Lecture #11: Earth: Geology and Interior Overview of Earth's basic properties. Earth's Interior and Surface: Composition and Structure. Surface Geologic Processes. Evolution of the Surface. Reading: Chapters 9.1, 9.2, 9.6. The Main Point Earth is a rocky planet with a differentiated interior i structure t and a dynamic surface that tis being modified by active volcanic, tectonic, impact, and erosional processes. Astro 102/104 1 Astro 102/104 2 Basic Properties of the Earth Average Distance from Sun: 1.5 x 10 11 m = 150 million km = 1 A.U. Period of Rotation around Sun: 365.25 days = 1 year. Period of Spin (wrt stars): 23 hours, 56 minutes, 4 sec = ~1 day. Mass = 5.98 x 10 24 kg = 1 M E. Radius = 6378 km = 1 R E. Density = 5.5 g/cm 3. Escape Velocity = 11.2 km/sec (~ 25,000 mph) = v E. Atmospheric Pressure = 10 5 N/m 2 = 1 bar = P E. Most of surface covered by liquid water. Atmosphere mostly N 2, but with abundant free O. Surface geologically active and young! Other Properties Earth has one large Moon. Earth has a strong Magnetic Field. Astro 102/104 3 Astro 102/104 4 1
What's Inside? High density implies rock+metal. "Onion Skin" layering, From differentiation. Crust, Mantle, Core. Crust is thin (5-10 km) under oceans. Crust is thick (20-70 km) under continents. Mantle is hot but solid (convection). Liquid outer core (convection). Solid inner core. Astro 102/104 5 How do we know what's inside? Seismic waves are elastic waves in the earth produced by an earthquake or by artificial means. Seismology is the science and study of earthquakes. We can directly measure the behavior of seismic waves that travel through the Earth's interior, and infer pressure, density, composition, temperature. P-waves (primary, pressure, pushing) propagate fastest. S-waves (secondary, shear, side-to-side) propagate only through solids because the bonds between neighboring molecules in a liquid or gas are too weak to transmit shear. Astro 102/104 6 Earth's Surface Topography Earth's Surface Topography Earth's topography is bimodal. Deepest: -11 km. Highest: +8.8 km. 71% under water. Astro 102/104 7 Astro 102/104 http://tesla.jcu.edu.au/schools/earth/ea1002/geomorphology/oceanography.html 8 2
Composition of the Crust Earth's crust is only ~ 0.4% of Earth's mass. But it is what we directly sample! Earth's crust consists of three rock types: Igneous: Rocks melted then cooled. Metamorphic: Rocks changed by heat+pressure. Sedimentary: Fragments created by erosion. Astro 102/104 9 Sedimentary Rock Layers, beds, dunes, etc. Formed by wind, water, ice. Earth Astro 102/104 Mars 10 Metamorphic Rock High pressure. High temperature. Compression and folding of previous igneous or sedimentary rocks. Igneous Rock Example: volcanic rock from Hawaii. Dozens of active volcanoes on Earth today! Not all igneous rock is erupted onto the surface. Astro 102/104 11 Astro 102/104 12 3
Volcanoes of the World Earth's Surface is Divided into Plates! Astro 102/104 13 USGS Astro 102/104 14 The Plates Move! Lots of Action at Plate Boundaries... The fact that the plates move was highly controversial when first proposed (Wegener, 1915) "Anecdotal" evidence: S. America & Africa fit like a jigsaw puzzle. Better evidence: Magnetic "stripes" on the seafloor. Other evidence: Similar fossils across continents. Similar rocks across continents. Earthquakes at plate boundaries. Very young age of seafloor. Astro 102/104 15 Astro 102/104 16 4
Abundant Geologic Evidence of Tectonic Forces at Work And these forces have been acting for long periods of time! Astro 102/104 17 Astro 102/104 18 Have there been impacts on the Earth? Yes! Several hundred impact craters identified on Earth. Most highly degraded by wind, water, ice, tectonics. Geologically rare. Astro 102/104 19 Summary The Earth is a complex rocky planet: Large Moon, strong magnetic field. Differentiated, rocky/metallic interior. Surface topography is bimodal: oceans, continents. Crustal rock types: igneous, metamorphic, sedimentary. Ample evidence of volcanic, tectonic, erosional, and even impact processes on Earth's surface. Unique aspect: surface divided into a few dozen moving tectonic plates. Astro 102/104 21 5
Next Lecture... Earth: Atmosphere and Climate. Earth's atmosphere. Weather and climate. The greenhouse effect. Cosmic influences on the biosphere. Reading: Complete chapters 10.1, 10.2, 10.6. Astro 102/104 22 6