Chapter 9: Plate Tectonics. Chapter 9: Plate Tectonics

Transcription

1 Continental Drift Sea-Floor Spreading Theory of Plate Tectonics Section 9.1 Continental Drift -- Section 9.1: Continental Drift -- Continental drift Hypothesis stating that the continents once formed a single landmass, broke up, and drifted to their present locations Pangaea The single landmass thought to have been the origin of all continents All lands Panthalassa The giant ocean surround Pangaea All seas Hypothesis developed by German scientist Alfred Wegener in 1915 Believed Pangaea began breaking up into smaller continents about 200 million years ago -- Section 9.1: Continental Drift -- Thought that this movement may have crumpled the crust in places, producing mountain ranges

2 -- Section 9.1: Continental Drift -- Evidence of continental drift Fossil remains Identical fossil remains of Mesosaurus, a small, extinct, land reptile from 270 million years ago, were found in both eastern South America and western Africa. Fossils of seeds of a small plant called Glossopteris were found in South America, southern Africa, and India. Fossils of the land reptile Lystrosaurus show a pattern of distribution similar to the Glossopteris fossils. There were no land bridges for reptiles to traverse and reptiles could not swim across the ocean, meaning South America and Africa were joined at one time. -- Section 9.1: Continental Drift -- Geologic evidence The age and type of rocks in the coastal regions of western Africa and eastern South America in Brazil matched closely. The Appalachians that end along the eastern United States are of similar age and structure of those found in the British Isles and Scandinavia. If the three landmasses are put together in a model of Pangaea, the mountains fit together in one continuous chain. Changes in climatic patterns Layers of debris from glacial deposits in southern Africa and South America, both of which now have warmer climates. These indicated ice sheets covering those areas 220 million and 300 million years ago. -- Section 9.1: Continental Drift -- Fossil evidence such as the coal deposits in the eastern United States, Europe, and Siberia indicate that tropical or subtropical swamps covered much of the land area in the Northern Hemisphere. Continents were once joined and positioned over the South Pole. Opposition to continental drift The force causing continental drift could not be explained. Evidence for continental drift was later revealed by seafloor spreading and the theory of plate tectonics. Section 9.2 Sea-Floor Spreading

3 -- Section 9.2: Sea-Floor Spreading -- Seafloor spreading Movement of the ocean floor away from either side of a midocean ridge Began with mapping of the ocean floor utilizing sonar (sound navigation and ranging) to discover two components of the ocean floor Deep-ocean trenches A surface feature in the seafloor produced by the descending plates Most occur around the edges of the Pacific Ocean with some also in the Indian and Atlantic oceans. Mid-ocean ridges System of undersea mountain ranges that wind around Earth 70,000 km long -- Section 9.2: Sea-Floor Spreading -- Mid-ocean Ridges -- Section 9.2: Sea-Floor Spreading -- Seafloor Spreading -- Section 9.2: Sea-Floor Spreading -- Mid-Atlantic Ridge Undersea mountain range with a steep, narrow valley along its center Part of the mid-ocean ridges Discovered that the ocean floor was very young compared with the age of continental rocks None of the oceanic rocks found were more than 180 million years old. The oldest continental rocks are about 4 billion years old. Renewal of the ocean floor Hypothesis by Harry Hess, American geologist The valley at the center of the Mid-Atlantic Ridge was actually a break, or rift, in Earth s crust and that magma was welling up through the rift because the ocean floor was moving away from both sides of the ridge.

