INTRODUCTION TO LITHOSPHERIC ENVIRONMENTS LESSON 1
Cutaway views showing the internal structure of the Earth. Below: This view drawn to scale demonstrates that the Earth's crust literally is only skin deep. Below is a view not drawn to scale to show the Earth's three main layers (crust, mantle, and core) in more detail You need to know the characteristics of the core, mantle and crust.
The Size and Structure of the Earth The Earth is about 12,750 kilometres (km) in diameter This was known by the ancient Greeks, but it was not until the turn of the 20th century that scientists determined that our planet is made up of three main layers: crust, mantle, and core. This layered structure can be compared to that of a boiled egg. The crust, the outermost layer, is rigid and very thin compared with the other two. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Alps or the Sierra Nevada, however, the base of the crust can be as deep as 100 km. Like the shell of an egg, the Earth's crust is brittle and can break.
Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle, which contains more iron, magnesium, and calcium than the crust, is hotter and denser because temperature and pressure inside the Earth increase with depth. At the centre of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy) rather than stony. The core is actually made up of two distinct parts: a 2,200 km-thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth's magnetic field. Not surprisingly, the Earth's internal structure influences plate tectonics. The upper part of the mantle is cooler and more rigid than the deep mantle; in many ways, it behaves like the overlying crust. Together they form a rigid layer of rock called the lithosphere (from lithos, Greek for stone). The lithosphere tends to be thinnest under the oceans and in volcanically active continental areas, such as the Western United States. Averaging at least 80 km in thickness over much of the Earth, the lithosphere has been broken up into the moving plates that contain the world's continents and oceans. Scientists believe that below the lithosphere is a relatively narrow, mobile zone in the mantle called the asthenosphere (from asthenes, Greek for weak). This zone is composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure over geologic time. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere.
Continental Drift v Plate Tectonics Alfred Wegener (1880-1930),
The changing positions of the continents as described by Wegener.
Evidence for continental drift and plate tectonics <ul><li>Evidence includes: </li></ul><ul><ul><li>Geological and structural evidence </li></ul></ul><ul><ul><li>Fossil evidence </li></ul></ul><ul><ul><li>Evidence from climate change </li></ul></ul><ul><ul><li>Palaeomagmetism </li></ul></ul><ul><ul><li>The age of basalt </li></ul></ul>
Evidence Supporting Continental Drift The Earth's crust is constantly moving, both vertically and horizontally, at rates of up to several centimetres a year. A widely-held theory that explains these movements is called " plate tectonics ." It was developed in the mid 1960s by geophysicists. The term "plate" refers to large rigid blocks of the Earth's surface which appear to move as a unit . These plates may include both oceans and continents. When the plates move, the continents and ocean floor above them move as well. Continental Drift occurs when the continents change position in relation to each other. While plate tectonics is a relatively new idea, scientists have been gathering data in support of the Continental Drift theory for a very long time. In 1912, Alfred Wegener and Frank Taylor first proposed the theory that 200 million years ago the Earth had only one giant continent, from which today's continents broke apart and drifted into their current locations. Wegener used the fit of the continents, the distribution of fossils, a similar sequence of rocks at numerous locations, ancient climates, and the apparent wandering of the Earth's polar regions to support his idea.
The Shapes Match The continents look as if they were pieces of a giant jigsaw puzzle that could fit together to make one giant super-continent. The bulge of Africa fits the shape of the coast of North America while Brazil fits along the coast of Africa beneath the bulge. The Plants and Animals Match Wegener noted that plant fossils of late Paleozoic age found on several different continents were quite similar. This suggests that they evolved together on a single large land mass. He was intrigued by the occurrences of plant and animal fossils found on the matching coastlines of South America and Africa, which are now widely separated by the Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms to have travelled or have been transported across the vast ocean. To him, the presence of identical fossil species along the coastal parts of Africa and South America was the most compelling evidence that the two continents were once joined.
