Hazardous Earth

crust- continental and oceanic                                                                                                (mantle-crust boundary is marked by the Moho discontinuity                                          continental- thickness = 35km-70km, density = 2.6-2.6 kg/m cubed, mineral comp = silicon, basalt and aluminium                                                                                                              oceanic - thickness = 5km-10km, density = 3kg/m cubed, mineral comp = silicon, basalt + Mg

Lithosphere- rigid, lies immediately above asthenosphere, varies in thickness

asthenosphere- semi Molton, thickness = 80-200km and flows slowly

Mantle- makes up 80% of the earth total vol, depth = 2600km, density of 3.3kg/m cubed at Moho and 5.6 kg/m cubed at the core, mineral comp = Mg and iron

outer core- liquid, 2260km thick, made of iron and nickel                                                              inner core- solid, 1220km thick, iron-rich

convection currents-  occur within asthenosphere, caused by vast amounts of heat generated deep in the mantle, as a result, the asthenosphere carries the lithosphere with it

Seafloor spreading

• lateral movement of new oceanic crust away from the mid-Atlantic ridge (constructive plate boundary)
• plates moved from mid-ocean ridges to subduction at ocean trenches

Paleomagnetism 

• traces of change in earth magnetic field in the alignment of magnetic minerals in the sedimentary and igneous rock
• iron particles in lava are aligned with the earth's magnetic field
• every 400,000 to 500,000 years the polarity reverses; this results in a series of magnetic stripes with sea-floor rocks aligned alternately towards north and south poles
• the stripped pattern suggests it's slowly moving away from the boundary

Age of seafloor rocks                                                                                                              

• ocean drilling recovered cores in water up to 7000m deep and the cores revealed a spatial pattern of sediments that supported seafloor spreading
• thickest and oldest sediment found closest to the continents
• cores showed that nowhere in the oceans was rock >2million years. confirming that ocean crust was continuously recycled

• During the late Paleozoic Era, massive glaciers covered huge continental areas of the southern hemisphere 
• Evidence of glaciation include layers of till (sediment deposited by glaciers) and striations in the bedrock beneath the till
• Mapping of glacial striations in the bedrock in Australia, India and South America indicates the glaciers moved from areas of present-day oceans onto land

Glossopteris Flora

• fossils of the Glossopteris flora are found in equivalent Pennsylvanian and Permian aged coal deposits on all five Gondwana continents
• the present day climates mean it is impossible to support Glossopteris type plates and so the continents must have once joined 
• pollen and spores of the plants can be dispersed over great distances by wind but the Glossopteris produce spores too heavy to be carried by wind thus giving evidence that continents must have been joined

Mesosauraus (freshwater reptile)

• fossils found in Permian aged rocks in parts of Brazil and South Africa and nowhere else in the world
• logical that Mesosauraus lived in lakes in what are now adjacent areas South America and Africa but were then united in a single continent

• The fit of shorelines of continents the appearance of same rock type and mountain ranges of the same age on continents ow widely separated 
• continental fit- reconstructions using the latest ocean basin data confirmed the close fit between continents when they're reassembled to form Pangea
• A good example of ‘continental fit’ is shown by the similarities in shape of the coastlines of South America and Africa 
• Coastlines of the Earth’s major continents appear to fit together like a jigsaw. 

• Detailed maps produced from seismic data worldwide showed that most earthquakes, and in particular high magnitude ones, were spatially concentrated in narrow bands.
• In between were relatively large areas that generated few earthquakes
• This suggested that rigid lithosphere and crust were broken up into tectonic plates
• However these slabs were not static, in some places, there were moving apart and in other areas, they were converging

• 2 plates pulling apart 
• magma rises up through the asthenosphere and forces its way to the surface, this puts the lithosphere under great stress and eventually fractures along parallel faults- underwater valleys
• the eruption of magma mostly occurs underwater, magma erupting directly onto seabed cools rapidly, forming pillow lavas
• Mostly takes place at the mid-Atlantic ridge, which is hidden 2.5km below the ocean surface. They consist of long chains of mountains rising 3000m above the seabed

• Mid-ocean ridges are not continuous; at frequent intervals, they are broken into segments by transform faults, which displace the ridge up to hundreds of km's sideways
• volcanic activity is absent  along transform fault, but as they slip energy is released in the form of an earthquake
• At mid-ocean ridges, seawater seeps into rifts and is superheated. As it rises towards the surface it causes chemical changes in basaltic rock. Black smokers sometimes reemerge 
• continental crust must thin considerably for rifting to occur

