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Rocks in the Field

Pictured below are some common rock types and geological features as you would see them in the landscape or in outcrop. The images are classified according to rock type (sedimentary, igneous, metamorphic). Each thumbnail has a short caption and links (by clicking on the thumbnail itself) to a medium-resolution image. Use the links below to jump down the page to the section you want:

The images on this page are all copyright D J and F G Waters, but may be used without charge for bona fide educational purposes.


Sedimentary Rocks

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Sandstone
The cliffs of Table Mountain, Cape Town, South Africa, are made of several hundred metres of clean white sandstones, mostly laid down as sand-bars near the shore of shallow seas about 450 million years ago.

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Sandstone
The "Maltese Cross", a well-known landmark in the Cedarberg Mountains, Western Cape Province, South Africa. These hard sandstones have been sculpted into tall pillars and other fantastic shapes by weathering along the vertical cracks in the rock.

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Sandstone (arkose)
Precambrian red-brown sandstones, laid down by river systems 1000 million years ago, make up the bulk of the mountain Quinag, Assynt district, Sutherland, Scotland. (An arkose is a sandstone that contains a lot of feldspar in addition to quartz grains.) These steep mountains were shaped by the movement of ice sheets and glaciers over the past million years.

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Sandstone, folded
Some mountain ranges are made of folded sedimentary rocks. This mountain peak, near Swellendam in the Western Cape Province, South Africa, is made up of crumpled layers of hard sandstone. The field of view is about 300 metres high.

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Sandstone
In this outcrop you see a sequence of sandstone layers. Each bed of sandstone, 15 cm to 1 m thick, was probably laid down in a fairly short space of time. Separating the sandstone beds are thin, soft layers of silt and shale. The beds are not all of even thickness; in the upper part of the picture they vary in thickness and often cut down into the bed below. This is typical of sand deposited in river channels. Precambrian, NW Scotland.

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Sandstone, with ripple marks
This view looks down onto the top of a layer of sandstone. The sandstone was laid down in water, possibly in a shallow lake. Water currents or wave action made ripples in the sediment surface, which were preserved when the next layer of sediment was laid down on top. Precambrian, NW Scotland.

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Siltstone, with filled cracks (desiccation cracks)
This layer of fine-grained sediment was deposited in water, but must have dried out soon afterwards, forming a pattern of cracks. The cracks then filled up with lighter-coloured sand, and finally the whole layer was covered by more sediment and hardened into rock. Precambrian, NW Scotland.

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Mudstone
The "Finger of God", also known as Mukorob, a rock pillar in southern Namibia, photographed in 1979. A 34 metre column of sandstone rests on softer mudstone, which has eroded away to a narrow neck. The pillar eventually blew down in a storm on 4 December 1988.

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Sandstone, coarse, with channels
These purplish-brown gritty sandstones were laid down by rivers 1000 million years ago. In close-up view you can see that many of the sedimentary layers are curved, rather than flat, and show where small channels formed, filled up, and moved across the surface as the river deposited its load of sand. This structure is called cross bedding. Precambrian, NW Scotland.

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Conglomerate
This coarse-grained conglomerate is made up of pebbles and small boulders of quartzite eroded from nearby mountains. You can see some layering in the outcrop, but the coarser material fills irregular channels that would have been cut by fast-moving flood water. Cretaceous, Western Cape Province, South Africa.

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Breccia
The blue-grey rock is a fine sandstone that has been broken into angular pieces in a zone of faulting, and then re-cemented in a rock that would be called a breccia. The rock fragments are not rounded enough to call the rock a conglomerate, and moreover they were probably never moved around on the surface as a sediment. Pembrokeshire, Wales.

Limestones

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Limestone (dolomite or dolostone)
This is the typical scenery of the Italian Dolomites, after which the mineral dolomite (calcium magnesium carbonate) is named. Here, very great thicknesses of dolomitic limestone laid down in the Triassic period now form steep-sided mountains on which plants cannot grow.

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Limestone
These tall limestone cliffs are part of the sacred Mount Parnassus in Greece. Below are the ruins of the Temple of Apollo at Delphi, built of the same material.

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Limestone
This relatively soft white limestone was formed when wind-blown dunes made of shell sand were hardened into rock, a few million years ago, on the southern coast of South Africa. Notice how wave action has cut a flat platform at sea level, made an undercut notch, and hollowed out an archway through the rock. Towards the top right you can see the bedding - the layering of sand deposited on the slope of a dune.

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Limestone
Malham Cove, Yorkshire. Limestone scenery in Britain. At the end of the Ice Age a river, produced by melting the ice sheet that covered the Pennines, tumbled over this 80-metre high cliff of Carboniferous Limestone.

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Limestone (caves and underground water)
Malham Cove, Yorkshire. Today there is no waterfall here, and the stream appears from a horizontal crack at the foot of the cliff, having flowed underground in a cave system for almost a mile.

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Limestone weathering
Malham Cove, Yorkshire. Above the cliffs of the Cove there is a large area of limestone pavement. This gets its characteristic appearance, of limestone blocks separated by deep fissures, from the chemical weathering of the limestone by rainwater. Rainwater is acid enough to dissolve limestone slowly, and the dissolution is concentrated along the regular pattern of cracks in the rock. Notice the ferns and other plants growing in the shelter of the cracks.

