Plate Tectonics Flashcards
(48 cards)
Natural Hazard
Naturally occurring event that threatens human lives and causes damage to properties
Characteristics of core layer
Approximately 1400 km thick, the inner core is in solid state due to extreme pressure exerted on it. The outer core is in liquid layer of approximately 2100 km thick. The core is composed mostly of iron and nickel. The temperature in the core is estimated to be between 3000C and 5000C
Characteristics of mantle layer
The mantle occupies 80 percent of the earth’s total volume and is approximately 2900 km thick and is made up mostly of solid rocks that flow under high temperatures and pressure. Its temperature ranges between 800C and 3000C
Characteristics of crust layer
The crust is the earth’s outermost layer and is also the Earth’s surface. It is less than 1% of the Earth’s volume. The crust is made up of the oceanic and continental crust. The crust can range in thickness from a few km to more than 70 km. Hot molten rocks known as magma is formed when the rocks in the lithosphere melt.
Oceanic and Continental Plates
Oceanic plates are made up of denser basaltic rocks and are beneath deep oceans. Its thickness is about 5 to 8 km and it is denser than continental plates.
Continental plates are made up of less dense granitic rocks and is beneath the earth’s continental land masses and under shallow seas close to continents. Its thickness is about 30 to 70 km and is less dense than oceanic plates.
Main plates
Pacific plate
North American plate
Nazca plate
South American plate
African plate
Eurasian plate
Indo-Australian plate
Antarctic plate
Convection currents
Convection currents are movements of heat within the mantle. Material in the mantle is heated by the core, causing the mantle material to expand, rise and spread out beneath the plates. This causes plates to be dragged along and to move away from each other. Then, the hot mantle material cools slightly and sinks, pulling the plates along. The sinking mantle material heats up again as it nears the core and the whole process repeats.
Slab-pull Force
Slab-pull force occurs when a denser oceanic plate is forced beneath a less dense continental plate or oceanic plate in a process called subduction. As the plate subducts, it pulls the rest of the plate along. The subducting or sinking plate drives the downward-moving portion of convection currents. The mantle material which is found away from where plate subducts drives the rising portion of convection currents.
Continental-Continental plate divergence
Convection currents cause two continental plates to move apart, forming a divergent plate boundary. The two continental plates are stretched, causing fractures to be formed at the plate boundary. As the crust continues to be pulled apart, the land in between the two continental plates sink, forming a linear depression known as a rift valley. The remaining highland next to the rift valley is known as a block mountain. An example is the East African Rift Valley, formed when the Nubian boundary of the African plate moved away from the Somalian boundary of the African plate. A number of active volcanoes and earthquake fractures can also be found along the East African Rift Valley.
Oceanic-Oceanic plate divergence
Convection currents cause two oceanic plates to move apart, forming a divergent plate boundary.
This causes fractures at the plate boundary. Magma rises from the mantle through the fractures to fill the gap between the plates and creates a ridge of new sea floor. This process is sea-floor spreading where the lave cools and solidifies after flowing out into the sea-floor. A mid-oceanic ridge is formed eventually, which is a central ridge structure that divides ocean basin in half. At various points along the mid-oceanic ridge, magma builds up and solidifies to form undersea volcanoes. When these volcanoes eventually grow above sea level, they are known as volcanic islands. An example is the Mid-Atlantic Ridge in the middle of the Atlantic Ocean, formed when the North American Plate and the Eurasian Plate moved away from each other.
Oceanic-Oceanic plate convergence
Convection currents cause two oceanic plates to move towards each other, forming a convergent plate boundary. The denser oceanic plate subducts under the less dense oceanic plate, forming a subduction zone. At the subduction zone, a depression in the sea floor called an oceanic trench is created. At the same time, the denser subducting plate causes mantle material above it to melt, forming magma. The magma rises through cracks in the crust to form undersea volcanoes. Eventually, the magma rises above the sea level to form a chain or arc of islands. Earthquakes may also occur at the boundary between the two plates due to the layer of rocks of the two plates rub against each other due to subduction. An example is the Pacific Plate converging with the slower-moving Philippine plate, where the Pacific Plate subducts beneath the Philippine Plate, forming the Mariana Trench and the Mariana Islands.
Continental-Continental plate convergence
Convection currents cause two continental plates to move towards each other, forming a convergent plate boundary. As both plates are too thick and buoyant, they collide with one another and resist subduction. This causes the plates to break and slide along fractures in the crust. Layers of rocks in the upper part of the continental crust are then compressed together and fold upwards or sideways, creating fold mountains. An example is the Himalayas, formed by the convergence of the Indian plate and Eurasian plate.
Oceanic-Continental plate convergence
Convection currents cause an oceanic plate and a continental plate to move towards each other, forming a convergent plate boundary. The denser oceanic plate subducts under the less dense continental plate. At the subduction zone, a depression in the sea floor called an oceanic trench is created and layers of rocks in the upper part of the continental crust are compressed together and folded, creating fold mountains. At the same time, the denser subducting plate causes mantle material above it to melt, forming magma. The magma rises through cracks in the crust to form active volcanoes. An example is the Sunda trench and Barisan Mountains, formed when the Australian plate subducts under a section of the Eurasian plate.
Transform plate movement
Plates slide past each other along transform plate boundaries. This movement results in the formation of a transform fault. In the process, tremendous stress builds up and is eventually released, often in the form of violent earthquakes. An example is the San Andreas Fault between the Pacific Plate and the North American Plate.
Comparison of fold mountains and block mountains
Both are formed at continental-continental plate boundaries. Block mountains are formed at divergent plate boundaries while fold mountains are formed at convergent plate boundaries. Fold mountains tend to be taller than block mountains. Fold mountains are formed by folding while block mountains are formed by faulting.