Good Fit What is some Evidence to support his Theory
The Rocks Match Broad belts of rocks in Africa and South America are the same type. These broad belts then match when the end of the continents are joined. The Ice Matches Wegener was aware that a continental ice sheet covered parts of South America, southern Africa, India, and southern Australia about 300 million years ago. Glacial striations on rocks show that glaciers moved from Africa toward the Atlantic Ocean and from the Atlantic Ocean onto South America. Such a glaciation is most likely if the Atlantic Ocean were missing and the continents joined. The Positions Don't Match If the continents were cold enough so that ice covered the southern continents, why is no evidence found for ice in the northern continents? The explanation is that the present northern continents were at the equator at 300 million years ago. The discovery of fossils of tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this frozen land previously must have been situated closer to the equator, in a more temperate climate where lush, swampy vegetation could grow.
Why Few People Believed Wegener Wegener's Continental Drift theory was not readily accepted by the science community of his day. It was difficult to conceive of large continents ploughing through the sea floor to move to new locations. What kind of forces could be strong enough to move such large masses of solid rock over such great distances? Wegener suggested that the continents simply ploughed through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plough through the ocean floor without breaking up. It was the work of Hess, a British geologist, who identified evidence from ocean floor exploration and other studies, that rekindled interest in Wegener's theory, and lead to the development of the theory of plate tectonics.
Evidence for Plate tectonics <ul><ul><li>Paeleomagmetism </li></ul></ul><ul><ul><li>The age of basalt </li></ul></ul>
The Pattern of Paeleomagnetic Reversal Either Side of a Mid-Ocean Ridge.
Computer generated image of a mid-ocean ridge. Brighter colours are higher.
A simplified model of the currents of semi-molten magma within the aesthenosphere. You need to understand that convection currents are generated by the heat produced from radioactive decay in the core. These convection currents are linked to the movement of plates leading to the creation of new oceanic crust from the mantle.
The locations of the main constructive plate margins marked by ocean ridges.
You need to know the names of the major plates, their direction of movement and their approximate rates of movement (cm/year). You need to know examples of each type of plate margin in order to illustrate the processes involved. You should be able to draw annotated diagrams of each type of plate margin.
<ul><li>You need to know the processes of constructive, destructive and conservative plate margins include: </li></ul><ul><ul><ul><ul><li>convection </li></ul></ul></ul></ul><ul><ul><ul><ul><li>divergence </li></ul></ul></ul></ul><ul><ul><ul><ul><li>subduction </li></ul></ul></ul></ul><ul><ul><ul><ul><li>earthquake activity </li></ul></ul></ul></ul><ul><ul><ul><ul><li>hot spot activity </li></ul></ul></ul></ul>
A Hot-Spot deep in the earth heats the semi-molten mantle which rises towards the surface (some escapes between the plates to the ocean floor forming underwater volcanoes) As it rises, the semi-molten rock spreads out and cools As the semi-molten rock spreads, the crustal plates are slowly wrenched apart The cooling mantle descends again. Lithosphere – solid mantle Asthenosphere – Semi-molten mantle Lighter, thicker and older “raft” of less dense continental crust Denser, thinner and younger oceanic crust Mid-Ocean Ridge with underwater volcanic activity and shallow earthquake activity caused as the plates are wrenched apart. Atlantic Ocean South American Plate African Plate The earthquakes are caused by the release in tension as the plates are forced apart. Mid-Ocean Ridge – A Constructive or Divergent Plate Boundary The lava is basic or basaltic . This is very fluid leading to gently sloping volcanoes if the lava breaks the sea surface. Underwater,“pillow” lava forms.
Iceland is one of the few places where volcanic activity along a mid-ocean ridge has breached the sea surface leading to basic lava volcanicity.