4 -- Section 9.2: Sea-Floor Spreading -- As the ocean floor moved away from the ridge, it was replaced by rising magma that cooled and solidified into new rock. Hypothesis by Robert Dietz, American geologist Named the movement of the ocean floor seafloor spreading Suggested that if the ocean floor was moving, the continents might also be moving Proof through paleomagnetism and earthquake patterns -- Section 9.2: Sea-Floor Spreading -- Continental Drift -- Section 9.2: Sea-Floor Spreading -- Paleomagnetism Study of the past magnetic properties of rocks When magma cools and solidifies, iron in the rock becomes magnetized and aligns when the rock hardens to point north. Many rocks from previous centuries have pointed south, suggesting Earth s magnetic field must have reversed itself several times. Verified by dating rocks with different magnetic orientations Found on the ocean floor by the Mid-Atlantic Ridge as patterns of alternating bands of normal and reverse polarity of magnetic orientations, suggesting spreading on the ocean floor that has occurred for a long time. -- Section 9.2: Sea-Floor Spreading -- Dates of reversals on land and in the ocean were exactly the same, proving seafloor spreading and continental drift.

5 -- Section 9.2: Sea-Floor Spreading -- Earthquake patterns Work of scientists Kiyoo Wadati and Hugo Benioff Found a pattern when plotting the depth of earthquakes in relation to their distance from deepocean trenches Occur in a belt about 50 kilometers thick No earthquakes detected below about 700 kilometers Result convinced scientists that major sections of ocean floor return to the mantle in subduction zones. Section 9.3 Theory of Plate Tectonics -- Section 9.3: The Theory of Plate Tectonics -- Plate tectonics Theory that the lithosphere is made up of plates that float on the asthenosphere and that the plates possibly are moved by convection currents Not only describes continental movement but also provides a possible explanation of how and why the continents move Tectonics: study of the formation of features in Earth s crust Two types of crust on Earth Oceanic crust: material that makes up the ocean floor Continental crust: material that makes up landmasses Movement of plates Lithosphere: thin outer shell of Earth consisting of the continental and oceanic crusts and the rigid upper mantle -- Section 9.3: The Theory of Plate Tectonics lithospheric plates have been identified to date. Movement of lithospheric plates» Moving away from each other» Moving toward each other» Sliding past each other Asthenosphere: zone of mantle beneath the lithosphere that consists of slowly flowing solid rock, like putty when under pressure

6 -- Section 9.3: The Theory of Plate Tectonics -- Plate Tectonics -- Section 9.3: The Theory of Plate Tectonics -- Lithospheric plate boundaries Divergent boundaries Boundaries formed by two lithospheric plates that are moving apart As plates move apart, magma from the asthenosphere rises and fills the space between the plates, cools, and hardens onto the edges of the separating plates, creating new oceanic crust. Most are found on the ocean floor and follow the midocean ridges. Rift valley Steep, narrow valley formed as lithospheric plates separate Often formed where continents are separated by plate movement -- Section 9.3: The Theory of Plate Tectonics -- Example: Red Sea occupying a huge rift valley by the African and Arabian plates separating -- Section 9.3: The Theory of Plate Tectonics -- Convergent boundaries Boundaries formed by two lithospheric plates that are moving toward each other Result in one of three kinds of collisions based on crusts involved Continental crust colliding with oceanic crust» Since oceanic crust is denser, it will be subducted, or forced, under the less dense continental crust» Subduction zone: region where one lithospheric plate moves under another» Ocean trench: deep valley in the ocean that forms along a subduction zone

7 -- Section 9.3: The Theory of Plate Tectonics --» Continental volcanic arc: mountains formed in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent» As the oceanic plate moves down into a subduction zone, it melts and becomes part of the mantle material.» Some of the magma formed rises to the surface through the continental crust and produces volcanoes. -- Section 9.3: The Theory of Plate Tectonics -- Continental crust colliding with continental crust» Neither plate is subducted since they have similar densities.» Edges are crumpled, uplifted, and create large mountain ranges.» Example: Himalayan Mountain Range Oceanic crust colliding with oceanic crust» Deep ocean trench is formed when one is subducted under another.» Part of the subducted plate melts, and resulting magma rises to the surface along the trench to form an volcanic island arc (chain of volcanic islands formed along an ocean trench) -- Section 9.3: The Theory of Plate Tectonics -- Convergent Plate Boundaries

