Lecture 27

1. Hydrosphere.Rocks. Earth’s Water Envelope Earth’s Surface Layer Types of Rocks Lecture 27 Chapter 13.13  13.14, 14.1  14.5

2. Hydrosphere ~70% of the Earth’s surface is covered by oceans and connected seas, which are full of life. Surface water evaporates and influences weather. Water absorbs solar energy and distributes it around the world. Oceans are large basins bounded by continents. An ocean bottom gradually slopes down to ~130m with an average width of 65 km (varying from 1 to 1000 km). This continental shelf turns into continental slope (down to ~2 km), which joins the abyssal plain via continental rise. Oceans average depth is ~3.7 km. Marianas Trenches – 11 km below the surface.

3. Ocean Currents • Two ways oceans affect climate: • Store heat (the first few meters contain more heat than the entire atmosphere). • Surface currents, produced by friction of air and water. Heat transfer between air and water can exceed the rate of arriving solar energy at the Earth. This is why continents are cooler in winters than coastal regions and islands. Surface currents are wind-driven and mostly parallel to major wind streams and form equatorial current. This current runs into South America in the Atlantics and into the East Indies in the Pacific.

4. Ocean Currents At each of the contacts the current divides into 2 parts. One flows to the south, the other one to the north. Then westerlies drive them eastward across the oceans. This mechanism produces 4 large whirlpools. The Gulf Stream Gulf Stream is a warm current moving partly into the Gulf of Mexico, partly straight north along the US eastern coast. It splits on the European side: one part moves south and completes the whirl, the other one moves into the Arctic Ocean. Gulf Stream makes climate of NW Europe milder.

5. Summary Earth’s atmosphere was developed over a large period of time and is hospitable for life due to the right distance from the Sun. Weather is mostly due to the large amount of vaporized water and winds, which exist due to the planet rotation and uneven distribution of continents and oceans. Water in oceans generally follows the wind directions and is affected by the current distribution of continents.

6. Crust The outer layer of the solid Earth’s surface consists of rocks and is called crust. Crust is about 40 km deep. Crust’s composition: Oxygen - 46.6% Silicon - 27.7% Aluminum - 8.1% Iron - 5.0% Calcium - 3.6% There almost no pure elements in crust, they are bound in compounds. Compounds of silicon account for 87% of the rock and soil of the Earth’s crust.

7. Minerals A mineral is a crystalline inorganic natural solid that has a specific chemical composition. Minerals have specific chemical formulas (e.g., CaCO3), but are usually identified by other names (e.g., calcite). Minerals made of the same compound may have different properties. Most minerals vary in composition, while composition of chemical compounds is invariable. Among ~4000 known minerals most a rare.

8. Properties of Minerals Two important properties of minerals: Crystal form arrangement of atoms in certain geometric patterns. Cleavage tendency of a substance to split along certain plains, which are determined by arrangements of atoms. Well-developed crystals are rare due to interference with neighboring crystals. Cleavage is not a characteristic property of all minerals. Mica can be peeled apart in thin sheets, while quartz has no cleavage at all.

9. Some Common Minerals Quartz (SiO2): six-sided prisms and pyramids, no cleavage. Used in jewelry, optical instruments, for making glass. Feldspar (silicates of K, Al - orthoclase, and of Na, Ca, and Al - plagioclase). Rectangular crystals with cleavage in 2 directions. Used in making porcelain and as a mild abrasive. Mica. Two main varieties: white (a silicate of H, K, Al) and black (a silicate of H, K, Al, Mg, Fe). Very soft, used as insulator in electrical equipment. Calcite (CaCO3)  hexagonal crystals, cleavage in 3 directions. Chief constituent of limestones and marbles.

10. Three Types of Rock There are 3 types of rock according to their origin. Igneous rocks - rocks cooled from a molten state. Volcanic origin. 2/3 of rocks in crust are igneous. Sedimentary rocks - materials derived from other rocks and deposited by water, wind, or glacial ice. Make 8% of the crust, but 3/4 of the surface rocks. Metamorphic rocks - igneous or sedimentary rocks that have been changed (metamorphosed) by heat and pressure deep under the Earth’s surface.

11. Igneous Rocks Appear in the form of irregular grains. Principal minerals contain silicon: quartz, feldspar, mica. If molten lava cools quickly, it forms natural glass called obsidian. Grain size in the igneous minerals contains information about both the cooling rate and the cooling environment. Fast cooling gives fine-grained rocks, which come to the surface with lava and cool there. Slow cooling gives coarse-grained rocks, which formed beneath the surface and became exposed due to erosion.

12. Sedimentary Rocks Sediments are deposited by water, wind, or ice, and become rocks through the pressure of the overlying deposits. The sedimentary rocks have nearly rounded grains that do not grow, like crystals of igneous rocks. There are two groups of sedimentary rocks: fragmental rocks and (bio)chemical precipitates. Fragmentals: conglomerate (wide variety of sizes), sandstone (small grains), shale (soft rock, consolidated mud). Precipitates: limestone (chemical precipitate or shell fragments), chalk (loosely consolidated variety of limestone), chert (microcrystalline quartz, used as tools by earlier people).

13. Metamorphic Rocks Both igneous and sedimentary rocks can be changed under high temperatures and pressures inside the Earth. Some minerals become unstable and form new substances by chemical reactions; others grow more as crystals. Many metamorphic rocks show a property called foliation. This is the arrangement of flat or elongated mineral grains in parallel layers due to a high pressure in one direction. Examples: Slate produced at low temperatures from shale, is harder that slate, and usually black or dark. Schist formed at high temperatures from shale or fine-grained igneous rocks, has large visible grains.