Download Geology of the, Maltese Islands

Geology of the Maltese Islands
There are 3 kinds of rocks –
Igneous rocks – rocks formed from lava/magma. They are composed of crystals and other minerals e.g. granite
Sedimentary rocks (gebel bijogeniku)– rocks formed from sediments and bodies of dead organisms which
settle on each other when dead (Limestone, Sandstone).
Geomorphic rocks – rocks baked by the heat of magma which may intrude into the rock layers (e.g. marble)
The rocks of the Maltese islands are all of the sedimentary type. They were almost all formed under the sea,
except for a small, localised layer formed during periods of heavy rainfall in the past. The rocks forming the
Maltese islands started forming around 25 million years ago, at the end of the Oligocene. The Maltese islands
surfaced above the sea around 10 million years ago.
There are 5 different layers of rock in the Maltese islands.
The Lower Coralline Limestone (Qawwi ta’ That, Zonqor). Where it is exposed it rises up to a height of
140m, for example at Ta’Cenc, Gozo, and Dingli, Malta. However this rock continues down even under the sea
bottom to a depth of over 600 meters. Even deeper, there are other rock layers such as the Dolomitic
Limestones. This rock formed in a shallow, warm sea. It is composed of Coralline Algae (algi korallini). These
had their outer skeletons made of calcium and when they died their skeletons piled up on each other forming
great banks. Between the Lower Coralline and the Globigerina there is a small layer of fossilised seashells,
known mainly as the Scutella Layer, up to 1 meter thick.
The Globigerina Limestone (Franka) – This layer is divided into three sub-layers. The globigerina Limestone
is made up the calcareous shells of very small organisms (Globigerina Foraminifera) which float in the sea.
When they died their skeletons fell to the bottom of the sea where they piled up on each other. This later on was
compressed by the weight of the overlaying layers of mud and become stone. The sea was deep at the time –
between 40 meters and 150 meters. The Globigerina Limestone is made up of three layers and so can be divided
into the Lower Globigerina Limestone, the Middle Globigerina Limestone and the Upper Globigerina
Limestone. Each layer is divided by a layer of phosphatic pebbles.
The Blue Clay (Tafli) – This rock is found mainly in Gozo, the Northwest and the West of Malta. There is no
other occurrence of it in the rest of the Maltese islands. It is around 65meters at its thickest. The Blue Clay is
made up of the same Foraminifera, when they died they accumulated at the bottom of the sea. With it there was
the arrival of clay from Sicily, brought by sea currents, which made up the clay content of the rock. This rock is
the only Impermeable layer of the Maltese islands. All other layers are permeable or pervious.
The Greensand (Gebla is-Safra, Rina) – This is a very thin layer found on the Blue Clay. It is not found in all
places the Blue Clay is found. It is thickest in Gozo, precisely at Gelmus (11meters thick). The sea bottom in
this period had risen and was relatively shallow and sandy. There were strong currents and there were lots of
animals, whose fossils are well represented in this layer. In fact it is one of the layers with the most fossils
inside. The Greensand has a green tinge when exposed to the air and water, but a mineral rich in iron corrodes
to take the rock as a reddish/orange colour.
The Upper Coralline Limestone (Qawwi ta’ Fuq) – The sea bottom become shallower still and very similar
to that at the time of the Lower Coralline. Conditions, and the flora and fauna present, were very similar to the
above mentioned period. This rock is almost as strong as the Lower Coralline Limestone.
Around 10 million years ago the Maltese islands rose out of the water. When this happened the land was
exposed to weathering and erosion, which helped to fashion the geomorphology of the Maltese islands as it is
presently. The weathering and erosion served to produce a number of localised deposits, such as are found at
Cirkewwa, Marfa, St. Thomas Bay, Benghisa and Dwejra, Gozo. During the Ice Ages, the Maltese Islands
underwent a period of intense rainfall, when rivers flowed in the different valleys (widien) that we have. These
rivers carried material with them, which they later deposited. These are known as Quaternary Deposits
(Depositi Kwaternarji). These contain large numbers of fossils which indicate what kind of flora and fauna was
living at the time.
Figure 1: The Geology of the Maltese Islands
Each rock produces different soils.
Table 1: Rocks and their respective soils
Rock
Soil Produced
Upper Coralline Limestone
Terra Rossa (hamrija Hamra)
Greensand
/
Blue Clay
Carbonate Raw Soil ( hamrija Taflija)
Globigerina Limestone
Xerorendzina (hamrija Bajda)
Lower Coralline Limestone
Terra Rossa (hamrija Hamra)
The Rocks also have different uses
Table 2: Uses of Maltese Rocks and their extraction
Rock
Uses of the rock
Extraction
Upper Coralline Limestone
Spall (zrar) for roads
Quarry (barriera) e.g. Ta Zuta
Greensand
/
/
Blue Clay
Pottery
Collection from cliffs
Globigerina Limestone
Building Stone
Quarry (barriera) Imqabba
Lower Coralline Limestone
Spall (zrar) for roads
Quarry (barriera) Dwejra, Ghawdex