Science Block 5 Quiz

Active volcanism (volcanoes) occurs in assocaition with

All of these (trenches, ridges, and hot spots).

Fossils of Glossopteris, an extinct plant, have bound in Antartica, India, and South Africa. The distribution of these fossils suggest that these areas were once

Connected to one another

Transform Faults

Conservative margins are also known as transform faults. Transform faults are mainly found on the ocean floor, where they offset mid ocean ridges and enable to ocean to spread at different rates. It was through the work of John Tuzo Wilson that these faults were recognised as the connection between the ocean ridges (divergent margins) and ocean trenches (convergent margins). At conservative margins, plates slide past each other, so that the relative movement is horizontal, and classified as either sinistral (to the left) or dextral (to the right). Lithosphere is neither created nor subducted, and whilst conservative plate margins do not result in volcanic activity, they are the sites of extensive shallow focus earthquakes, occasionally of considerable magnitude. Case study:

----------- is the process by which hot material from deep within the Earth rises while cooler material near the surface sinks, creating a circular pattern.

Convection

Scientists think that the main driving force of plate tectonics is

Convection in the asthenosphere

Thin, rigid layer that forms the outermost part of the lithosphere is called Earth's

Crust

How did the discovery of magnetic reversals helped to develop the theory of plate tectonics

It provided more evidence that plates move away from mid-ocean ridges.

Divergent Boundary

Divergent Boundaries Places where plates are coming apart are called divergent boundaries. As shown in the drawing above, when Earth's brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere). The sinking of the block forms a central valley called a rift. Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface. Where a divergent boundary crosses the land, the rift valleys which form are typically 30 to 50 kilometers wide. Examples include the East Africa rift in Kenya and Ethiopia, and the Rio Grande rift in New Mexico. Where a divergent boundary crosses the ocean floor, the rift valley is much narrower, only a kilometer or less across, and it runs along the top of a midoceanic ridge. Oceanic ridges rise a kilometer or so above the ocean floor and form a global network tens of thousands of miles long. Examples include the Mid-Atlantic ridge and the East Pacific Rise.

Which of the following is also known as the spreading center?

Divergent boundary

Earth's Layers center to surface

Inner Core, Outer Core, Mantle, Crust

A continental-continental collision causes

Mountain ranges to form

Which of the following is not a major tectonic plate?

Nazca

The ancient landmass that once contained all the landmasses is

Pangea

Convergent Boundary

Places where plates crash or crunch together are called convergent boundaries. Plates only move a few centimeters each year, so collisions are very slow and last millions of years. Even though plate collisions take a long time, lots of interesting things happen. For example, in the drawing above, an oceanic plate has crashed into a continental plate. Looking at this drawing of two plates colliding is like looking at a single frame in a slow-motion movie of two cars crashing into each other. Just as the front ends of cars fold and bend in a collision, so do the "front ends" of colliding plates. The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend. All that folding and bending makes rock in both plates break and slip, causing earthquakes. As the edge of the oceanic plate digs into Earth's hot interior, some of the rock in it melts. The melted rock rises up through the continental plate, causing more earthquakes on its way up, and forming volcanic eruptions where it finally reaches the surface. An example of this type of collision is found on the west coast of South America where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean.

Transform Boundary

Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions. Perhaps the most famous transform boundary in the world is the San Andreas fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!

Plate Tectonics

Plate tectonics is the theory that Earth's outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earth's mantle. This strong outer layer is called the lithosphere. Developed from the 1950s through the 1970s, plate tectonics is the modern version of continental drift, a theory first proposed by scientist Alfred Wegener in 1912. Wegener didn't have an explanation for how continents could move around the planet, but researchers do now. Plate tectonics is the unifying theory of geology, said Nicholas van der Elst, a seismologist at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York. The driving force behind plate tectonics is convection in the mantle. Hot material near the Earth's core rises, and colder mantle rock sinks. "It's kind of like a pot boiling on a stove," Van der Elst said. The convection drive plates tectonics through a combination of pushing and spreading apart at mid-ocean ridges and pulling and sinking downward at subduction zones, researchers think. Scientists continue to study and debate the mechanisms that move the plates. Mid-ocean ridges are gaps between tectonic plates that mantle the Earth like seams on a baseball. Hot magma wells up at the ridges, forming new ocean crust and shoving the plates apart. At subduction zones, two tectonic plates meet and one slides beneath the other back into the mantle, the layer underneath the crust. The cold, sinking plate pulls the crust behind it downward.