8 -- Section 9.3: The Theory of Plate Tectonics -- Transform fault boundaries Boundaries formed by two lithospheric plates that are sliding past each other Do not slide smoothly, scrape each other, get caught up to build up force, and eventually release, causing minor tremors to major earthquakes Example: San Andreas Fault -- Section 9.3: The Theory of Plate Tectonics -- Transform Plate Boundaries Section 9.4 Mechanisms of Plate Motion -- Section 9.4: Mechanisms of Plate Motion -- Causes of plate motion According to the theory of plate tectonics, the lithosphere is broken into separate plates that ride on the denser asthenosphere with the asthenosphere being warped and moved based on convection currents (movement in a fluid caused by uneven heating in which hotter magma rises and cooler magma sinks) Convection: transfer of heat through the movement of a fluid material, such as magma Heat from Earth s core and mantle cause some magma in the lower asthenosphere to become hotter and less dense, causing it to rise above the more dense material. When the hot magma reaches the base of the lithosphere, it cools, becomes more dense, and sinks again.

9 -- Section 9.4: Mechanisms of Plate Motion -- The cooling magma is pushed to the side by new hot magma that rises, causing the lithospheric plate to be carried along with the moving magma. Evidence for convection currents Recent studies of the ocean floor in which heat flow is higher along plate boundaries where two plates are separating that elsewhere on the ocean floor, explaining temperature differences. Questions of whether convection currents alone are strong enough to move the plates at rates suggested by geological evidence. The Rock Cycle Igneous Rocks Sedimentary Rocks Metamorphic Rocks Section 3.1 The Rock Cycle -- Section 3.1: The Rock Cycle -- Magma Hot, molten rock that originates inside the earth Parent material for all rocks Types of rocks Igneous rock Rock formed from cooled and hardened magma Comes from the Latin meaning from fire Is called lava if it cools at the earth s surface Examples: basalt, obsidian, pumice, granite Sedimentary rock Rock formed from hardened deposits of sediments Fragments of rock that result from the breaking of rocks, minerals, and organic matter Carried away and deposited by water, ice, and wind

10 -- Section 3.1: The Rock Cycle -- The hardened deposits are cemented together over time. Examples: conglomerate, sandstone, gypsum, halite, coal, limestone Metamorphic rock Rock formed from other rocks as a result of intense heat, pressure, or chemical processes Metamorphic = changed form Examples: marble, quartzite, schist, gneiss, slate The rock cycle Series of processes in which rock changes from one type to another and back again Steps of the rock cycle Once igneous rock has formed, a number of processes break down the rock into sediments. -- Section 3.1: The Rock Cycle -- The sediments are compacted and hardened to form sedimentary rocks. The sedimentary rocks are subjected to high temperature and/or pressure within the earth to become metamorphic rocks. Heat and pressure increase to melt the metamorphic rock into magma to later form igneous rocks. Not all rocks go through a complete rock cycle. Igneous rock may never be exposed to Earth s surface and therefore never be broken down into sediments to form sedimentary rock. Some igneous rock may receive enough pressure and temperature early on to become metamorphic rock. Other exceptions -- Section 3.1: The Rock Cycle -- The Rock Cycle Section 3.2 Igneous Rocks

11 Two groups of igneous rock Intrusive igneous rock Rock formed from the cooling of magma beneath Earth s surface Intrudes, or enters, into other rock masses beneath Earth s surface Examples: granite Extrusive igneous rock Rock formed from molten lava that rapidly cools and hardens on Earth s surface Extrudes, or exits, onto Earth s surface Examples: basalt Texture of igneous rocks The size of the crystalline grains in igneous rock Largely determined by the cooling rate of the magma or lava that formed the rock Texture differs in intrusive igneous rock and extrusive igneous rock Intrusive igneous rock Formation: magma cools and hardens slowly deep underground Crystalline grains Coarse-grained Large because of long cooling time, allowing large crystals to form Examples: granite Granite (Intrusive Igneous Rock) Extrusive igneous rock Formation: lava cools and hardens rapidly on Earth s surface Crystalline grains Fine-grained Small because of quick cooling, not allowing large crystal grains to form Examples: basalt

