Lecture 10_Earth structure_rev.pptx

GEOG 111: INTRODUCTION TO PHYSICAL GEOGRAPHY Lecture 9: Lithosphere: Earth Structure and Materials Instructor: Dr. Peter Bilson Obour 1  Learning Goals  Identify layers of earth In...

GEOG 111: INTRODUCTION TO PHYSICAL GEOGRAPHY Lecture 9: Lithosphere: Earth Structure and Materials Instructor: Dr. Peter Bilson Obour 1  Learning Goals  Identify layers of earth Interior  Explain forces that shape the surface of the Earth  Describe occurrence of minerals on earth  The Earth's Interior  The deep interior of the Earth is still of a mystery  We have only penetrated the outermost portion with our deep drilling exploration.  What we know today have come from seismic wave data or lava that has extruded onto the earth surface  What we do know is that the Earth's interior is somewhat like a concentric series of rings, progressing from the dense and intensely hot inner core toward the brittle outer shell of the crust The Earth  Inner Core  Depth of 5,150-6,370 kilometers  The inner core is made of solid iron and nickel and is unattached to the mantle, suspended in the molten outer core  It is believed to have solidified as a result of pressure-freezing which occurs to most liquids under extreme pressure  Outer Core Depth of 2,890-5,150 kilometers The outer core is a hot, electrically conducting liquid (mainly Iron and Nickel) This conductive layer combines with Earth's rotation to create a dynamo effect that maintains a system of electrical currents creating the Earth's magnetic field It is also responsible for the subtle jerking of Earth's rotation This layer is not as dense as pure molten iron, which indicates the presence of lighter elements Scientists suspect that about 10% of the layer is composed of sulphur and oxygen because these elements are abundant in the cosmos and dissolve  D" layer  Depth of 2,700-2,890 kilometers  This layer is 200 to 300 kilometers thick  Although it is often identified as part of the lower mantle, seismic evidence suggests the D" layer might differ chemically from the lower mantle lying above it  Scientists think that the material either dissolved in the core, or was able to sink through the mantle but not into the core because of its density  Lower Mantle  Depth of 650-2,890 kilometers  The lower mantle is probably composed mainly of silicon, magnesium, and oxygen  It probably also contains some iron, calcium, and aluminium  Scientists make these deductions by assuming the Earth has a similar abundance and proportion of cosmic elements as found in the Sun and primitive meteorites  Transition Region  Depth of 400-650 kilometers  The transition region or mesosphere (for middle mantle), sometimes called the fertile layer and is the source of basaltic magma  It also contains calcium, aluminium and garnet, which is a complex aluminium-bearing silicate mineral.  This layer is dense when cold because of the garnet  It is buoyant when hot because these minerals melt easily to form basalt which can then rise through the upper layers as magma  Upper Mantle  Depth of 10-400 kilometers  Solid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions  Olivine (Mg,Fe)2SiO4 and pyroxene (Mg,Fe)SiO3 have been found  These and other minerals are crystalline at high temperatures  Part of the upper mantle called the asthenosphere might be partially molten  Oceanic Crust  Depth of 0-10 kilometers  The majority of the Earth's crust was made through volcanic activity  The oceanic ridge system, a 40,000 kilometer network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, covering the ocean floor with an igneous rock called basalt  Hawaii and Iceland are two examples of the accumulation of basalt islands.  Continental Crust  Depth of 0-75 kilometers  This is the outer part of the Earth composed essentially of crystalline rocks  These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates)  The crust is the surface of the Earth. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).  The Origin of the Earth  The age of the Earth was once, and still is, a matter great debate  In 1650 Archbishop Ussher used the Bible to calculate that the Earth was created in 4004BC  Later on in the mid-nineteenth century Charles Darwin believed that the Earth must be extremely old because he recognized that natural selection and evolution required vast amounts of time.  The Origin of the Earth  It wasn't until the discovery of radioactivity that scientists began to put a timescale on the history of the Earth  Rocks often contain heavy radioactive elements which decay over long periods of time, the decay is unaffected by the physical and chemical conditions and different elements decay at different rates  These rates are slow and half-life of several hundred million years are not uncommon  Throughout this century the race has been on to discover the oldest rocks in the world.  