Plate Tectonics

Continental Drift Continents can be made to fit together like pieces of a picture puzzle The similarity of the Atlantic coastlines of Africa and South America has long been recognized In the early 1900s Alfred Wegner made a strong case for continental drift: he noticed that Africa, South America, India, Antarctica and Australia had almost identical late Paleozoic rocks and fossils Wegner assembled the continents to form a giant supercontinent Pangaea

Palaeomagnetism The Earth has a natural magnetic field with a north and south pole, like a magnet. It is this field to which the north arrow of a compass needle aligns to point northwards. When rock is molten (such as lava) any magnetic minerals also align to this natural magnetic field. When the rock cools, these miniature "compasses" are frozen in that direction

When geoscientists look at igneous rocks, they find that some show that the Earth's magnetic field is opposite to what it is today. In these rocks the minerals show that the north magnetic pole was once at the south magnetic pole, and the south magnetic pole at the north magnetic pole. This change in poles is known as Polar Reversal and has taken place many times over the age of the earth

The ages of many Polar Reversals are well known through studies of basalts making up the ocean floors

palaeomagnetic methods allow an understanding of changing continental positions over time. Geoscientists can use this magnetic information to locate the position of the Earth's magnetic pole at the time the lava cooled. When the pole is plotted relative to a continent over time, it appears that the pole has moved. This phenomena is known as "polar wander". Geoscientists assume that the pole has been reasonably fixed and that polar wander is really the result of continent drift.

A relatively recent theory that the Earth's crust is composed of rigid plates that move relative to one another. Plate movements are on the order of a few centimeters/year - about the same rate as your fingernails grow! Plate Tectonics There are 3 types of plate boundaries: 1. divergent 2. convergent 3. transform

What is Plate Tectonics?

If you look at a map of the world, you may notice that some of the continents could fit together like pieces of a puzzle.

Plate Tectonics The Earth’s crust is divided into 12 major plates which are moved in various directions. This plate motion causes them to collide, pull apart, or scrape against each other. Each type of interaction causes a characteristic set of Earth structures or “tectonic” features. The word, tectonic, refers to the deformation of the crust as a consequence of plate interaction.

Modern Plate Map

What are tectonic plates made of? Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

What lies beneath the tectonic plates? Below the lithosphere (which makes up the tectonic plates) is the asthenosphere.

Plate Movement “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

What happens at tectonic plate boundaries?

Divergent Convergent Transform Three types of plate boundary

Spreading ridges – As plates move apart new material is erupted to fill the gap Divergent Boundaries

Age of Oceanic Crust Courtesy of

Oceanic Divergent Boundary Example: Mid-Atlantic Ridge

Iceland has a divergent plate boundary running through its middle Iceland: An example of continental rifting

Continental Divergent Boundary Example: Red Sea / E. African Rift

There are three styles of convergent plate boundaries – Continent-continent collision – Continent-oceanic crust collision – Ocean-ocean collision Convergent Boundaries

Forms mountains, e.g. European Alps, Himalayas Continent-Continent Collision

Himalayas

Called SUBDUCTION Continent-Oceanic Crust Collision

Oceanic lithosphere subducts underneath the continental lithosphere Oceanic lithosphere heats and dehydrates as it subsides The melt rises forming volcanism E.g. The Andes Subduction

When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone. The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench. The worlds deepest parts of the ocean are found along trenches. – E.g. The Mariana Trench is 11 km deep! Ocean-Ocean Plate Collision

Where plates slide past each other Transform Boundaries Above: View of the San Andreas transform fault

Continental Transform Boundary - Example: San Andreas

…what’s the connection? Volcanoes and Plate Tectonics…

Volcanism is mostly focused at plate margins Pacific Ring of Fire

- Subduction - Rifting - Hotspots Volcanoes are formed by:

Pacific Ring of Fire Hotspot volcanoes

Hot mantle plumes breaching the surface in the middle of a tectonic plate What are Hotspot Volcanoes? Photo: Tom Pfeiffer / The Hawaiian island chain are examples of hotspot volcanoes.

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes. The volcanoes get younger from one end to the other.

…what’s the connection? Earthquakes and Plate Tectonics…

As with volcanoes, earthquakes are not randomly distributed over the globe At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Figure showing the distribution of earthquakes around the globe

Plate Tectonic Theory Plate boundaries are marked by – Volcanic activity – Earthquake activity At plate boundaries – plates diverge, – plates converge, – plates slide sideways past each other

Plate Tectonic Theory Influence on geological sciences: Revolutionary concept – major milestone, comparable to Darwin’s theory of evolution in biology Provides a framework for – interpreting many aspects of Earth on a global scale – relating many seemingly unrelated phenomena – interpreting Earth history

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 plates that 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 to the margins of the tectonic plates

Harry Williams, Historical Geology44 PLATE TECTONICS We are concerned primarily with plate collision and orogenesis:

Harry Williams, Historical Geology45 OROGENESIS AT CONVERGENT BOUNDARIES 1. Passive Margins: prior to orogenesis, the continental boundary is a PASSIVE margin. Sedimentation at passive margins reflects the progressive increase in water depth. Nearshore deposits are coarser - sand grading to silt and clay; further out on the continental shelf in clean shallow water, carbonate reefs form in tropical regions. A common feature of continental margins undergoing extensive sedimentation is SUBSIDENCE, due to the weight of sediment ISOSTATICALLY DEPRESSING the crust; in this way shallow water deposits (e.g. 100’ depth) can build up thicknesses of 1000's of feet.

Harry Williams, Historical Geology46 mountains hills cliffsridges Andean-Type Orogenesis 1. Passive stage (pre-convergence) -> marginal deposits form.