12 Basalt (Extrusive Igneous Rock) Porphyry Igneous rock composed of large and small crystals Formed from when magma cools slowly at first and then more rapidly as it gets closer to the surface, causing large crystals caught in a mass of smaller crystals Other textures Obsidian, or volcanic glass Extremely rapid cooling occurs where crystals cannot form Glassy appearance Pumice During extremely rapid cooling, gases escaping from the magma get trapped in the rock and form bubbles. Some pumice can float on water because of the air in it causing it to be less dense than water.

13 Composition of igneous rocks Mineral composition of an igneous rock is determined by the chemical composition of the magma from which the rock develops. Three families of igneous rocks Granite Basalt Diorite Formation: Granite formed from felsic magma, which is high in silica Mineral contents: Orthoclase feldspar and quartz (light colored minerals) Quartz May also have plagioclase feldspar, hornblende, and muscovite mica Color: generally light-colored Examples: granite (cg), rhyolite (fg), obsidian ( volcanic glass ) Granite Rhyolite Obsidian

14 Formation: Basalt formed from mafic magma, which is high in iron but low in silica Basalt Gabbro Mineral contents: Plagioclase feldspar and augite May also have contain olivine, biotite mica, and hornblende (dark colored ferromagnesian minerals) Color: generally dark-colored Examples: basalt (fg), gabbro (cg) Diorite Mineral contents: Plagioclase feldspar, hornblende, augite, and biotite mica Contains little to no quartz Diorite Andesite Color: generally medium-colored Examples: diorite (cg), andesite (fg)

15 Igneous Rock Structures Intrusions (underground rock masses made up of intrusive igneous rocks) Batholith Largest type of igneous intrusion, covering over 100 km 2 and reaching a depth of thousands of meters Means deep rock Form the cores of many major mountain ranges, such as the Sierra Nevada in California and the Coast Range in British Columbia Batholith Stock Igneous intrusion with an area less than 100 km 2 Similar to a batholith but much smaller Composite of formations

16 Laccolith Flat-bottomed intrusion that pushes overlying rock layers into an arc Means lake of rock Sometimes can be identified by the small domeshaped mountains they push up on Earth s surface Many are located beneath the Black Hills of South Dakota. Sill Sheet of hardened magma that forms between and parallel to layers of rock Vary in thickness from a few centimeters to hundreds of meters and can extend horizontally for several kilometers Many are located in Big Bend National Park in Texas. Sill Dike Igneous intrusion that cuts across rock layers Common in areas of volcanic activity Differ from sills because they cut across rock layers instead of parallel to them.

17 Dikes Summary of Intrusions Extrusions (surface rock masses made up of extrusive igneous rocks) Volcano Formed when lava erupts onto Earth s surface and forms a cone of extrusive igneous rock surrounding a central vent through which lava can flow The Ring of Fire Volcano

18 Volcanic neck Solidified central vent of a volcano Created when a volcano stops erupting for a long period of time, the lava in the central vent cools and solidifies, and the cone surrounding it deteriorates to leave the hardened lava Shiprock in New Mexico is an example. Dike and Volcanic Neck Lava plateau Raised, flat-topped area made of layers of hardened lava Formed from lava that flows out of long cracks in Earth s surface, spreads out over a vast area, and fills valleys and covers hills Arizona Lava Plateau

19 Section 3.3 Sedimentary Rocks Formation of sedimentary rock Sedimentary rock is formed from the accumulations of different types of sediment. Processes that form sedimentary rock Compaction Process in which air and water are squeezed out of sediments Caused by the weight of overlying sediments to exert pressure to squeeze out the air and water Cementation Process in which dissolved minerals left by water passing through sediments hold the sediments together Glue between rocks Classes of sedimentary rock Clastic sedimentary rock Rock made up of fragments of pre-existing rocks Fragments carried away from their source by water, wind, or ice that are then deposited Clastic sedimentary rocks are classified by the size of the sediments they contain. Gravel-sized fragments Sand-sized fragments Clay-sized fragments Gravel-sized fragments Conglomerate» Sedimentary rock composed of rounded gravel or pebbles cemented together by minerals» Individual pieces of sediment can easily be seen.

