Plate Tectonics An overview on earth science OBJECTIVES

Plate Tectonics An overview on earth science.

OBJECTIVES • View the earth’s interior and how the earth structures were formed. • Describe the rock life cycle. • Explain the historical theories of the earth’s continent formation (continental drift). • Compare historical theories to the presently accepted theory (plate tectonics). • Examine the different crustal plate boundaries (transform, divergent and convergent).

EARTH’S INTERIOR • The earth is composed of three layers: CRUST -- The crust (1%) -- The mantle (84%) -- The core (15%) • It is important to understand the properties of these layers and how each layer are connected to the others. CORE MANTLE

THE EARTH’S CRUST • The crust is the outermost part of the Earth. • The crust thickness ranges from 3 to 25 miles thick. • There are two types of crust : CONTINENTAL & OCEANIC Oceanic Continental What do you see is different between the two types of crust?

CONTINENTAL vs. OCEANIC CRUST CONTINENTAL Crust OCEANIC Crust • Thicker than oceanic crust • Composed of mainly granite • Thickness is about 35 to 40 kilometers (or 20 to 25 miles) • Density = 2. 7 g/cm 3 • Since density is less, there has to be more crust to balance on the mantle • Covers 40% of the earth’s surface • Thinner than continental crust • Composed of mainly basalt • Thickness is less than 10 kilometers (or 6. 2 miles) • Density = 3. 3 g/cm 3 • Since density is more and water sits above, there has to be less crust to balance on the mantle

THE EARTH’S MANTLE • The mantle is about 100 -200 kilometers (62 to 124 miles) miles under the crust. • The mantle has never been drilled into. All information about the mantle is learned from studying earthquake waves, heat flow, magnetics, gravity, laboratory experiments on rocks and minerals from volcanic eruptions. • The upper mantle is composed of molten rock and is responsible for moving the crustal plates. • The lower mantle is closer to the core and is composed mainly of iron and magnesium silicate molecules.

THE EARTH’S CORE • The core, like the mantle, is divided into two sections. • The inner core is composed of completely solid iron with about 10 percent alloy of oxygen, sulfur or nickel. • The outer core is DATA ON THE EARTH’S INTERIOR also composed Earth Type Thickness (km) Types of Rock Found of these elements silicic rocks, andersite and Crust 30 but is a liquid basalt instead of solid. peridotie, eclogite, olivine, Upper Mantle 720 spinel, garnet, pyroxene, pervoskite and oxides • Since the core is not magnesium and silicon Lower Mantle 2, 171 connected to the oxides iron + oxygen, sulfur, mantle, it rotates Outer Core 2, 259 nickel alloy at a different rate iron + oxygen, sulfur, Inner Core 1, 221 nickel alloy than the rest of the Total Thickness 6, 401 earth.

THE ROCK CYCLE • The rock cycle is important to understand when learning about plate tectonics. • There are three types of rocks that form : igneous, igneous sedimentary, sedimentary and metamorphic • Rock formations occur by different earth processes • Rocks are constantly changing due to plate tectonics and other mechanisms.

IGNEOUS ROCKS • Metamorphic and sedimentary rocks can transition to igneous through two processes. PROCESS 1: 1 If pushed deep into the earth’s surface, as a result of plate tectonics, some rocks melt into magma, cool and solidify into an igneous rock. PROCESS 2: 2 Lava reaching the earth’s crust and erupting due to a volcanic reaction can cool and solidify into an igneous rock. • Igneous rocks are know as extrusive or volcanic volcani rocks. COOLING

SEDIMENTARY ROCKS • There are three types of sedimentary rocks: clastic, clastic biogenic, biogenic and precipitate • These rocks are all formed by lithification, lithification – Lithification is where loose materials are converted into solid rock by either compaction or cementation. • Clastic rocks are fragments of either metamorphic or igneous rocks. • Biogenic rocks are fragments from living organisms or fossils. • Precipitate rocks are formed from the chemical reaction (precipitation). • Chemical precipitation is a process in which a solid is separated from solution.

METAMORPHIC ROCKS • Igneous and sedimentary rocks can transition to metamorphic rocks by two main processes. • The first example is regional metamorphism or by the formation of mountains. These rocks typically exhibit the characteristic of foliation, foliation meaning where clear bands of minerals and/or color can be seen. • The second process is contact metamorphism. The rock is formed by recrystallizing as a result of an igneous intrusion heating the surrounding area.

CONTINENTAL DRIFT • • • The theory of continental drift has been around as early as 1596, first suggested by the Dutch map maker Abraham Ortelius. He proposed that the Americas, Eurasia and Africa were once joined together but are now separate as a result of “earthquakes and floods”. The only proof he had for his claim was the observation that the present day continents looked like the had once fit together, like a puzzle.

CONTINENTAL DRIFT Since 1596, more proofs have been documented. – Fossils of the same plants and animals from similar time periods have been found on different continents. • An example of this is the ancient reptile Lystrosaurus found in South America, Antarctica and Africa!

CONTINENTAL DRIFT Since 1596, more proofs have been documented. – Matching rocks and glacier deposits from similar time periods have been found on different continents.

CONTINENTAL DRIFT More evidence was developed by Alfred Wegener in 1912. – Mountain chains are also connected across one continent. Rocks have been age-dated along with isotopic comparisons to show similar origin.

CONTINENTAL DRIFT Additional timeline images showing continental drift. Glacial Evidence

PLATE TECTONICS • Though continental drift was accepted as a theory, there were still limitations in the models. The major limitation was no mechanism for actual plate movement was described or understood. • Harry Hess in the 1960 s was the first to propose a mechanism with the understanding of sea floor spreading – Mapping sea floor depths and earthquake locations revealed the earth’s major spreading center : the Mid-Atlantic Ridge. – Convection was described as the force (or mechanism ) causing the crustal plates to move and spread.

PLATE TECTONICS • The red dots signify earthquake locations. • The visible trend is earthquakes align along the crustal plate boundaries – NOT only along the continents. • The lithosphere includes the continental and oceanic crust. • The lithosphere is broken into 7 major plates and many minor plates. • There are 3 types of plate boundaries.

PLATE TECTONICS

PLATE TECTONICS • The 3 boundary types are – 1. Convergent • plates come together, collide, forced up and/or down, crust is destroyed 2. Divergent • plates move apart, divide, new crust is formed 3. Transform DIVERGENT BOUNDARY • plates slide past one another, crust remains intact CONVERGENT BOUNDARIES

PLATE TECTONICS TRANSFORM BOUNDARY • Two tectonic plates grind past one another. • These boundaries are usually along faults • This movement causes friction between the two plates resulting in a release of energy called strain. • The strain cause earthquakes. • The most common fault is the San Andreas Fault in California.