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Competition & Quiz Solutions

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July 2021

We asked you to:

Take a look at these thin sections of rocks viewed using a petrological microscope under cross-polarised light.  All of these rocks can be described as being rich in minerals from which mineral group?

The answer is:

The Olivine Group

1. Olivine-dolerite with ophitic pyroxene.

2. Olivine-teschenite with sector zoned titanium augite (TI augite pink in thin section).

3. Olivine rich Dunnite adcumulate.

Adcumulates are rocks containing ~100–93% accumulated magmatic crystals in a fine-grained groundmass.

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Determine the relative sequence of events in the diagram.

Therefore if you think that unit F was the earliest event, followed by unit B you would start your answer: F, B... and so on until you list your final/latest event.

As a bonus OGG Members can win a copy of Philip Powell's 'The Geology of Oxfordshire'. Entry details will be sent in the next circular to members.


H,I,J,K,M,L (units H-L tilted) (erosion) A, B, C, D (tilting), G intrudes, (erosion), E, F, (uplift and erosion).

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Yup, simple instructions this month (the answer may be trickier). We would like you to do all, or some of the following (we will accept part-answers).

a. Describe the organism that you see in the image.

b. Suggest the geological period that you think this organism is from.

c. Identify the genus.


Paleophragmodictya is thought to be the oldest fossil sponge, from a group called hexactinellida, the 'glass sponges'.



A rock specimen has been obtained 300 km from the oceanic ridge and was dated at 9 Ma.

i. Calculate the rate of sea floor spreading in centimetres per year.

ii. Identify the location of the ridge axis on the diagram.

iii. Explain why the boundaries between normal and reversed polarity are irregular.


i. 3.3 cm/yr.

ii. axis through the central normal polarity region (white stripe on left)

iii. Spreading occurs at different rates along each segment affected by local faulting, transform faults and localised volcanic activity.

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The diagram shows a thin section drawings of two metamorphic rocks and their sedimentary parent rocks.


i. Complete the statements by entering the correct rock id letters:

Rock..... is the parent of rock.....

Rock..... is the parent of rock.....

ii.  Describe how Rock L forms.

iii.  Rock N has symmetrical ripple marks on the bedding planes.  Describe the environment in which rock N was deposited.


i. L is the parent of O and N is the parent of M.

ii. The rolling of a seed grain (eg oolith, shell fragment or sand grain) in carbonate rich mud which forms concentric layers of Calcium Carbonate. The resulting voids are later cemented in a carbonate matrix.


iii. under water affected by bi-directional currents; inter-tidal in marine or lacustrine environment affected by wave action.

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Here's a super easy one for the festive season. Can you spot the identity of this mystery fossil?


It's not difficult as you may think and if you've been on one or two OGG Field Trips the chances are that you'll have seen components of this fossil in the rocks you have looked at.  


For those of you in Oxford, if you were to pass under the Bridge of Sighs (Hertford College) into Queen's Lane, the building stone on your right hand side contains disarticulated fragments of this fossil.



This is a close up of a crinoid: Actinocrinus indianensis.  Source: Berkeley University of California



January 2022

A nice simple question to help kick start the year. 

Which of the three diagrams represents a graben?

It's not difficult as you may think. Examples of a graben are:  The Central North Sea, The Rhine Valley and the Baikal Rift Zone. 


The Rima Aridaeus on the Moon is also considered to be a Graben.


Enough of the chit chat - answer the question!!

The Answer is:




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