Geological Forensics 3
Part of a series of EarthCaches that demonstrate how Geology and Forensics are often use together to explain features we find in rocks.
Geology is the study of the Earth; its rocks; its structure; everything within it - and also the processes that shape its surface. Forensics is the use of science and technology to investigate and establish facts. Geologists, and especially palaeontologists, use the evidence found in rocks to deduce the environment that a sedimentary rock or fossil was deposited in, or the 'story' of how the crystals in a metamorphic or igneous rock formed.
As stated, there are three main types of rock:
1 – Igneous Rocks
- These form when molten rock (known as ‘magma’), cools. As a general rule, the slower it cools, the larger the crystals found within it, and vice versa.
- Common examples include:
- Granite - formed when magma cools slowly underground over millions of years, granites typically have contain large crystals, giving some quite pretty textures
- Basalt – formed by magma being extruded as lava at the Earth’s surface from a volcano. These typically cool very quickly, giving us igneous rocks which contain small crystals.
- Obsidian – formed in the same way as basalt, these cool even more rapidly, forming microscopic crystals, often giving a glassy appearance.
2 – Sedimentary Rocks
- These form when older rocks have been weathered and eroded, then transported, and then deposited, usually in layers.
- Individual layers can often show evidence of what happened at the ancient surfaces; footprints, worm trails, ripple marks created by waves lapping the shore, and many other such features (including the presence of fossils), can all help us deduce the actual environment the layers were deposited in.
- Over time, these layers are then squashed by the weight of overlying sediments, slowly turning the sediments into sedimentary rocks.
- Common examples include:
- Mudstone and sandstone - the names are determined by the size of the ‘grain’ in the rocks (mud being a smaller grain size than sand). Could have been deposited on dry land, in rivers, or in the sea.
- Limestone – generally formed in ancient warmer, shallow seas (e.g. like the present day barrier reef off NW Australia), these can be formed around beautiful coral reefs, where fish, snails, worms, and various shells all lived, inter-acting with each other as a community, living in and on the mud and sand that made up the ancient sea-floor.
- Coal – formed in ancient (up to 300 million year old!) swamps, full of massive trees, ferns and other plants.
3 – Metamorphic Rocks
- These form when older rocks are buried deep within the Earth’s crust over millions of years, squashing and heating the rocks, so deforming them and causing the original structure within the rock to crystalise or re-crystalise, depending on the original rock. Common examples include:
- Marble – formed when ancient limestones are crystalised, destroying the delicate fossils that may have been present
- Slate – formed when ancient mudstones are crystalised, squashing the mud grains together, then turning these grains into flatter minerals, all elongated in the same direction, so giving slate its characteristic property of being about to be easily split (e.g. for roof tiles)
- Schist – formed when slate is further deformed, causing the already-elongated minerals to grow larger and more prominent
- Gneiss (pronounced ‘nice’) – formed when schist is further deformed, causing the already-elongated minerals to grow even larger still. These crystals can then often become bands of colour within the rock, these themselves also being deformed, taking on the appearance of wavy lines of differing colour to the ‘main’ mass of crystals that surround it
When at the coordinates, one will be upon the 5m wide walkway along the top of the sea wall. Locally these old sea-walls are known as the ‘Hot Walls’, because in the summer they provide a sun-trap for the small pebbly beach at its base, luring locals and tourists alike for a spot of sun-bathing.
SAFETY NOTE – there is a vertical and >10m high drop to the pebbly beach below from the seawall. DO NOT under any circumstances let children/anyone/dogs, etc. up on to this wall. The land-side wall also has a considerable drop down to the roadside.
To make the pathway look authentic and blend in with the centuries-old buildings in the immediate vicinity, the walkway has been covered with stone slabs. Access to this walkway is via steps either to the north-west of or adjacent to the Square Tower from the north-western side, or from a ramped (but no steps) walkway from the south-east. Once upon the walkway, one has great views out across the Solent to the Isle of Wight, of the Spinnaker Tower, and of every single vessel that leaves or enters Portsmouth Harbour.
As usual with EarthCaches there is no physical cache. To Log this EarthCache, please study the slabs that make up this walkway, then email the answer to these questions:
- Describe the slabs – What general colour are they? Are the slabs made up of grains or crystals? What fossils can you see/identify?
- From your answer to Question 1, do you think the slabs are made of igneous, sedimentary, or metamorphic rock?
- Look at a selection of the slabs. They are full of ‘swirly’ features that appear to cut through the other textures and features within each individual slab. Describe these features. What size are they? What 3D shape would they have?
- Once you have completed Question 3, what do you think the features are? (If stuck, please see the Hint)
- From working out your answers to the above Questions, how wide and long do you think the biggest ‘culprits’ that made the ‘swirly’ features were? (and once you have worked out the answer to this, then think of what exists at the present day that we have to compare with!! Wow!)
Please do not post photos of the slabs, but please do post photos of yourself and the many views and/or vessels that you will see from this great spot.
Good luck!