Wednesday, 30 November 2011

Changing sea levels

Isostatic and eustatic sea-level changes

Changes in sea level due to local subsidence or uplift are referred to as Isostatic. For example, Scotland is rising as it is rebounding now that it is free of several kilometres of ice which has weighed it down since it was deposited during the last ice age. In contrast , the south east of England is dropping by almost a millimetre each year.

Changes in sea level due to changes in the volume of water in the oceans are called eustatic. This can be caused        
by melting of the polar icecaps releasing more water into the oceans. When the ice advances and more water is held in the ice caps then sea level falls. Eustatic change is also caused by rapid rate of sea floor spreading, causing the MOR to swell with magma. This causes sea level to rise.

Measuring past sea levels

  • Using seismic evidence to find unconformities where transgressing seas have resulted in the sea laying down younger beds. When the sea retreats regressions can also be identified as unconformities. 
  • Using exposed geology to estimate the area of flooded continents through time. When sea levels falls there will be raised beaches and cliff lines. When sea levels rises there will be submerged and marine organisms can be found in younger sediment. 
  • Using oxygen isotopes ratios to assess the past temperatures and therefore the amount of ice. 

Milankovich cycles

The cycles are caused by changes in the amount of radiation reaching the earth from the sun over time. This is not because the Sun changes its output of energy, as it has remained relatively constant, but because the earths orbit around the sun varies in three predictable cycles.

  • Eccentricity: the earths orbit changes shape to become more elliptical over a period of 100,000 years. At present the eccentricity is almost at a minimum with a difference of around 6% in received radiation between January and July. At maximum eccentricity this difference increases to between 20% and 30%, which has a massive effect on climate. 
  • Obliquity: the tilt of the earths axis, which is responsible for our changing seasons, changes up to 3 degrees with a cycle of 41,000 years. A smaller tilt promotes the growth of ice sheets as warmer winters result in more moisture ans snow.  
  • Precession: eccentricity and obliquity together cause this further cycle where the inclination of the Earths axis changes in relation to where it is on the orbit. The cycle operates in periods of 19,000 and 23,000 years. At the moment we are closest to the sun so northern winters are slightly warmer than 10,000 years ago when the planet was furthest from the sun. Slow changes in the direction of the axis of the earth as it orbits results in greater seasonal contrasts.

Evidence for Milankovitch cycles

The blue Lias and Kimmeridge clay

For many years geologists studying the Blue Lias rocks to the lower Jurassic in Lyme Regis noticed the way the layers of rock changed from clay to limestone and back again. The pattern of beds created seemed so regular that it must have an explanation.

Analysis of these rocks has shown that the change in environment from a clay-rich sea to a limestone-producing sea happened on a roughly 41,000 year cycle. This correlates with the obliquity of orbit predicted by Milankovitch cycles.

Attempts to identify a 100,000- year cycle have been met with scepticism from some experts, but work carried out on clay found at Kimmeridge bay in Dorset on Upper Jurassic rocks has identified regular Milankovitch cycles.

Tuesday, 29 November 2011

Climate change over geological time

We currently live in an icehouse where large continental ice sheets exist at both poles. The onset of this icehouse started in Antarctica 34 ma and in the Arctic 2ma. At least 3 times during earths history, the planet has been in a 'deep freeze', when ice sheets extended from the poles to the tropics.


Icehouse events are characterised by lower temperatures, ice caps and glaciers. The ice sheets from the last period of glaciation are still present. The huge increase in ice coverage then increases the drop in in global temperatures by reflecting more of the suns radiation back into space.


Greenhouse events are characterised by a lack of ice coverage and an overall increase in global temperatures. They can be caused by an increase in the amount of solar radiation reaching the Earth or a change in the concentration of gases in the atmosphere.

Extinction events

The extinction of species can be influenced by climate change. Most organisms thrive in a relatively limited range of conditions, and if the conditions in an area change then the species living there will alter. If the changes happen on a global scale then whole species or groups could be wiped out. An example of this is the mass extinction at the Permian-Triassic  boundary where 96% of marine life extinct and 70% of terrestrial life became extinct. This followed a major glaciation that affected Antarctica, Africa and South America when they all joined as Gondwanaland. It was similar to the current icehouse, with continental ice sheets in the southern hemisphere and low atmospheric carbon dioxide concentrations.

Irregular echinoids


Irregular echinoids are characterized by having the anus outside of the apical system. The anus has moved to the edge of the test, or towards the posterior. This means that irregular echinoids are heart shaped and have bilateral symmetry. these adaptations are to allow the irregular echinoid to live in a burrow. Examples include the Cretaceous form, Micraster and the modern day sand dollar.