The oldest volcanic rock found so far has been dated at 3.75 billion years old, but this is not the whole story  Radioactive dating shows that they are about 4.55 billion years old.  The Origin of the Earth THE EARLY ATMOSPHERE  The present composition of the atmosphere – 21% OXYGEN – 78% NITROGEN – 0.04% CARBON DIOXIDE – ~0.9% ARGON  The atmosphere was not like this when the Earth was created over 4½ billion years ago.  The Origin of the Earth THE FIRST BILLION YEARS  The Earth's surface was originally molten, as it cooled the volcanoes belched out massive amounts of CARBON DIOXIDE, STEAM, AMMONIA and METHANE. There was NO OXYGEN. The STEAM condensed to form water which then produced shallow seas.  Evidence points to bacteria flourishing 3.8 billion years ago so this means that life got under way about 700 million years after the Earth was created. Such early forms of life existed in the shallow oceans close to thermal vents, these vents were a source of heat and minerals.  The Origin of the Earth THE NEXT BILLION YEARS  These primitive life forms then took the next evolutionary step and started to PHOTOSYNTHESISE (using sunlight to convert carbon dioxide and water to food energy and oxygen).  This was an important turning point in Earth history because the carbon dioxide in the atmosphere was being converted to oxygen.  These green plants went on producing oxygen (and removing the CO2). Most of the carbon from the carbon dioxide in the air became locked up in sedimentary rocks as carbonates and fossil fuels.  The Origin of the Earth  Carbon dioxide also dissolved into the oceans. The ammonia and methane in the atmosphere reacted with the oxygen.  Nitrogen gas was released, partly from the reaction between ammonia and oxygen, but mainly from living organisms such as denitrifying bacteria. (remember that nitrogen is a very unreactive gas and it has built up slowly). THE LAST 2½ BILLION YEARS OR SO  As soon as the oxygen was produced by photosynthesis it was taken out again by reacting with other elements (such as iron).  This continued until about 2.1 billion years ago when the concentration of oxygen increased markedly.  Forces That Shape the Surface of the Earth The crust of the Earth is constantly being changed by forces from within and outside the Earth system These forces that are constantly altering the crust are classified into – Endogenic Processes/Forces – Exogenic Processes/Forces  Endogenic Forces Processes or forces (that operate within the interior of the earth) are driven by the Earth's vast heat engine are called endogenic forces or processes. – The movement of tectonic plates is a product of convection currents in the mantle – Deep within the Earth's core, heat is generated by the radioactive decay of elements – The heat is transferred upward to warm the mantle causing it to slowly circulate and tug on the plates above  Endogenic Forces – As the crustal plates are moved about, they interact by colliding, sliding by, or diverging from one another – They produces faults and earthquakes, volcanoes, the creation of mountain systems, or deep valleys and trenches – The great mountain systems like the Himalayas are a product of the collision of lithospheric plates. – The huge trenches found on the ocean floor, like the Marianas Trench, are caused by plate interaction.  Exogenic Processes Those processes acting at the surface of the earth and primarily driven by solar energy are called exogenic processes – Wind is created by the variation in pressure over distance (pressure gradient force). Pressure variations are, in part, created by the variation of surface heating due to the unequal distribution of solar energy receipt. As wind blows it exerts an erosive force on the surface to detach and transport soil particles. Wind erosion is therefore an exogenic process. – Erosion by rain is driven by the initial evaporation due to absorption of energy and subsequent conversion into precipitation by condensation processes. – Glaciers form when summer temperatures decrease to the point where the previous winter's snowfall does not melt and accumulates over time eventually compacting and metamorphosing into ice. The accumulating ice spreads out as a great sheet sculpting the surface beneath it  Minerals  A mineral is a naturally occurring inorganic substance with a unique crystal structure  The physical and chemical composition of a mineral is consistent between samples  There are nearly 3000 minerals, only 20 are common, and 10 make up 90% of the minerals of the crust  Minerals may be composed of a single element, like gold (Au) or a combination of elements End of Class 27