interior As the seismic waves pass through layers of the Earth, the energy changes speed and direction. This is due to the materials having different densities.
Earth’s Interior Understanding the Earth’s interior is vital to our understanding of crustal plates and their movements. So why do we care?
Earth’s Interior We 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-2 Crustal Plate Review
The Theory of Plate Tectonics If 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 Drift Alfred 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 Drift Wegener’s Hypothesis lacked a geological mechanism to explain how the continents could drift across the Earth’s surface as he had proposed. In 1929 Arthur Holmes elaborated one of Wegener’s many hypothesis:
The Theory of Continental Drift Arthur 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 Drift Not until the 1960’s 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 Drift To understand this theory better we need to look at the geological processes taking place and the evidence which supports it. We’ll start by looking at the crust.
Earth’s Crust The crust covers the mantle and is the Earth’s hard outer shell, the surface we are living on . Compared to the other layers the crust is much thinner.
Earth’s Crust The 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 Crust This 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 Crust Continental crust is the part of Earth’s 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 Boundaries The Theory of Plate Tectonics builds on Wegener’s Theory of Continental Drift. This theory states that the Earth’s crust or Lithosphere is broken up into “tectonic plates”
Plate Boundaries If 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 Boundaries If we imagine the tectonic plates like blocks of wood, there are three ways they can interact.
Plate Boundaries Remember 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 Boundary Divergent 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 Boundary Convergent 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 boundaries which we will discuss more later.
Transform Plate Boundary Transform 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 Text Eg: Tectonic Impacts Vocab List • Plate: Large pieces of the Earth’s crust
Lesson-3 Plate Interactions
Divergent Plate Boundary This is where two plates separate. These spreading zones are where new crust is being generated. Mid-ocean ridges mark divergent plate boundaries.
Divergent Plate Boundary These 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 Boundary Lava usually erupts in long cracks in the Earth’s surface called fissures rather than mountains. This is because the crust is being pulled apart rather than forced together.
Divergent Plate Boundary Rocks 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 Boundary General Composition of Igneous Rocks: Oceanic Divergence • Basaltic • Low silica content • High concentration of dark coloured mafic minerals Continental Divergence • Rhyolitic • Rich in silica and felsic minerals • Light in colour
Convergent Plate Boundary This is where two plates come together ‘collide’. There are three types of convergent zones: oceanic-oceanic oceanic-continental Continental-continental
Oceanic-Oceanic Convergence This 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 Convergence As 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 Convergence The volcanoes that are created at this boundary form a curved line out of the sea and scientists call these island arcs.
Oceanic-Oceanic Convergence Examples of island arcs include the Philippine Islands, Japan and Indonesian Islands.
Oceanic-Oceanic Convergence Metamorphism 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 Boundary General Composition of Igneous Rocks: • Mainly Basalts • Increase in Andesites as island arcs mature • High silica granite and less silica gabbro intrusions can be found within the crust
Oceanic-Continental Convergence Because 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 Convergence Where 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 Convergence As the subducting oceanic plate is forced beneath the overriding continental plate, the continental plate is lifted and folded upwards producing mountains.
Oceanic-Continental Convergence An example of a oceanic-continental convergent plate boundary is along the west coast of South America. Here we find the Andes Mountains.
Oceanic-Continental Convergence The Oceanic Nazac plate is slamming into the Continental South American plate