Mouth and adjacent area

The mouth is still on the underside of the test, but often it has move away from the centre. The mouth lacks jaws and the periphrastic girdle found in regular echinoids. Instead the animal takes in particles from sea water and filters these. There is a large lip called the labrum, projecting on the lower side of the mouth. The labrum is used to direct currents and prevent unwanted sediments getting into the mouth. Behind the labrum there is a modified set of interambulacral plates, forming the plastron. The plastron has small tubercles for attachment of spines. These small spines are used to help dig a burrow or for movement within it.

Petaloid ambulacra 

The ambulacra do not extend all the way down from the top to the mouth, but form a flower-shaped structure called the petaloid ambulacra. These have many small pore pairs for tube feet on top of the echinoid. The petaloid ambulacra at the anterior of the animal are larger than the others, and form the anterior groove. This is lined by cilia, which beat to generate currents to pass food particles to the mouth, and is called the fasciole. Very long tube feet extend from the anterior ambulacra, which are used to help dig the burrow and keep it stable

Mode of life

Irregular echinoids live in soft sediment and in a low-energy enviroment(infaunal). They do not have jaws and have a reduced sized mouth called the peristome because they filter feed. Instead they dig burrows, using the spines on the plastron.  

Microfossils (Conodonts and Radiolaria)


These microfossils range from 200um to 5mm are the teeth of a soft bodied creature. These are composed of calcium phosphate, apatite, occur in pairs are known as conodont elements. The earliest Conodonts are found in Precambrian rocks and died out in the Permo-Triassic event.



These are marine, planktonic animals, which range from 30um to 2mm. They are composed of silica and occupied niches near the surface to hundreds of metres depth. They are preserved at depths below the CCD and are easy to recover. They are excellent stratigraphic and palaeo-environmental tools.

Microfossils (Foraminifera)

These are mostly simple single-celled creatures with a protective shell or test. They range from size 1um to around 110mm. Early forms had tests of particles glued together for protection, while more advanced forms secrete an amazing diversity of shells. Modern formanifera capture their food using thread-like structures, which extend through holes in the test. Most forms are benthonic but a few such as Glabigerina are planktonic.

They range from early Cambrian to the present day, although the common forms were not common until the Mesozoic. This group has proven to be an excellent stratigraphic tool used extensively in the oil industry. Research has enabled detailed ranges to be determined for many different species. This also provides evidence of how evolutionary changes occur.

Microfossils (Ostracods)

What are microfossils?

There is no difference between microfossils and macrofossils, other than size. If you need a microscope to see it properly, it is a microfossil. 

Microfossils can be used to correlate rocks. They can be found in the chippings produced when drilling boreholes and these contain thousands of undamaged microfossils. Some rocks such as chalk or chert are composed almost entirely or microfossil remains. Their preservation in ocean floor sediments makes them ideal for investigating evolutionary theory.  


These are complex crustaceans, related to crabs and trilobites. They have two valves, a hinge with teeth and sockets and adducter muscles to close them, which is similar to bivalves. They are usually less than 2mm in length. The shell or carapace is made of chitin or calcium carbonate. 

Osracods range from Cambrian to the present day, although the earliest groups are now extinct. They have a long stratigraphic range and mainly a benthonic mode of life, which means they are poor zone fossils. They are superb paleo - environmental indicators, having different forms in waters of the entire range of salinity.T

Saturday, 26 November 2011

Chronostratigraphic correlation

This is matching events which may be possible over large areas. Sometimes worldwide changes in sea level result in unconformities over large area, which can then be correlated.

Using tuffs from a volcanic eruption

A tuff is the rock resulting from pyroclastic deposition. when a violent eruption occurs , ash is blasted tens of kilometers into the atmosphere and is laid down over enormous areas exactly the same age- geologically instantaneously.

The tuff that forms from the ash is the best of all rocks to use for correlation because:

  • You can carry out chemical analysis - determine exact composition 
  • Its laid down over large areas rapidly
  • Radiometric dating K-Ar
  • Can be used for both relative and absolute dating  
Using varves from glacial lakes

In the summer, glacial ice melts faster and the increased flows carry down silts into the lake to make a thin pale layer. During the winter there is no meltwater and the lake itself may freeze over. The result is very low-energy deposition of clay particles and organic matter that grows under the ice to produce a thin dark layer.  One year is recorded as a pair of layers on the lake bed. this means you count layers to find out how many larers are represented in the sequence. The correlation is provided by thicker layers resulting from hotter summers, the pattern of thick and thin bands being the same for all lakes in the same area.

Lithostraticgraphic correlation

This is based on recognising rock types, or more usually, a sequence or succession of rocks.