• oceanic-continental = ocean plate subducts, at an angle of 30-70 degrees, this is caused by the deepening of the ocean at the plate boundary and forms an ocean trench (long, narrow depression with depths of 6000-11,000m). Layers of sediment and sedimentary rock develop on oceanic plate and buckle as it subducts, these sediments and rock crumple, fold and uplift along the leading edge of the continental plate. In addition, the continental plate buckles and uplifts, and molten materials are ejected into it resulting in mountain chains e.g. the Andes. Faulting and fracturing occur in the Benioff Zone, where the decending plate is at an angle of 45 degrees. Oceanic plate melts
• Oceanic-Oceanic = two oceanic plates meet the slightly denser one will subduct, creating a trench. As the descending plate melts magma rises to the surface and forms chains of volcanic islands known as island arcs e.g. North American subducted beneath the Caribbean plate 
• Continental-continental = two continental plates converge, little if an subduction takes place because two plates are similar densities. e.g. collision of African and Eurasian plate created the Alpes

• Plates slide past each other in a shearing motion
• volcanic activity is absent
• frictional resistance to movement causes a build-up of pressure 
• pressure causes the rock to fracture, releasing energy as earthquakes
• E.g. North American and Pacific plate slide past each other along the San Andres Fault

• Higher Viscosity magma (acid, high silica content)
• Rhyolitic and andesite lava
• the violent bursting of gas bubbles when magma reaches the surface
• vent and top of cone often shattered
• Material erupted: Gas, Dust, Ash, Lava bombs, Tephra
• long periods with no activity
• steep-sided stratovolcano (made up of layers of ash and lava); caldera

stratovolcano

• complex internal networks of lava flows which form sills and dykes
• vents of stratovolcanoes fill with a mass of solidified magma which acts as a plug  and prevents magma rising freely
• enormous pressure builds up until eventually, it erupts

caldera

• volcanic craters- more than 2km in diameter 
• develop when eruption destroys most of the cone and the underlying magma chamber is largely emptied. The side of the volcano collapses to form a caldera

• lower viscosity magma  (basic lava- low silica content, higher temperature at eruption)
• basaltic lava
• gas bubbles expand freely; limited explosive force
• Material: gas and lava
• frequent eruption

Lava Plateaux

• basic magma erupts from multiple fissures, vast areas covered by magma (flood basalts)
• e.g. Deccan Plateaux which covers more than 500,000km squared

shield volcano

• Gentle sloping sides result from basic lava 
• if successive flows accumulate for long enough, huge volcanos extending horizontally for 10's of km

• Hawaiian chain of the island (formed as Pacific plate slowly moved NW over Hawaiian hotspot, vast amounts of basaltic have accumulated) lies at the center of the Pacific Plate, thousands of km away from a plate boundary. 
• As Pacific plate continues to move away from Hotspot, volcanoes loose source of magma and become extinct
• Hotspot- a fixed area of intense volcanic activity where magma from a rising mantle plume reaches the Earth's surface
• Running through East Africa is a 4000km long rift valley containing several active volcanoes
• over the past 30million years, the crust has been stretched, causing tension within local rocks
• resulting in rifting, with magma forcing itself to the surface and creating a line of active volcanoes
• This is how Mount Kilimanjaro was formed

• A volcano that erupts more than 1000km cubed of material in a single eruption
• Giant calderas
• Yellowstone has a caldera measuring 75km in diameter
• Toba in Indonesia erupted 75,000 years ago
• Most resent = Taupo, North Island, New Zealand 25,000 years ago

• It combines magnitude (amount of material erupted) and intensity (the speed at which the material erupts) into a single number on a scale from 0 (least) to 8 (most)
• Each increase in the number represents a 10 fold increase in explosivity
• not useful for effusive eruptions 

assessing explosivity, things to take into account 

• vol. of erupted material
• height ejected material reaches
• duration in hours
• various qualitative description

E.g. 

Montserrat, 1995 = 3 VEI

Krakatoa, 1883 = 6 VEI

Yellowstone (640,000 before present) = 8 VEI

Lava flow

• Basic Lava (basaltic)- free-flowing and can run for considerable distances (Hawaii in July 2015 a lava flow extended for 20km before stopping
• Acid lava (rhyolitic)- thick and pasty so don't flow easily 
• Burnt, buried or bulldozed infrastructure, property, and crops

Pyroclastic Flow

• Combination of hot gas (500+ degrees), ash and rock fragments traveling at high speed (100km/h) following the contours of the ground and destroying everything in their path
• Inhalation causes death
• AD 79 Mount Vesuvius, Pompeii 

Gas Emissions 

• CO2 , CO1, SO2 
• SO2 combines with atmospheric water = acid rain, this enhances weathering and damages crops

