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Transcript
Chapter 2 Journey to the Center of the Earth • The Earth’s magnetic field is a dipole magnet. This means it has a north and a south pole. The force generated by a magnetic field repels charged particles. • Magnetic field lines show trajectories along which magnetic or charged particles would flow. 1 • The solar wind contains electrically charged particles that are deflected by the Earth’s magnetic field. The magnetosphere is the region inside this shield. • The Van Allen radiation belts (located 3,000 km and 10,500 km out from the Earth) are important because they trap those solar wind particles and cosmic rays that get through the magnetosphere that would otherwise bombard the Earth with dangerous radiation. The atmosphere is a very thin layer that surrounds the earth 2 The atmosphere is mostly nitrogen and oxygen with a small proportion of trace gases (labeled as other gases on the diagram). Trace gases consist of compounds like carbon dioxide (CO2) and methane (CH4). Although trace gases are present in very small amounts, they play important roles in climate warming because they tend to absorb and reflect back to the surface long-wave radiation, thus promoting heating.. Venus and Mars both have atmospheres made up of CO2 gas. Pressure and temperature increase with depth in the Earth. At the center of the Earth, pressure is 3.6 million times greater than at the surface and temperatures can reach 4,300oC, nearly as hot as the Sun’s surface. The rate of temperature change with depth is called the geothermal gradient. 3 • The troposphere is the lowest layer of the atmosphere and this is where air undergoes convection. Convection refers to the circulation that occurs when different parts of a fluid have different temperatures. Warm air rises, cooler air sinks. Convective cells also occur deep within the earth where warm, melted rock rises, then sinks as it cools. • Land covers about 30% of the Earth’s surface, oceans 70%. • Groundwater is all surface water plus the water that fills the openings underground. The hydrosphere is that plus the water vapor in the atmosphere. •The topography of the Earth shows different kinds of landscapes such as mountains, valleys, and plains. There are also submarine plains, oceanic ridges, and deep trenches beneath the ocean. 4 •The solid earth is mostly made of iron (35%), oxygen (30%), silicon (15%), and magnesium (10%). The remaining 10% consists of the other 88 naturallyoccurring elements. The lighter elements were blown away by the solar wind during the formation of the Earth and other planets. Categories of earth materials • Carbon-containing compounds are generally called organic chemicals (oil, protein, fat, plastic, and rubber). Carbon-containing compounds such as pure carbon (C), carbon dioxide (CO2), carbon monoxide (CO), lime (CaO)and calcium carbonate (CaCO3) are considered inorganic. • Minerals are a solid inorganic substance where the atoms are arranged in an orderly pattern or crystalline lattice (example: quartz, feldspar, diamond, calcite). Precipitation occurs when dissolved atoms come together to form a solid. A crystal is a single coherent example of a mineral. A grain is a fragment of a mineral. Rocks are aggregates of crystals or grains. Igneous rocks crystallize from molten rock. Sedimentary rocks arise from the cementation of loose grains (sand, mud, etc.) and through chemical precipitation (from the ocean or bodies of water). Metamorphic rocks arise from heat and pressure-induced alteration of existing rock (without melting). • • • • • • • 5 Categories of earth materials (cont’d) • Glasses are physical solid structures in which the atoms are disordered (for example, obsidian, commercial glass). These materials were cooled very quickly from the molten to the solid state thus not allowing the atoms to order themselves into a lattice structure. • Metals are solids comprised of metallic elements only such as gold, iron, and copper. • Melts are hot liquids that crystallize at surface temperatures to form igneous rocks. When they occur within the Earth, they are called magma. Extruded melts are called lava. • Volatiles are substances that are stable in a gaseous state at the Earth’s surface. • The Earth’s Interior – a layered sphere • The Earth’s interior consists of a series of layers that are called (1) the crust, (2) the mantle, and (3) the core. The crust is a relatively thin skin (7-10 km beneath oceans, 25-70 km beneath the land surface) over the mantle. There are two kinds of crust: • Oceanic crust consists of basalt (mafic, meaning iron-rich, rock). • Continental crust is intermediate to silicic. • The mantle, (ultramafic rock) can be divided into 3 layers: upper mantle, transition zone, and lower mantle. The crust and the upper mantle are known as the lithosphere. The underlying, warmer and more plastic portion of the mantle is known as the asthenosphere. • The inner core can be divided into an outer core of liquid iron alloy and an inner core of solid iron alloy. Within the Earth’s outer core, there is convective flow and that flow, it is believed, is what creates the Earth’s magnetic field. 6 7 What is an earthquake? • When rock within the crust suddenly breaks, an earthquake results. An earthquake is a vibration. This vibrational energy moves as seismic waves. Earthquakes can produce a new fracture in the earth’s crust or can cause sliding or movement on an existing fracture. Sliding on a existing fracture is called a fault. • The speed at which seismic waves travel can change abruptly beneath the surface. The boundary at which the velocity changes is called a seismicvelocity discontinuity. This represents a pronounced change in the nature of the material comprising the earth, such as a change in density or compressibility. • Major discontinuities define the boundaries between the mantle and the core. The Moho seismic discontinuity defines the boundary between the crust and the mantle.. 8 The Crust • Oceanic crust underlies the sea floor and is less than 10 km thick. It is composed primarily of mafic (ironrich) igneous rocks. • Continental crust ranges from 25 km thick to more than 70 km thick and can contain a variety of rock types but, on average, is less mafic and more silicic than oceanic crust. Rocks that are more mafic tend to be heavier than silicic rocks. The continental crust “float” on the asthenosphere because it is less dense and lighter in relation to the more mafic and heavier oceanic crust. Continental crust usually consists of rocks different from the mantle. 9 The Mantle • The Earth’s mantle is about 2,885 km thick and is the largest part of the Earth. The mantle is made up of ultramafic rock called peridotite, a rock rich in iron and magnesium but poor in silica. • The mantle consists of the upper mantle (to 400 km), the transition zone (400 km to 670 km), and the lower mantle (670 km to the core boundary). Mantle rock is relatively solid but can also flow very slowly (like softened wax). • There is a region below the ocean basins called the low-velocity zone where up to 2% of the mantle has melted and exists as droplets of molten rock between solid grains. • Temperature in the mantle generally increases with depth but not uniformly. This creates a convective flow pattern. • The mantle composition consists mostly of iron and magnesium-rich minerals. The Core • There is an outer core (between 2,900 and 5,155 km deep) and an inner core (from 5,155 to 6,371 km). • The outer core is a liquid iron alloy because of the extremely high temperatures in that region. • The flow of the outer core generates the Earth’s magnetic field. • The inner core is a solid iron alloy, despite the high temperatures, because of the tremendous pressure it is subjected to. 10 End of Chapter 2 11