The East African Rift Valleys are thought to be embryonic constructive plate margins. The Valleys are zones of active volcanic and earthquake activity
Subduction Zone Pacific Ocean South American (continental) Plate moving westward Nazca (oceanic) Plate moving eastwards Deep Peru-Chile Ocean Trench Andes – Range of Fold Mountains Extrusive Volcanic Activity at the Surface Intrusive Volcanic Activity creating solid batholiths of granite. Friction between the two plates creates heat which melts the crust creating molten magma. The oceanic crust melts back into the mantle. Asthenosphere – Semi-molten mantle Solid Lithosphere Lithosphere A Subduction zone – A Destructive or Convergent Plate Margin Earthquake Focus Earthquake Epicentre Oceanic Crustal Plate Earthquakes result from the release in pressure as the huge plates try to move past each other. Earthquake shock waves radiate away from the focus. The volcanic lava is acidic or andesitic . This is viscous and gassy leading to steep sided, explosive volcanoes.
<ul><li>Destructive Plate Margins destroy oceanic crust and can lead to large scale landforms including: </li></ul><ul><li>Fold mountains </li></ul><ul><li>Ocean trenches </li></ul><ul><li>Volcanic Chains </li></ul><ul><li>Island Arcs </li></ul><ul><li>The are also active earthquake zones characterised by both shallow and deep earthquakes down to a depth of about 70km. </li></ul>
Where two oceanic plates converge, island arcs are generated.
Where two continental plates converge at a collision zone, high mountain ranges and plateaus are formed.
Everest, and the rest of the Himalayas, are a result of a collision zone.
Half Dome in Yosemite National Park, rises more than 1km above the valley floor. The granitic rocks that form Half Dome and other spectacular Park features represent unerupted magma later exposed by deep erosion (Photograph by Carroll Ann Hodges, USGS.)
The San Andreas Fault and its sister faults represent a conservative plate margin. Landforms are low key (ranges of hills and shallow trenches) but earthquakes are common and potentially devastating.
Impact of earthquake activity on the San Andreas Fault
The range of plate boundaries: transform (conservative), divergent (constructive) and convergent (destructive).
Ocean trenches, island arcs and island chains in the Pacific.
These four diagrams illustrate the shrinking of the formerly very large Farallon Plate, as it was progressively consumed beneath the North American and Caribbean Plates, leaving only the present-day Juan de Fuca, Rivera, and Cocos Plates as small remnants. Large solid arrows show the present-day sense of relative movement between the Pacific and North American Plates.
<ul><li>You need to know the resulting global patterns of landforms, including: </li></ul><ul><ul><ul><li>fold mountains </li></ul></ul></ul><ul><ul><ul><li>ocean trenches </li></ul></ul></ul><ul><ul><ul><li>island arcs </li></ul></ul></ul><ul><ul><ul><li>ocean ridges </li></ul></ul></ul>
The Hawaiian Islands are especially vulnerable to destructive tsunamis generated by major earthquakes in the circum-Pacific Ring of Fire. Travel times (in hours) are shown for the tsunamis produced by the 1960 Concepción, Chile, earthquake (purple curves) and by the 1964 Good Friday, Valdez (Anchorage), Alaska earthquake (red curves). The 1960 tsunamis killed 61 people and caused about $24 million in damage
A giant wave engulfs the pier at Hilo, Hawaii, during the 1946 tsunami, which killed 159 people. The arrow points to a man who was swept away seconds later. ( Retouched photograph courtesy of NOAA/EDIS.)
Aerial view of the city of Armero, Colombia, devastated by mudflows triggered by the eruption of Nevado del Ruiz in November 1985. The mudflows destroyed everything in their paths and killed about 25,000 people. ( Photograph by Darrell G. Herd, USGS.)
Wahaula Visitor Center, Hawaii Volcanoes National Park, was one of more than 200 structures overrun by lava flows (foreground) from the 1983-present eruption at Kilauea Volcano. (Photograph by J.D. Griggs, USGS.)