Tephra- Material ejected from a volcano

Lahars

• Mudflow (soil, ash and rock fragments)
• the consistency of wet concrete 
• Snow and ice on a volcano summit melt during an eruption and flow rapidly down the cone
• Travel at speeds up to 50 km/h
• everything in their path is destroyed or buried 
• 1984, the eruption of Nevado del Ruiz- Lahars killed 23,000 people 

Floods

• Volcanic eruptions beneath an ice field or glacier cause rapid melting 
• In Iceland- several active volcanoes lie under the Vatnajökull ice field
• During an eruption, vast quantities of water accumulated until they find an exit from under the ice

• Massive displacement of ocean water
• Tsunami waves are capable of traveling at 600km/h
• height at deep water = 1m and wavelength = 200km (long)
• Increase dramatically as they approach the shore and when they break they transfer vasts amounts of water along the shore inland 
• The tsunami created by Krakatoa (1883) drowned 36,000 people 

Focus- Origin of the earthquake

Epicenter- Point on earth surface located directly above the focus

Shallow-Focus

• 70km below the surface
• They occur in cold, brittle rocks resulting from fracturing of rock due to stress within the crust
• Release low levels of energy, although high-energy shallow quakes are capable of causing severe impacts
• Majority at convergent place boundaries 

• 70-700km below the surface
• with increasing depth pressure and temperature increases to very high levels 
• Minerals change type and volume 

Richter scale

• measures hight of the wave, it's logarithmic
• Each whole number increase represents a tenfold increase in the amplitude of the seismic wave. This represents a 30 fold increase in the release of energy

Modified Mercalli Scale

• Measures earthquake intensity and its impact
• It related ground movement to impacts that can be felt or seen
• Qualitative assessment based on observation and description

Moment Magnitude scale

• Measures energy released more accurately than Richter scale
• uses the amount of physical movement caused, which is a direct function of energy
• Amount of energy released is related to geological properties such as rock rigidity, the area of fault surface and amount of movement on the fault

Earthquakes are associated with the formation of entire mountain chains such as Himalaya-Karakorman Range in Asia

Rift valley

• Steep-sided valley formed when plates move apart
• Rift valleys along mid-ocean spreading ridges, in East Africa and elsewhere are evidence of the effects of earthquakes
• Rift valleys on the continents are altered by weathering and erosion 

Inward-facing fault scarps (small step on ground surface) and escarpments (long, steep slope) of rift valleys mark the location of faults caused by tension and compression within the crust

overtime fault scarps are worn away, blending into the landscape

Vertical and Horizontal movement of the ground

Severity depends on

- Earthquake magnitude                                                                                                                       -Distance from the epicenter                                                                                                                       -Local geology

Locations close to the epicenter of a high magnitude quake and where the surface layers are relatively unconsolidated and have a high water content will experience extreme ground shaking e.g. Parts of Mexico City in 1985

Buildings can withstand vertical movement better than horizontal movement 

Ground movements can rip apart pipelines, sewers, railway tracks and roads and cause buildings to collapse

Also disrupts natural drainage, diverting streams and rivers and affecting the movement of ground water in aquifers

When an earthquake strikes an area with surface materials of fine-grained sands, alluvium, and landfills with high water content, the vibrations can cause these materials to behave like liquids

Materials lose their strength; river banks collapse and structures tilt and sink at their foundations

E.g. Major issue during the Kobe earthquake, just under 200 berths in the port were destroyed (much was built on reclaimed land), affecting Japanese economy and wold trade

Slope failure

Steep slopes in mountainous regions like the Himalaya-Karakoram range are notoriously unstable and vulnerable to landslides

The vulnerability is increased by deforestation and heavy monsoon rain

Landslides block transport routes in mountainous regions where accessibility is already limited 

Movement of soil and rock can also block rivers creating temporary lakes, which threaten areas downstream with catastrophic floods were the dams to fail

E.g. Kashmir in 2005 

Upland valleys are favored sites for reservoirs (artificial lake). Should an earthquake create a landslide on slopes above a reservoir, the displacement of water and the wave generated could weaken and overtop the dam e.g. Italy, 1963, generated a 100m wave which drowned 3000 people

Underwater earthquakes cause the seabed to rise vertically

This displaces the water above, producing powerful waves at the surface which spread out at high velocity from the epicenter

• Low height and long length so can pass underneath a ship at sea without being noticed
• wave height increases as to enter the shore
• before it breaks water in front is pulled back out to sea (drawdown)
• Finally, the Tsunami wave rushes in as a wall of water that can exceed 25m heigh
• E.g. Tsunami off the coast of Aceh province in Sumatra in Dec 2004, delivered 1000 tonnes of water per meter of shoreline

Height is affected by the shape of the seabed and coastline

Underwater landslides can also create tsunami waves that radiate outwards when a large volume of rock is shaken and slides downslope, it drags water behind it from all sides and collides with center