Earth’s Structure View of Earth from moon. We know what the surface of the Earth looks like. What does the inside look like?

Origin of the Earth Earth is 4.6 billion years old Meteors and Asteroids bombarded the Earth Earth is 4.6 billion years old Our early Earth grew from a barrage of extraterrestrial impacts, increasing in mass over time. In the early stages of planetary accretion, the Earth was much less compact that it is today. The accretionary process led to an even greater gravitational attraction. Gravitational energy converted into heat. Radioactive elements are unstable, and over time become more stable. Radioactive decay processes releases heat. U-238 to Pb-206. Initial accretion led to a homogeneous sphere that eventually became density stratified with the heavier iron and nickel sinking to the center of the earth, and the lighter silicates (rocky) floated to the top.

Earth’s Interior Core Mantle Crust dense Iron and Nickel Inner Core - solid Outer Core - liquid Mantle Less dense than core Iron and Magnesium silicates Mostly solid Upper mantle is partially molten Core – composed of two parts. The core is very dense and composed mostly of Iron and Nickel. The inner core is solid. This is because there is so much pressure exerted that it can only exist in the solid form. The outer core is liquid iron and nickel. Again, it is very dense but because it is in its liquid form it is less dense than solid and forms the outer core. Mantle is composed of Iron and Magnesium silicates (Si + O). It is mostly solid, but the uppermost part of the mantle is partially molten (slush – where you do have liquid and solid mixed). Now, this is very important in plate tectonics. Crust –This is the outermost layer. It is very thin and rigid. Rigid means that it is a solid. If you can imagine an apple. It would have the thickness of the skin of an apple. There is two types of crust – continental crust and oceanic crust. Important notes: Thin crust + uppermost mantle is rigid. This forms the “lithospheric plates”. partially molten mantle acts as a lubricant. This enables the plates to move freely along the surface of the Earth. Crust Outermost layer Very thin and rigid Continental – granite Density = 2.8 g/cm3 Oceanic – basalt Density = 3.0 g/cm3

Earth’s Interior Lithosphere - The rigid portion of the Earth’s crust (both oceanic & continental) that is broken into plates. “Litho” comes from the Greek word meaning rock Asthenosphere – the slushy (partially liquid & solid) in the upper mantle on which the plates “float” “Asthenes” comes from the Greek word meaning weak

Types of Seismic Waves P waves S waves Primary waves Push and pull movement Travel fastest (~ 6 km/sec) Travel thru solids and liquids S waves Secondary waves Move side-to-side Slower (~ 4 km/sec) Travel thru solids only

Continents Today

Continental Drift Theory In 1912 a German meteorologist, astronomer and explorer named Alfred Wegener presented a theory that the continents were once joined together in one land mass and have over millions of years slowly drifted apart into their present positions as we see them today. He named the supercontinent “Pangaea” after the Greek word meaning “all land”.

Continental Drift Geographic Fit About 210 Ma, Pangaea began to break apart. Continents seem to fit together like pieces of a puzzle

Continental Drift Theory Through Wegener’s research he found evidence to support his theory: Fossils of Glossopteris, a seed fern, was found in India, Australia, South America, Africa and Antarctica Fossils of Mesosaurus, a small reptile were found in freshwater deposits in Africa & South America.

Continental Drift Fossils Studies were done on land animals that could not swim. Once continents split, evolution occurs. Organisms begin to differentiate. Climate is different, food sources are different, predators are different. Similar distribution of fossils such as the Mesosaurus

Glossopteris Fern

Continental Drift Layers of the mountains found in S.E. Brazil, South America and S.W. Africa line up layer for layer in exact format. Continents look like puzzle pieces that could fit together.

Continental Drift Mountains If you were to put the puzzles back together, mountain chains match up, i.e., Appalachian Mtns, British Isles, and Caledonian Mtns. Mountain ranges match across oceans

Continental Drift Glaciation Glaciers typically occur near the poles. We are in a warm period, but in the past it was cool and ice caps grew. Deposits related to glaciers were deposited. You also find glacial deposits in Africa near the equator. How to you get glacial deposits near the equator? Glacial ages and climate evidence

Continental Drift Glaciation Glaciers typically occur near the poles. We are in a warm period, but in the past it was cool and ice caps grew. Deposits related to glaciers were deposited. You also find glacial deposits in Africa near the equator. How to you get glacial deposits near the equator? Glacial ages and climate evidence

Continental Drift Model Problems Alfred Wegener Presented research to professionals Did not provide a plausible mechanism to explain how continents drifted Wegener was a scientist. Scientists go to scientific meetings to present their research. When Wegener presented his research to other physicists, they did not agree. They wanted to know how do continents drift? There was no driving mechanism to support continental drift. Wegener could not answer “how” so his Continental Drift Model was not accepted.

Seafloor Spreading Continental drift reexamined in 1960’s with new information New theory developed – Seafloor spreading Supporting evidence for seafloor spreading World seismicity Volcanism The 1960’s is relatively recent. Geology is a new science compared to Physics, Chemistry and even Biology. People were doing separate studies about earthquakes, volcanoes, age of sea floor, paleomagnetism. Someone put it all together. A new theory developed – Seafloor Spreading. There were a lot of evidence to support Seafloor spreading. Age of seafloor Paleomagnetism Heat flow Theory combining continental drift and seafloor spreading termed “Plate Tectonics”

Principles of Plate Tectonics Earth’s outermost layer composed of thin rigid plates moving horizontally Plates interact with each other along their edges (plate boundaries) Plate boundaries have high degree of tectonic activity mountain building earthquakes volcanoes