Methods of lithostratigraphic correlation

  1. You could correlate using a sequence of beds
  2. Looking at the thickness of beds -thickness of thin and thick bars. such a correlation might work for identifying where you are in a coal field
  3. Composition of beds may be distinctive if a rare mineral present in a bed. The Cretaceous- Tertiary boundary rocks are correlated on the presence of iridium     

Problems of lithostratigraphic correlation
  1. lateral variation - where sediment change type horizontally - deltas- sediments at source may be thicker, further away thinner 
  2. Diachronous beds - where sediment type is laid down at different times e.g delta over the years, a continuous layer of sands is left behind. these sands get younger from land to sea -  they are definitely not the same age.

Biostratigraphic correlation

If two widely separated rock units contain a sequence of identical zone fossils, then the rocks have the same relative age. Using graptolites, ammonites and some microfossils, rocks can be correlated worldwide. 

Methods of biostratigraphic correlation
  1. First or last appearance - of zone fossils, but when fossil groups first appear they can be hard to find at first point, as they maybe very rare initially. The same applies towards the end of a fossil's range 
  2. The range - can be very helpful when used with other fossils. Some fossils have a short time range these fossils are good zone fossils.
  3. A fossil assemblage - when a number of different fossils are found in one bed 

Problems of bistratigraphic correlation 
  • Many fossils are restricted to particular environments
  • Some kinds of fossil are very longed- ranged - they dont evolve quickly so are not good zone fossils
  • Derived fossils confuse the true sequence of beds
  • Not all sedimentary rocks contain fossils. Inparticular, rocks laid down in glacial, fluvial and desert environments on land are unlikely to contain fossils

Cretaceous-Tertiary mass extinction

what do we know? 

This is a large extinction event and 75% of species became extinct around 65million years ago. This marks the end of the Mesozoic era. Once again the result was gradual, showing a decline in species over several million years, leading finally to an abrupt extinction event. Marine casualties include ichtyosaurs, brachiopods, forminifera, belmnites and some bivalves. Loses on land include dinosaurs, pterosaurs and plants. The extinction of dinosaurs did leave a large ecological niche, which mammals largely filled.

Hypothesis to explain the Cretaceous-Tertiary mass extinction

Impact of an asteroid or meteorite- this remains a popular theory and there is a lot of evidence to suggest that a large object did hit the earth 65ma.

  • A layer of iridium can be found concentrated in clay's at the boundary. Most iridium comes from space
  • shocked grains of quartz are found at the boundary as a thin layer within sediments 
  • the presence of tektites 
  • large scale sedimentary evidence in Texas that there was a huge tsunami at this time 
  • a large meteorite crater can be found on the Yucatan Peninsula in Mexico, at Chicxulub. Although there are other contenders this is the most likely impact site for the meteorite. 
Major volcanic activity( Deccan traps)-  there was another enormous series of eruptions in India, covering an area of roughly 500,000km^2. The eruptions took place over about 30,000 years. The effects of this volcanism was probably the same as with the Siberian Traps.

Permian-Triassic mass extinction

what we know? 

This was a massive extinction event, the biggest in geological history, which occurred around 251million years ago, marking the end of the Palaezoic year. This event itself was not abrupt, there being a gradual decline in species over several millions of years.

  • Around 95% of all marine invertebrates became extinct  e.g trilobites,tabulate corals , rugose corals and many braciopods 

Hypotheses to explain the Permian-Triassic mass extinction 

Supercontinent formation - at the end of the Permian, Pangaea was formed when all the world continents collided together. The evidence to support this landmass comes from plate reconstructions and the stratigraphic record. This presence of one large landmass had several effects: 
  • There where fewer continental shelves - lack of habitat for shallow marine dwellers, which is backed up by the evidence of 95% of marine life becoming extinct. 
  • Presence of single continent caused rapid flunctions in climate, and unstable weather patterns 
  • A single continent reduced the imput into the oceans from rivers and estuaries. this would have significantly decreased the amount of nutrients  available for shallow marine life and may have altered the salinity of the oceans.
  • Widespread glaciations occured in the southern hemisphere . caused sea level fall (regression) 

Major volcanic activity (Siberian traps) - this is believed to be the largest volcanic eruption in Earth history. It is probably not a coincidence that  they correspond with the largest extinction event in geological history. The volcanic rocks are largely flood basalt's, thought to be form a large mantle plume intersecting the surface. Today the volcanic rocks cover an area the size of Europe about two million km^2 . The eruptions were thought to last for around one million years. The volcanic activity had several effects:

  • Emission of poisonous gasses would kill many animals and plants in close proximity.
  • The gases and ash would have initially lowered global temperatures by blocking the heat from the sun. This cooling event would have lasted for hundreds of thousands of years. 
  • The emission of greenhouse gasses such as CO2 ans SO2 could have caused an increase in global temperatures, after the cooling period had ceased. this effect is believed to have lasted for hundreds of millions of years.

