GEOG 3251 summer 10 Adina Racoviteanu Objectives Understand

GEOG 3251 summer ‘ 10 Adina Racoviteanu

Objectives • Understand how Earth is dynamic and how moving plates form ocean basins, mountain ranges, islands, volcanoes, and earthquakes • Identify the three general categories of plate boundaries recognized by scientists: convergent, divergent, and transform • Understand how theory of plate tectonics was developed and supported

Why is the Earth so restless? • ground shakes violently • volcanoes erupt with explosive force • mountain ranges rise to incredible heights

Catastrophism world view Until 1700 s: common belief: • Biblical Flood shaped Earth's surface • All earthly changes were sudden and caused by a series of catastrophes.

Uniformitarian principle • 1785 : James Hutton, a Scottish geologist proposes new world view: The present is the key to the past. • geologic forces and processes acting on the Earth today are the same as those that have acted in the geologic past.

EARTH’S LAYERS • Three layers: – the core, – the mantle and – the crust • Chemical & mechanical differences Like boiled egg

THE CORE • Composition: mostly iron and nickel • Temperature: very hot, even after 4. 5 billion years of cooling. • divided into two layers: – a solid inner core – liquid outer core.

CORE GENERATES CURRENTS • Core is so hot it radiates natural heat to the upper layers. • convection currents generated. • The convection currents cause the movement of the tectonic plates.

MANTLE • Composition: mafic minerals - rich in the elements iron (Fe) magnesium (Mg), silicon, and oxygen • Dense, hot layer of semi-solid rock • Upper part of the mantle is cooler and more rigid than the deep mantle

CRUST • Composition: oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. • rigid and very thin

LITHOSPHERE • Greek (lithos = stone) rigid outermost layer made of crust and uppermost mantle • broken up into the moving plates that contain the world's continents and oceans

ASTHENOSPHERE • Greek (asthenos = weak) = part of the mantle that flows, a characteristic called plastic behavior. • The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithospheric plates.

CRUST/MANTLE AGAIN • lithosphere – – hard ~100 km thick crust floats on top continental crust 20 to 70 km thick – oceanic crust ~ 8 km thick • asthenosphere – soft – ~3000 km thick – “fluid-like”

Crust: 2 types • Oceanic crust is made of relatively dense rock called basalt • Continental crust is made of lower density rocks, such as andesite and granite.

Oceanic vs. continental crust Oceanic Continental basaltic rocks granitic rocks "mafic” Mg, Fe “felsic” (Fe-Si, Al) denser less dense thinner thicker

EARTH STRUCTURE SUMMARY • • The Earth is sphere with a diameter of about 12, 700 Kilometres. Temperature and pressure rise with depth. • CORE: – – temperature is believed to be an incredible 5000 -6000°c. Inner core: solid FE and NI Outer core: liquid Fe and Ni Complex convection currents give rise to a dynamo effect which is responsible for the Earth's magnetic field. • MANTLE: – It's made of solid rock and behaves like an extremely viscous liquid - (This is the tricky part. . . the mantle is a solid which flows? ? ) • CRUST: – very thin (average 20 Km) – OCEANIC CRUST: thinner (10 kilometres) – CONTINENTAL CRUST: thicker (35 kilometres on average). • The continental crust in the Himalayas is some 75 kilometres deep.

THEORIES OF MOUNTAIN ORIGIN • CONTINENTAL DRIFT – Alfred Wegener (1880 – 1930) • PLATE TECTONICS – Harry Hess and G. Mason (1962)

VIDEO • PLATE TECTONICS INTRO

CONTINENTAL DRIFT

FOSSIL EVIDENCE • Fossils of the same species found on several different continents. • Wegener’s idea: species dispersed when the continents were connected and later carried to their present positions as the continents drifted. For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and Australia.

TORN NEWSPAPER IDEA • Rock sequences in South America, Africa, India, Antarctica, and Australia show remarkable similarities. • Wegener’s idea: the same three layers occur at each of these localities.

SHIFTING CLIMATE

SHIFTING CLIMATE ZONES • Wegener’s interpretation: • climate zones remained stationary and the continents drifted to different locations. • The drift of the continents caused the apparent movement of the climate zones.

ARGUMENTS AGAINST • What mechanism moves the continents? ? Wegener’s idea: Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges.

Continental Drift Evidence Summary • • Puzzle idea: continents fit together Torn newspaper: matching geology Fossils: same fossil on different continent Paleoclimate

SEA-FLOOR SPREADING In the late 1950's, scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean.

SEA FLOOR SPREADING HO When mapped, the anomalies produce a zebra-striped pattern of parallel positive and negative bands. The pattern was centered along, and symmetrical to, the mid-ocean ridge

Computer-generated detailed topographic map of a segment of the Mid-Oceanic Ridge. "Warm" colors (yellow to red) indicate the ridge rising above the seafloor, and the "cool" colors (green to blue) represent lower elevations.

New discoveries: Sea-floor spreading • 1962: Hess (1962): New ocean floor is formed at the rift of mid -ocean ridges. VIDEO

SEA FLOOR SPREADING

SEA FLOOR SPREADING HO

SEA FLOOR SPREADING • So, continents are no longer thought to plow through oceanic crust • Continents are part of plates that move on the soft, plastic asthenosphere. • driving force: convection currents

If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere.

PLATE TECTONICS SUMMARY • Plate = large, rigid slab of solid rock • Tectonics (Greek) = to build • PLATE TECTONICS: states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.