Fold Mountains
Over million of years, layers of rocks when compressed and folded form fold mountains. Fold mountains can grow to impressive heights and lengths such as the Himalayas. Fold mountains are formed along convergent plate boundaries where plates move towards each other and collide. The resultant compressional force creates immense pressure which causes the layers of rocks to buckle and fold. This process is known as folding. The upfold is called the anticline and the downfold is known as the syncline. When there is increasing compressional force on one limb of a fold, the rocks may buckle until a fracture forms. The limb may then move forward to ride over the other limb.
Rift Valley and Block Mountains
River valley and block mountains are formed at divergent plate boundaries where plates are pulled apart, giving rise to faults. A fault is a fracture in the rocks along which the rocks are displaced. The tensional forces from these movements result in parts of the crust being fractured. This process is called faulting. Along these faults, sections of the crust can extend. A rift valley is a valley with steep sides that is formed when sections of the crust extend along fault lines. The central block off land subsides between two parallel faults due to tensional forces. The sections which are left standing beside the rift valley are block mountains.
Volcanoes
A volcano is a landform formed by magma ejected from the mantle onto the earth’s surface. Magma is molten rock found below the earth’s surface and builds up within the crust to form a magma chamber, which is a reservoir of molten rock beneath the earth’s crust. Volcanoes can be found at divergent and convergent plate boundaries where there is subduction. At these subduction zones, magma rises and accumulates in the magma chamber, where pressure builds up until the magma forces its way onto the earth’s surface through vents. Vents are openings in the earth’s surface with a pipe leading into the magma chamber. Magma that is ejected onto the surface is known as lava, which builds up around the vent to form a volcano.
Shield Volcanoes
Shield volcanoes have gently sloping sides and a broad summit. These volcanoes are formed when low-silica lava has been ejected. Low-silica lava flows easily and spreads out over a large area before solidifying. Since the lave does not trap much gas, eruptions are usually not explosive. With successive eruptions, the base of the volcano increases in size as lava accumulates. Shield volcanoes are common near divergent plate boundaries where magma can rise directly from the mantle. An example is Mount Washington in the United States of America.
Stratovolcanoes
Stratovolcanoes develop from successive eruptions of lava and ash. Ash deposits can also contain coarser rock fragments ejected from the vent. After an initial eruption, the subsequent eruption ejects lava which covers the soft ash and prevents it from being eroded away. Over time, successive eruptions build a high volcano with a slightly concave profile, which means the volcano is steeper at the top and gentler at the base. Secondary cones may develop as magma from the vent seeps into the sides of the cone and erupts. Pyroclastic flow and lahars may result from volcanic eruptions. Pyroclasts refer to hot rock fragments and super heated gases ejected during a volcanic eruption. The movement of such rock fragments and gases down the slopes of an erupting volcano is known as pyroclastic flow. Lahars refer to a mixture of pyroclasts and melted ice from the mountains, which are wet volcanic debris flowing down the slopes of an erupting volcano. The eruptions of Mount Pinatubo, a stratovolcano in the Philippines in 1991, emitted 10 million tonnes of lava, 20 million tonnes of sulfur dioxide and millions of tonnes of ash. It was the second largest volcanic eruption that occurred on land in the 20th century.
Massive destruction by volcanic materials
When an volcano erupts, lava and pyroclasts are released that consists of ash, rock fragments and volcanic bombs. These volcanic materials can cause widespread damage of property. Lava of high temperatures between 500C and 1400c can burn down the areas it flows through. Low-silica lava flows easily and quickly over long distances and can destroy larger areas. A pyroclastic flow contains hot rock fragments ranging from ash to boulders. It moves at speeds of more than 80km/h and damages everything in its path. When people breathe in hot ashes and gases, they can suffer serious injury or even die. The large volcanic bombs of heated rocks can destroy property in the area where they fall. The ongoing eruption of Kilauea in Hawaii has damaged many houses and infrastructure since 1983. This causes loss of lives, homelessness and economy impact as people and government need to pay for repairs.
Landslides
Landslides can occur due to the structural collapse of a volcanic cone during a volcanic eruption. Large scale landslides can block the flow of river and result in flooding, road blockage and bury villages and farmlands. In 1995, a pyroclastic flow from the eruption of Nevado del Ruiz in the Andes mountains of South America triggered lahars which buried the town of Armero and killed more than 20 000 people. This causes loss of lives, homelessness and economy impact as people and government need to pay for repairs.
Pollution
A volcanic eruption can release ash particles which can disrupt human activities over large distances from the volcano. This is because fine ash particles of less than 0.001 mm can spread easily over thousands of km by winds to affect areas further away from the eruption source. When thick plumes of ash settle on the ground, they can block sunlight, suffocate crops and trigger severe respiratory problems in humans and animals. Volcanic eruptions can release various gases such as CO2, SO2, H2 and CO which are harmful to people. In 2010, the eruption of Eyjafjallajökull in Iceland emitted large amounts of volcanic ash clouds containing tiny particles of abrasive glass, sand and rock. These tiny particles can damage aircraft engines and structures. This caused the extensive closure of air space in many European countries. The airline industry suffered economic losses of about US$1.8 billion due to worldwide connecting flight cancellation.
Fertile soil
When lava and ash from volcanic eruptions break down, they form fertile volcanic soils and can support agriculture. Volcanic rocks contain minerals. However, these minerals in the newly formed rocks are not available to plants. After they are weathered and broken down for a very long period of time, the minerals will then be available to plants. many people have been living near the volcanic areas in Java and Bali because the fertile volcanic soils support the cultivation of crops such as tea, coffee and rice. The soils remain fertile than in most non-volcanic areas even after many rounds of cultivation.