Harry Williams, Historical Geology47 canyons valleys beaches deltas 2. early subduction -> marginal deposits are deformed by compression. Folds and thrust faults are formed. 3. Volcanic arc forms. 4. Lateral growth by accretion; emplacement of igneous masses; metamorphism; further deformation of marginal deposits.

Harry Williams, Historical Geology48 5. Continued uplift and deformation results from continuing plate convergence. Examples = Andes of western south America; Rockies of North America (older, inactive mountain belt).

Harry Williams, Historical Geology49 The Sierra Nevada and the Coast Range are good examples of inactive Andean-type orogenesis. The Sierra Nevada batholith is a remnant of a continental volcanic arc. The Coast Range consists of accretionary wedge sediments that have been deformed and uplifted by plate convergence (Note: the subduction zone between the Pacific and North American plates that formed these features has since changed into a transform fault – the San Andreas).

Harry Williams, Historical Geology50 Continental Collision-Type Orogenesis About 20 million years ago - India was separated from Asia by a progressively narrowing ocean basin - the Tethys Sea. The “collision” begun with the subduction of the oceanic plate beneath the Tethys Sea. This caused the onset of orogenesis in Tibet (uplift, folding, faulting, metamorphism, volcanism). Initial growth was similar to Cordilleran-type orogenesis; however, when the continents collide one of them can not be subducted (too thick and buoyant), therefore the plates are welded together forming a SUTURE ZONE and producing a large mountain chain, containing sedimentary, igneous and metamorphic rock a

Harry Williams, Historical Geology51 The period of mountain building is termed an OROGENY. The Himalayas are very high because they are very young (geologically). Uplift continues and erosion hasn’t had long to wear the mountains down.

Harry Williams, Historical Geology52 Suture zones usually have symmetrical patterns of rock types (folded sedimentary, metamorphic, granite) and the high rugged mountains resulting from orogenesis are subject to intense erosion, shedding sediment to both sides of the mountain range. sediment

Harry Williams, Historical Geology53 The present-day relief of mountain belts is a result of erosion: in North America, the Appalachians being much older than the Rockies are more subdued and lower and consist mainly of eroded folds. AppalachiansRockies

Harry Williams, Historical Geology54 folds

Harry Williams, Historical Geology55 Young (relatively) mountain belts like the Rockies are higher and more rugged. Fault block mountains are also common UPLIFT

Harry Williams, Historical Geology56 The Grand Tetons of Wyoming are an example of a block of crust uplifted along a fault. faults

Harry Williams, Historical Geology57 All orogenies have common features: 1. compression, buckling and uplift at the edge of the continent. 2. emplacement of igneous rock masses 3. folding, faulting and metamorphism 4. erosion of the growing mountains and deposition adjacent to the mountains (including the back-arc basin). 5. scraping off, folding and uplift of sea floor rock and sediments onto the edge of the continent - resulting in: sediment

Harry Williams, Historical Geology58 Continental Growth By Accretion Caused by the "plastering on" of material at the edge of a continental plate adjacent to a subduction zone. The material that accumulates in this fashion can be small continental masses MICROCONTINENTS or oceanic features such as volcanic arcs and seamounts submarine volcanoes). The Seychelles Bank in the Indian Ocean is an example of a microcontinent, which has apparently become detached from Africa. These larger masses of rock are scraped off the subducting plate and plastered onto the continent to form ALLOCHTHONOUS or EXOTIC TERRANES (meaning they had originated elsewhere).

Harry Williams, Historical Geology59 Example: Wrangellia consists of basaltic island arc volcanic rocks, deep marine shales and shallow marine limestones. This material originated in the area presently around New Guinea in Triassic time (225 million years ago) - travelling some 10,000 km to its present location. The collision of these smaller masses with the larger continent also causes orogenic activity, resulting in coastal mountain ranges.

Mountain Building at Convergent Boundaries 11.3 Mountain Formation  Ocean-Ocean Convergence Ocean-ocean convergence mainly produces volcanic mountains.  Ocean-Continental Convergence The types of mountains formed by ocean-continental convergence are volcanic mountains and folded mountains. An accretionary wedge is the accumulation of different sedimentary and metamorphic rocks with some scraps of ocean crust.

Ocean-Ocean Convergence

Ocean-Continental Convergence

Mountain Building at Convergent Boundaries 11.3 Mountain Formation  Continental-Continental Convergence At a convergent boundary between two plates carrying continental crust, a collision between the continental fragments will result and form folded mountains.

Continental-Continental Convergence

Mountain Building at Divergent Boundaries 11.3 Mountain Formation  The mountains that form along ocean ridges at divergent plate boundaries are fault-block type mountains.

Mountain Building by Continental Accretion

Non-Boundary Mountains 11.3 Mountain Formation  Not all mountains are formed by plate boundaries. Some are formed by hot spots or regional extension or stretching.

Continental Accretion 11.3 Mountain Formation  Terranes Terranes are any crustal fragments that have a geologic history distinct from that of the adjoining fragments.  Accretion is a process that occurs when crustal fragments collide with and stay connected to a continental plate. Terranes occur along the Pacific Coast.

Accretion in Western North America

Principles of Isostasy 11.3 Mountain Formation Isostasy is the concept that Earth’s crust is floating in gravitational balance upon the material of the mantle.  Isostatic Adjustment for Mountains Because of isostasy, deformed and thickened crust will undergo regional uplift both during mountain building and for a long period afterward. Isostatic adjustment is the process of establishing a new level of gravitational equilibrium.

Isostatic Adjustment

Isostatic Adjustment in Mountains