Igneous Intruders EarthCache

This Earthcache looks at two Igneous rock types, both Dykes, a giant Gabbro and some Dolerite swarm dykes

It is tidal, and involves a walk of about 1km along the shoreline. Part sandy and stretches with shingles, you'll also need to climb over some small rock outcrops en route. It is not very strenuous & sensible shoes should be worn to avoid slipping and getting injured.

An Introduction

Rocks can be found in all sorts of shapes, sizes, colours, toughness, textures, mineral content and age, they are all classified into 3 categories depending on how they were formed.

• Igneous. (Latin, Ignis= fire)

• Metamorphic. (Greek meta = change, morphe = form)

• Sedimentary.

Here in an area where the 'host' rock is Sedimentary and known as the Mullaghmore Sandstone formation with its associated mudstone dating from the Viséan stage of the lower Carboniferous period, 330 to 346 million years ago we find a couple of examples of igneous rock together at the coordinates.

They are both Dykes, one is a great big giant GABBRO Dyke, about 400m wide and about 5km long. These rocks along with the swarm dykes are probably the youngest rocks in the county and date from the late Cretaceous or early Tertiary period and are thought to have formed during rifting as the Earth’s crust began to split, separating Europe & North America and eventually leading to the formation of the Atlantic which is still ongoing at a rate of about 20mm per year along the mid-Atlantic ridge.

Gabbro is generally described as being coarse grained, with this particular example having really big crystals because it formed at depth in the earths crust and cooled there very very slowly.

At roughly the same time and for similar reasons intense volcanic activity was taking place in the north east of the island causing masses of lava to come to the surface and leading to the formation of the Giants Causeway and to 'flood 'the region with Basalt, not unlike what is happening in Iceland from time to time at the present.

A Dyke is a vertical or almost vertical sheet of rock that formed in a crack or fault in a pre-existing rock body. However, if this occurs horizontally along a weak bedding plane between two layers for example in Sedimentary rock then it is called a Sill.

It has been speculated that the reason this Gabbro dyke got so big may be due to the length-ways splitting along an earlier feeder dyke and the intrusion of magma into this breach.

The other or others are part of a DOLERITE swarm of dykes dated to about 58Ma±, they are often several kilometres long in a roughly east west direction and can also be traced across the bay at Enniscrone where they cut through the Limestone there and here at the Earthcache site through the earlier Gabbro intrusion.

These swarm dykes later 'intruded' into the main Gabbro dyke where they cooled much faster and we know this because if you examine one you will see their grains are much much smaller.

These two types of INTRUSIVE or PLUTONIC Igneous rock are closely related because they both have very similar mineral compositions, the main difference between them is the size of the ‘grain’ in the rock, by grain I mean the individual crystals, the crystals are formed as a result of cooling rates, as mentioned the bigger the crystals the slower the cooling rate. Plutonic describes igneous rock which has solidified at depth over a long period of time and is named after the Greek god of the underworld.

In their composition they are pretty much the equivalent of BASALT, which has a very fine grain due to a much faster cooling rate because it occurs during a volcanic eruption on the surface and is said to be EXTRUSIVE.

Now the Gabbro and Dolerite can also be described as PHANERITIC (crystals visible to the naked eye). The reason they are on the surface today is because of erosion

All of the igneous rocks mentioned are MAFIC rocks - Ma(magnesium)fic(Ferric-iron). That means they are rich in PYROXENE, a dark mineral. The pale crystals are PLAGIOCLASE, Feldspar a very common rock forming mineral, less common minerals include Olivine(olive green), Amphibole(dark), and Biotite(dark).

Chemically Mafic rocks are on the other side of the rock spectrum to FELSIC rocks, an example of which can be examined in the form of a Granitic glacial Erratic which was left at a virtual stage waypoint of the same name at the end of a previous ice age and came from the Ox mountains to your south and east across the bay.

• You will need a measuring tape or ruler
• Also you must be aware of the TIDES. It is probably better to visit this Earthcache at low tide because access is much simpler both for getting there and for examining the Dykes. It is better NOT to do it an hour and a half either side of the high tide and definitely NOT when the tide is very high or during a storm

Information for your answers can be found here at 3 sets of coordinates or reference points and the other answers online or wherever.

• Q1 Reference Point. You should be standing on a narrow Dolerite dyke which splits in two like a zipper as it meanders through the main Gabbro dyke, you will also see a number of core sample holes drilled by Geologists in it and elsewhere close by.
• Q2 Reference Point. You should be standing on another narrow Dolerite dyke which can be followed from the shoreline all the way up through the Gabbro, now with your back to the ocean you can see from the bottom up as it rises that it is not very straight and is inclined at an angle.
• Q3 Reference Point. Here you should find yourself well above the waterline where the Gabbro is very weathered and the crystals are especially big and the surface is crumbling

Please send the answers using the message center and not by email because I may miss them there.

1. What diameter are all the core sample holes here and they can also be seen elsewhere in the area?
2. Is this angle obtuse (tilting right) or acute (tilting left) in the 2m rock face in front of you and measure the dolorite dyke width here and decide which of the following the most accurate 0.15m, 0.55m, 0.87m or 1.20m?
3. Here examine the grains or crystals, do they have reflective surfaces? Yes or No, also there are three colours to be seen, black, pale grey and what is the third colour?
4. Gabbro was named by the German geologist Christian Leopold von Buch after a town in which country?

Needless to say you may log the find but I will delete any logs that I have received no answers for or are inaccurate. photos that don't give clues to the answers are most welcome but not obligatory

I'd like to thank www.gsi.ie for allowing me to use some site specific information from some of their 'Bedrock Geology Map Series' Booklets

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