Types of Rocks · 2 Types of Rocks Rocks are not all the same! The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have

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Types of Rocks Rocks are not all the same! The three main types, or classes, of rock are sedimentary, metamorphic,

and igneous and the differences among them have to do with how they are formed.

IGNEOUS ROCK Igneous rocks are formed when magma (molten rock deep within the earth) cools and

hardens. Sometimes the magma cools inside the earth (INTRUSIVE) and other times it

erupts onto the surface from volcanoes (in this case, it is called lava) (EXTRUSIVE).

When lava cools very quickly, no crystals form and the rock looks shiny and glasslike.

Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny

holes and spaces in the rock.

Examples of this rock type include basalt, obsidian and pumice

SEDIMENTARY ROCK

Sedimentary rocks are formed from particles of sand, shells, pebbles, and other

fragments of material. Together, all these particles are called sediment. Gradually, the

sediment accumulates in layers and over a long period of time hardens into rock.

Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You

can often see sand, pebbles, or stones in the rock, and it is usually the only type that

contains fossils.

Examples of this rock type include conglomerate and limestone.

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Metamorphic Rock

Metamorphic rocks are formed under the surface of the earth from the

metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The

rocks that result from these processes often have ribbonlike layers and may have

shiny crystals, formed by minerals growing slowly over time, on their surface.

Examples of this rock type include gneiss, marble and granite.

Maltese Rock Layers

The Maltese Islands started to form around 30 to 25 million years ago when sediment

and fossils were deposited on the bottom of the sea. The different rock layers

resulted from different environments and geological times in which the sediments and

fossils were deposited. Around 10 million years ago, these rock years rose to the

surface due to African plate and the Eurasian plate moving towards each other

causing the sediment in between to uplift. Therefore, all of the Maltese rock types

are sedimentary.

The image of the Maltese Islands,on the next page is a

geological map. Each colour shows a particular type of rock that

makes up the islands. The Maltese Islands are made up of 5

different rock types. These are namely; Upper Coralline

Limestone, Greensand, Blue Clay, Globigerina Limestone and

Lower Coralline Limestone.

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Blue Clay

This image shows the locations on Malta

where this type of rock may be found in

Maltese it is called l-Qawwi ta' fuq. It is

found on top of hills and plateaus in

Malta and Gozo. It is quite a hard rock

having a white and/or grey colour. It is

the youngest rock found on the Maltese

Islands meaning that it was the last to

form. It is also porous, which means that

it lets water pass through it.

Upper Coralline Limestone

This image highlights the locations on

Malta where this type of rock may be

found. In Maltese we find this rock named

as il-Gebla is-Safra. It is a very thin layer

of rock usually having an orange colour.

It is highly porous and it easily crumbles

into sand.

Greensand is known to contain a lot of marine

fossils such as that depicted in this photo.

Greensand

This image shows the locations on Malta where

this type of rock may be found. Blue Clay is

known as it-Tafli in Maltese. As its name implies

it has a bluish shade. It is also a soft rock that

erodes quickly. It is porous but it is not

permeable.

Blue Clay was commonly used for pottery making.

However, this tradition has diminished.

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Globigerina Limestone

Lower Coralline Limestone

The Maltese rock sequence is relatively simple being divided into five main layers. See picture below.

This image shows the locations on Malta

where this type of rock may be found.

In Maltese it is found as Franka. It is

softer than Coralline Limestone but it is

harder than Blue Clay and Greensand.

It is a porous rock usually having a

white/yellow colour. It is used mainly

for buildings and making sculptures.

This image shows the locations on Malta

where this type of rock may be found.

Lower Coralline Limestone is called

il-Qawwi t'isfel or Zonqor in Maltese.

It is the oldest rock type of the Maltese

Islands. This means that it was the first

to form. It is a hard rock, porous and has a

white/grey colour. The Lower Coralline

Limestone is mainly used for road

surfacing and in concrete mixtures.

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