InformationSheet No

Landscape The rocks of the Burren

How rocks are formed

5 The Earth is a dynamic planet. Its surface, and its interior, are constantly changing. One of the effects of this is the formation of new rock.

Rocks are formed in three ways:1. Through the laying down of sediment

particles and the remains of plants and animals (PRODUCES SEDIMENTARY ROCKS, e.g. limestone, mudstone, sandstone)

2. Through the melting of existing rocks deep underground and from volcanic activity (PRODUCES IGNEOUS ROCKS, e.g. granite, basalt)

3. Through the distortion of existing rocks deep underground (PRODUCES METAMORPHIC ROCKS, e.g. slate)

How the rocks of North Clare were formedAll of the rocks in North Clare are SEDIMENTARY ROCKS. They underwent four major stages to produce the rock formations we see on the surface today.

Step 1. Sediment particles such as silt, clay and parts of plants and animals were deposited on the sea floor. These sediments would have been quite soft and pliable.

Step 2. The sediments were buried as more sediment was laid down on top. The sediments were compacted, the particles were pushed closer together and water was squeezed out. The particles became cemented together as salts formed in between them. At this point, the sediments had been turned into rock.

Step 3. The rocks of North Clare were gradually uplifted (pushed upwards) by tectonic forces.

Step 4. The rocks of North Clare were exposed at the Earth’s surface and were eroded and weathered by wind, rain and temperature changes. This is why there are no siltstones, sandstones or shales in the north part of the Burren. They have been eroded away exposing the older, underlying limestones.

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B 3

Sedimentary Rock

Metamorphic Rock

Igneous Rock

Crystallisationof Magma

Weathering ofRocks at Surface

Depositionof Sediment

Burial &Compaction

Deformation &Metamorphism

Melting

Uplift

Erosion &SedimentTransport

Metamorphic Rock

050100150200250300350400

Bur

ial

million years ago

Laid down on the

sea floor

Uplifted by tectonic forces

Exposed at the Earth’s

surface

InformationSheet No

Landscape The rocks of the Burren

6How the limestones of North Clare were formedDuring the Carboniferous Period, in Earth’s history (between 359 and 299 Ma*), the land that would become Ireland was located 10ºS of the equator (Ref: Module A1 Information Sheet No. 4).

The Carboniferous limestones of the Burren (and Ireland), were formed in a warm, shallow, tropical sea. This sea was similar to the Bahamas or Persian Gulf today.

The sea was full of marine life such as corals and shellfish.

When these marine animals and plants completed their natural life cycle and died, their remains floated down to the sea floor. Their soft body parts rotted away over time, leaving the hard skeletons.

At the same time particles of calcium carbonate, deposited out of the sea water, were also building up in layers on the sea floor. The hard skeletons of the marine plants and animals became embedded in the layers of calcium carbonate. Over a long time these layers compacted to form limestone and the skeletons of the organisms became fossilised within those rocks.

Around 326 Ma, the sea deepened very rapidly. The formation of the limestone ended abruptly, as it can only be formed in shallow seas, where the light reaches the sea floor.

Only about 500m of the Burren limestone is visible on the surface. We have to go down another 300m before we reach the base. It took approximately 20 million years for all this limestone to form.

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* Mega Annum, or millions of years

Limestones are grey coloured

rocks made up mostly of calcium

carbonate (lime). They can be

dissolved slowly by rainwater.

InformationSheet No

Landscape The rocks of the Burren

How the shales, sandstones and siltstones of North Clare were formed

7

Direction of delta movement from ancient continental land mass during late Carboniferous

Between 326 and 318 million years ago, the sea that covered North Clare was deep, dark and quiet; no animals lived there as there was very little oxygen. There was an ancient continent 100–200 km to the south-west (off today’s coast).

As time passed, small amounts of very fine clay particles, originating from rivers located on that far away continent, were carried into the deep sea basin by coastal currents. These clay

particles settled on the seabed on top of the limestones. They eventually became compacted into black shale rocks (known to geologists as the Clare Shales).

About 318 million years ago large volumes of silt, fine sand and clay were quickly washed in on top of the shales from large rivers that flowed from the continent to the south west. These sediments formed a massive river delta, similar to the Mississippi today.

This delta covered an area of several hundred square kilometres, and stretched all the way from north Clare (at Doolin) to Limerick and Kerry.

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B 3

A river delta is formed at the mouth

of a river where it enters the sea. The

water carried in by the river breaks

up into little streams, in a triangular

shape, giving the appearance of an

uprooted tree. The streams carry clay,

gravel and sand out with them and

as the current of water in the streams

slows down, these sediments are

dropped building the delta outwards.

Ireland 330 million years ago

Area covered by the ancient delta, 316million years ago

The Clare Shales are fine-grained, black coloured rocks that can easily be broken into distinct layers. They lie on top of the limestone in the south Burren and can be seen at Poulacapple, Slieve Elva and the Fisherstreet cliffs at Doolin.