At transform boundaries, two plates

Scarpe past each other

Earth's magnetic reversals are recorded in

Sea-floor rock

Most earthquakes occur along or near the edges of the Earth's

Tectonic Plates

At divergent plate boundaries

Tectonic plates move apart allowing magma to move to the surface

Continental/Continental Divergent

The Himalayan mountain range and Tibetan plateau have formed as a result of the collision between the Indian Plate and Eurasian Plate which began 50 million years ago and continues today. India migration Northward migration of India 225 million years ago (Ma) India was a large island situated off the Australian coast and separated from Asia by the Tethys Ocean. The supercontinent Pangea began to break up 200 Ma and India started a northward drift towards Asia. 80 Ma India was 6,400 km south of the Asian continent but moving towards it at a rate of between 9 and 16 cm per year. At this time Tethys Ocean floor would have been subducting northwards beneath Asia and the plate margin would have been a Convergent oceanic-continental one just like the Andes today. Between 40 and 20 Ma the rate of northward drift slowed as the two continental plates collided and the former Tethys Ocean closed. Neither continental plate could be subducted due to their low density/buoyancy. This caused the continental crust to thicken due to folding and faulting by compressional forces. The continental crust here is twice the average thickness at around 75 km. The thickening of the continental crust marked the end of volcanic activity in the region as any magma moving upwards would solidify before it could reach the surface. The Himalayas are still rising by more than 1 cm per year as India continues to move northwards into Asia, which explains the occurrence of shallow focus earthquakes in the region today. However the forces of weathering and erosion are lowering the Himalayas at about the same rate. The Himalayas and Tibetan plateau trend east-west and extend for 2,900 km, reaching the maximum elevation of 8,848 metres (Mount Everest).

Continental-Oceanic Convergent

The Nazca Plate is moving eastwards, towards the South American Plate, at about 79mm per year. Where the two plates meet, the denser oceanic lithosphere of the Nazca Plate is forced down and under the more buoyant continental lithosphere of the South American Plate, descending at an angle into the mantle in a process called subduction. This is marked on the ocean surface by the presence of the Peru-Chile (or Atacama) Trench. The friction between the plates prevents the subducting oceanic plate from sliding smoothly. As it descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeperas the ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone. As subduction of the Nazca Plate continues, some of the sediments that accumulated on the ocean floor (together with some of the ocean crust) are scraped off and forced (accreted) onto the South American Plate in a process called "obduction". This forms an accretionary wedge (or 'prism'), where layers of the deformed and metamorphosed sediments and ocean crust are thrust onto the South American Plate along faults - or thrust planes - adding to the size of the continent.

Oceanic-Oceanic Convergent

The South American Plate is moving westwards due to sea floor spreading at the Mid Atlantic Ridge. Where it meets the Caribbean Plate, it descends (subducts) beneath it. This is because the oceanic lithosphere of the South American Plate is cooler and denser than that of the Caribbean Plate. The subduction causes low density ocean floor sediment to be scraped off the surface of the South American Plate and thrust onto the Caribbean Islands as accretionary wedges, in a process called obduction. The line of subduction is marked by the deep sea Puerto Rico Trench. As the South American Plate descends, it drags against the overlying plate, causing both to fracture and deform. This results in frequent shallow focus earthquakes that get deeper as the ocean plate descends further, defining a zone of earthquake foci known as a Benioff zone. Continued subduction of the South American Plate brings sea water, locked in the ocean crust, deep into the mantle. As the plate heats up the water is liberated, lowering the melting point of the mantle and causing partial melting. This produces magma, which rises and may be erupted explosively as andesite at the surface. Andesitic magma is less dense than the surrounding material, and can have a temperature of 1000oC. It is viscous, trapping gases as it rises. The water and gases in andesitic magma account for the explosive activity of andesitic volcanoes, which typically lie dormant for many hundreds or thousands of years. These volcanoes typically produce ash and pyroclastic flows, as well as small amounts of andesitic lava. The eruptions on Montserrat during the 1990s are a good example of this type of activity.

How do the earth's crust and mantle differ?

The crust is much thinner and less dense than the mantle

Crust is neither destroyed nor formed along

Transform boundaries

Many scientists rejected continental drift when it was first proposed because

Wegener could not explain how Earth's continents moved

Subduction occurs

When an oceanic plate and another plate converge

Mid-plate tectonic activity

Whilst most tectonic activity is focused on plate margins, it is possible for earthquakes and volcanoes to occur far from the edges of tectonic plates. The causes of intraplate earthquakes are not fully understood. Some are associated with pre existing weaknesses in plates which can become reactivated. The series of earthquakes which took place in New Madrid, Missouri, in 1811-12 and the 2001 Gujarat, NW India earthquake are examples of intraplate earthquakes which caused significant destruction. Intraplate volcanoes are thought to be associated with 'hot spots' in the mantle, which remain stationary as plates move over them. The mechanism by which such hot spots produce volcanic activity is a subject of much debate amongst Earth scientists. Some have suggested they are caused by mantle plumes - cylindrical bodies of material hotter than the surrounding mantle. The plumes are thought to originate at the outer core/lower mantle boundary, rising upwards to the base of the lithosphere over many millions of years.

Oceanic Crust at a mid-ocean ridge is

Younger than crust near a trench

A tectonic plate is a slab of Earth's

crust and outermost mantle

Early Studies of the ocean floor helped to develop the theory of plate tectonics becasue they showed that

the age of oceanic curst increases with distance from a mid-ocean ridge