Mass extinctions through geological time

A mass extinction is when there is a massive decrease in the number of different species, over a relatively short period of time, perhaps spanning several thousand or a few million years. For any one species, extinction is catastrophic. the normal process of extinctions occurs continually, generating a regular change of all the species living on earth, called background extinction. Sometimes, however,extinction rates rise suddenly for a relatively short time - as a mass extinction event.

The most famous of the mass extinctions is the Cretaceous - Tertiary mass extinction- because it is when the dinosaurs became extinct. This was the most recent large-scale mass extinction and has been well documented. Mass extinction events are not rare and some environmentalists and biologists believe that we are in the middle of another major mass extinction event, fuelled by mans effect on the environment.

5 major mass extinctions

  • The Ordovician-Silurian boundary 
  • an event towards the end of the Devonian
  • Triassic-Jurassic boundary 
  • Cretaceous-Tertiary boundary 
  • Permian- Triassic boundary

Saturday, 5 November 2011

Radiometric dating

Radiometric dating is the way we find the age of a rock that has unstable isotopes. we find the half-life to find the ratio of parent atoms to daughter atoms.

e.g's of radiometric dating

K^40 -> Ar^40   This is used in sedimentry and metamorphic rocks  halflife= 1260ma

Uranium-238 -> lead-206   This is used to find the age of igneous rocks halflife= 4500ma

Rb^87 -> Sr^87   This is used in very old igneous rocks - very long halflife = 50,000ma 

Problems of radiometric dating 

  • Sedimentry- hard to date-> they erode easily, contain different minerals, layed down different times (Diachronus) and they need the mineral glauconite. 
  • Argon is a gas so its almost impossible to find out the age in K^40 dating
  • There is a margin of error in the dating-  +- 15ma
  • Matamorphic - not a closed system. when it melts the time clock resets back to zero

Relative dating

Original Horizontality

Most sedimentry rocks are originally deposited in shallow seas. For example,clasts carried down by rivers are deposited as beds, with brakes in deposition showing up as bedding planes . These beds are commonly laid down horizontally or very close to horizontal. It is therfeore assumed that if layers of rock are tilted, then they have moved from this original position.

The Principle Of Superposition

The principle states that the rocks at the bottom of a sequence are always the oldest and younger rocks are laid down on top of older ones. for example, rocks at the bottom of the cliff are older than those at the top. This assumes the rock have not been tilted upside down.

Way-up criteria

These structures are only formed one way up and so if these are present we can tell if the rocks have been turned upside down.

  • Desication cracks
  • Graded bedding
  • Cross bedding
  • Fossils can sometimes be found in life position and so indicate the way they lived
Included Fragments

Fragments eroded from an older rock can be found within younger rock. The fragments have to be older than the rock they are found in.


  • Xenoliths found in igneous rocks have to be older, as they are fragments of country rock that fell into the magma due to stoping.
  • Derived fossils are also older than the sediment they are found within. they have been eroded from older beds and redeposited in younger beds
  • Pebbles in a conglomerate are older rocks eroded then redeposited

Cross- cutting relationships

Features which cut through rocks must be younger thanthe rocks they cut in. An example would be a dyke, which cuts through sediments. The sediments had to be there first for the dyke to be able to intrude them. Similary structures such as faults that cross-cut strata are, by definition younger than the beds they cut.

Wednesday, 2 November 2011

regular echinoids

Main Points

  • They have 5 fold symmetry
  • Contain 5 ambulacra and 5 interambulacra 
  • The ambulacra are narrow, consist of 2 rows of plates and contain pore pairs which have tube feet  
  • Interambulacra- wider, two rows of plates with tubercles 
  • Tube feet have three main points - movement, respiration and attachment
The water vascular system - hydraulic system used for animal to extend its tube feet by forcing water itnto its tissues. this is controlled by the mandreporite

The mouth 

  • The mouth is surronded my a membrane called the peristome
  • Edges of the test turned inwards, to produce a lip called the perignathic girdle
  • There are 5 jaws each with a sharp tooth supported inside the mouth, called Arisotles lantern 

Spine Attachment

the tubercles consist of two parts the boss a, a wide base and the mamalon , a nipple like structure in the centre of a boss. muscle attaches the spine to the boss, and as a muscle , it can contract and cause the spine to move in a coordinated manner. this means the echinoid can move it spine for walking.

Mode Of Life 

regular echinoids live on rocky shores, a high energy enviroment, which is reflected by their robust test and spines.