Density andPressure alsoIncrease as youmove towardsthe core.
How Do We Know?Scientists use seismographic (energy waves produced byearthquakes) data recorded at research stations positioned allover the world to piece together the earths internal structure.
Earth’s Interior• Core– High Density– Iron and Nickel– Inner Core - solid– Outer Core - liquid– Less dense than core– Iron and Magnesium silicates– Mostly solid– Upper mantle is partially molten• Mantle– Outermost layer– Very thin and rigid– Continental – granite– Density = 2.8 g/cm3– Oceanic – basalt– Density = 3.0 g/cm3• Crust
The Inner Core Center of the earth Deepest layer, 6250 km underthe surface. A solid ball of an iron alloy(sulphur & nickle traces). Solidified due to intensepressure. 2400 km in diameter. Approximately 6000°C- Hotter than the surface of thesun.
The Outer Core• Layer of molten (liquid) Ironbetween Inner Core andMantle.• 2,300 km thick.• Cooler than inner core, butstill scorching hot at 4,500°C• Mostly iron, plus sulphur &nickle.• Constantly in motion as theearth spins.– Generates Earth’selectromagnetic field.
The Mantle• Middle layer (Between Crust and Core)• 2900 km thick (approx 84% of earth’s volume).• Temperature:o 500°C near crusto 4,000°C near outer core.• Composed of Magnesium, Iron, Aluminum,Silicon and Oxygen.
Convection Currentsin the Mantle• The mantle is made of“solid” rock.– The rock is extremely hotbut the high pressurekeeps it from melting.• The rock is “plastic” anddoes flow and movearound, like hot asphalt.– Hot rock rises and coolerrock descends creatingslow moving convectioncurrents.
Earth’s Crust• Thin Outermost Layer (0-40km)– Made up of low density materials.– Solid Rock (Broken up into large plates)• Divided into 2 Types:– Continental Crust:• Made primarily of Granite.• Density of 2.7 g/cm3 (less dense)• Thickness of 40km• Oldest Crust– Oceanic Crust:• Made Primarily of Basalt• Density of 2.9 g/cm3 (more dense)• Thickness of 8km• Youngest Crust• Bill Nye
“The Origin of the Continentsand Oceans” 1906• Wegner laid out his theory ofplate tectonics and continentaldrift.– Proposed that the continentswere actually large mobileplates of crust.– Could not explain themechanism by which thecontinents move.• Wegner’s theory was largelyignored and written off by thescientific community for nearly50 years.
Plate Tectonics• The Earth’s crust is divided into 12 majorplates which are moved in variousdirections.• This plate motion causes them to collide,pull apart, or scrape against each other.• Each type of interaction causes acharacteristic set of Earth structures or“tectonic” features.• The word, tectonic, refers to thedeformation of the crust as a consequenceof plate interaction.
What are tectonic plates made of?• Plates are madeof rigidlithosphere.The lithosphere is made upof the crust and the upperpart of the mantle.
Plate Movement• “Plates” of lithosphere are moved around by theunderlying hot mantle convection cells
• Divergent• Convergent• TransformThree types of plate boundary
• Spreading ridges– As plates move apart new material is erupted to fill the gap• In plate tectonics, a divergent boundary is a linear feature thatexists between two tectonic plates that are moving away from eachother. These areas can form in the middle of continents or on theocean floor.• As the plates pull apart, hot molten material can rise up this newlyformed pathway to the surface - causing volcanic activity.Divergent Boundaries
Age of Oceanic CrustOcean Ridges: This map shows the age of the oceanic crust.Red: shows the youngest ages.Blue: shows the oldest ages (around 200 million years old).New crust is created at divergent plate boundaries
• Iceland has a divergentplate boundary runningthrough its middleIceland: An example of continental rifting
• Convergent boundaries are where the platesmove towards each other.• There are three types of convergent boundary,each defined by what type of crust (continentalor oceanic) is coming together.• There are three styles of convergent plateboundaries– Continent-continent collision– Continent-oceanic crust collision– Ocean-ocean collisionConvergent Boundaries
Forms Mountains (Example: European Alps and Himalayas)Continent-Continent Collision• When continental crust pushes against continental crust both sides of theconvergent boundary have the same properties.• Neither side of the boundary wants to sink beneath the other side, and as aresult the two plates push against each other and the crust buckles andcracks, pushing up (and down into the mantle) high mountain ranges.
• At a convergent boundary where continental crust pushes againstoceanic crust, the oceanic crust which is thinner and more dense thanthe continental crust, sinks below the continental crust.• This is called a Subduction Zone. The oceanic crust descends into themantle at a rate of centimeters per year.• Subduction is a way of recycling the oceanic crust. Eventually thesubducting slab sinks down into the mantle to be recycled.• It is for this reason that the oceanic crust is much younger than thecontinental crust which is not recycled.Continent-Oceanic Crust Collision
• Oceanic lithosphere subductsunderneath the continentallithosphere• Oceanic lithosphere heats anddehydrates as it subsides• The melt rises formingvolcanism• E.g. The AndesSubduction
• When two oceanic plates collide, one runs over theother which causes it to sink into the mantle forminga subduction zone.• The subducting plate is bent downward to form avery deep depression in the ocean floor called atrench.• The worlds deepest parts of the ocean are foundalong trenches.– E.g. The Mariana Trench is 11 km deep!Ocean-Ocean Plate Collision
• Where plates slide past each other.• Also known as strike-slip faults.• Frequently form earthquakes.Transform BoundariesAbove: View of the San Andreastransform fault
Volcanism ismostly focused atplate marginsPacific Ring of Fire
- Subduction - Rifting - HotspotsVolcanoes are formed by:
• Hot mantle plumes breaching the surface inthe middle of a tectonic plateWhat are Hotspot Volcanoes?Photo: Tom Pfeiffer / www.volcanodiscovery.comThe Hawaiian island chain are examplesof hotspot volcanoes.
Hot Spot VolcanoesThe tectonic plate moves over a fixedhotspot forming a chain of volcanoes.The volcanoes get younger from one end to the other.
As with volcanoes, earthquakes are not randomlydistributed over the globeAt the boundaries between plates, friction causes themto stick together. When built up energy causes them tobreak, earthquakes occur.Figure showing thedistribution ofearthquakesaround the globe
Where do earthquakes form?Figure showing the tectonic setting of earthquakes
Plate Tectonics Summary• The Earth is made up of 3 main layers (core,mantle, crust)• On the surface of the Earth are tectonic platesthat slowly move around the globe• Plates are made of crust and upper mantle(lithosphere)• There are 2 types of plate• There are 3 types of plate boundaries• Volcanoes and Earthquakes are closely linked tothe margins of the tectonic plates