20 Breccia» Elastic sedimentary rock composed of angular fragments cemented together by minerals Sand-sized fragments Called sandstones Quartz is the major component of sandstone. Has pores through which groundwater and crude oil can move Clay-sized fragments Called shale Particles cemented together under pressure into flat layers that are flaky and will easily split apart. Chemical sedimentary rock Rock formed from minerals that have been dissolved in water Types Rocks formed from dissolved minerals that precipitate (settle) out of the water as a result of a change in temperature» Limestone

21 Evaporites» Rocks formed when water evaporates and leaves behind the minerals dissolved in the water» Examples - gypsum (on the left) and halite (on the right) Biochemical sedimentary rock Rock formed from the remains of organisms Examples Coal (shown on the left) Limestone (shown on the right) Chalk Sedimentary rock features Stratification Layering of sedimentary rock Occurs when there is a change in the kind of sediment being deposited Beds (stratified layers) vary in thickness depending on how long each type of sediment was being laid down. Cross-bedding Layers not stratified horizontally Often occur as a result of sediments being deposited by wind Cross-bedding

22 Graded bedding Occurs as different sizes and shapes of sediment settle to different levels Largest and heaviest grains generally settle at the bottom. Ripple mark Formed by the action of wind or water on sand May be preserved when sand becomes sandstone Mud crack Occur when muddy deposits dry and shrink, causing the mud to crack Occur in river flood plains and dry lake beds When area is flooded again, new deposits fill in the cracks and preserve them as the mud hardens to become solid rock. Fossil Trace or remains of an organism in sedimentary rock Occurs as a result of sediments burying remains or leaving impressions in the rock

23 Concretion Nodule of rock with a different composition from that of the main rock body Geodes Crystal cavities found in rocks Formed by groundwater depositing dissolved quartz or calcite inside cavities of rocks that then crystallize Section 3.4 Metamorphic Rocks -- Section 3.4: Metamorphic Rocks -- Metamorphism Changing one type of rock to another by heat, pressure, and chemical processes Most occurs deep beneath the surface of Earth where heat and pressure are the greatest. All metamorphic rock is formed from existing igneous, sedimentary, or metamorphic rock. Types of formation of metamorphic rock Contact metamorphism Change in the structure and mineral composition of a rock surrounding an igneous intrusion Heat from magma of the intrusion changes the structure and mineral composition of the surrounding rock. -- Section 3.4: Metamorphic Rocks -- Regional metamorphism Metamorphism that affects rocks over large areas during periods of tectonic activity Occurs over an area of thousands of square kilometers Tectonic plates generate great heat and stress to change rock. Relates to contact metamorphism since volcanism and magma movement often go with tectonic activity Classification of metamorphic rocks Foliated rock Rock that has visible parallel bands Ways to form

24 -- Section 3.4: Metamorphic Rocks -- Extreme pressure may flatten mineral crystals in the original rock and push them into parallel bands. Minerals of different densities separate into bands, producing alternating light and dark bands. Examples Slate Schist Gneiss -- Section 3.4: Metamorphic Rocks Section 3.4: Metamorphic Rocks -- Foliated gneiss Foliated slate -- Section 3.4: Metamorphic Rocks -- Slate can turn into schist with greater heat and pressure, and even more heat and pressure changes schist into gneiss. Unfoliated rock Rock that does not have visible bands No bands of crystals Examples Quartzite (from the sedimentary rock sandstone) Marble (from the sedimentary rock limestone)

25 -- Section 3.4: Metamorphic Rocks -- Quartzite Marble