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Limestone weathering
Malham Cove, Yorkshire. This detailed view of the limestone pavement shows how the blocks of limestone in between the deep straight fissures have been shaped by rainwater, which has slowly dissolved curving, fluted channels as it drains off towards the deeper cracks.


Igneous Rocks

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Volcanoes
A view of the cones and craters of extinct volcanoes near the Puy de Dome, Auvergne, central France.

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Basalt lava flow
The surface of a basalt lava flow, near Kilauea on the Big Island of Hawaii. This is the type of lava texture known as ropy lava or pahoehoe, formed when the molten lava is very fluid or runny.

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Basalt lava flow
The surface of a basalt lava flow, near Kilauea on the Big Island of Hawaii. Here, the crust formed on the top of the flow has cracked and collapsed as molten lava flowed out from underneath.

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Santorini volcano
This view, from a viewpoint above the town of Fira, looks out across the caldera, the huge collapsed crater, of the Santorini volcano, which was formed after the last major eruption in about 1645 BC. In the centre of the deep lagoon is the new volcanic island of Nea Kameni, built up over the past 500 years by new lava.

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Lavas and tuffs, Santorini
This view of the inside wall of the caldera (collapse crater) on the island of Thira, shows the layers of lava and tuff that make up the cone of the old volcano, blown apart by the eruption of 1645 BC.

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Granite boulders
Granite weathers slowly, by chemical weathering. Bodies of granite commonly have only a few, widely-spaced fractures, so that a typical style of weathering produces smoothly-curved rounded shapes. These boulders of granite are about 4 metres in diameter and sit on smooth bare rock surfaces at Spitzkoppe, Namibia. In the distance are the smooth peaks of the Erongo Mountains, also made of granite.

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Granite scenery
This shows a typical landscape formed out of granite in a dry climate. Smooth rounded slopes are scattered with large boulders. The granite weathers to a warm orange-brown colour. Brandberg, Namibia.

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Granite (detail)
Granites are coarse-grained rocks, as a rule, having cooled slowly at some depth. Commonly they consist of two different sizes of crystals - larger feldspars set in finer-grained quartz and feldspar. Here, brick-shaped feldspars are lined up by flow of the partly-crystallised granite magma as it was intruded. Lands End Granite, Cape Cornwall, England.

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Basic dyke, cutting granite
The dark rock is an intrusion of basic magma intruded up a rather irregular crack through paler granite. Lion's Head, Cape Town, South Africa.

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Inclusion of metamorphic rock in lava (xenolith)
It is quite common to find lumps of other rocks trapped inside bodies of igneous rock. These are called xenoliths (literally "foreign rocks"). They may be blocks of the surrounding rock near the surface, but in other cases they may have been brought up from the great depths at which the magma was formed. This xenolith is a partly-melted metamorphic rock from the source region of the lava in which it occurs. El Joyazo, SE Spain.

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Gabbro
The Black Cuillin mountains of Skye are made of dark gabbro, and form much more rugged topography than their rounded granite neighbours, the Red Cuillins. This view looks north-west into the heart of the Cuillins, towards Loch Coruisk.

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Gabbro (detail)
Gabbro, a basic rock, is made of roughly equal amounts of dark pyroxene and lighter feldspar. In this outcrop, part of the gabbro has developed a much coarser grain size and the two minerals can be easily seen and distinguished. Manacle Point, Lizard peninsula, Cornwall.

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Gabbro (detail)
This example of gabbro contains light-coloured feldspar and two dark minerals, pyroxene and olivine. Weathering has given the darlk minerals, particularly the olivine, a dark reddish colour. Coverack, Lizard peninsula, Cornwall.

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Peridotite (detail)
This is the type of rock that makes up much of the Earth's mantle. The darker crystals that stand out on the surface are pyroxene, but most of the rock is made of the mineral olivine, weathering to a light brown colour on the rock surface. Here, near Ronda in southern Spain, mantle peridotite has been brought to the Earth's surface.

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Chromite (chromium ore)
Chromite is the most important ore of the metal chromium. It occurs in some basic igneous rocks, and is sometimes concentrated into layers during the crystallisation of large magma bodies. The black bands in this photograph are layers of chromite over half a metre thick, in the Bushveld intrusion, South Africa.

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Kimberlite pipe (source of diamonds)
A view into the "Big Hole" at Kimberley, South Africa
. Diamonds are formed in the Earth's mantle, and are brought up to the surface in the volcanic rock called kimberlite. Kimberlites form pipe-shaped intrusions, and diamond miners dug vertically downwards, following the pipe downwards into the Earth. Mining has now continued underground, and the abandoned hole has partly filled with water.

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Vein
Hot fluids that pass through fractures in rock deep in the crust can crystallise out minerals that fill the fracture, and form a mineral vein. This vein contains feldspar and quartz and looks like a coarse-grained granite. It cuts a metamorphosed conglomerate. Veins like these may contain minerals of economic importance.


Metamorphic Rocks

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Metamorphic rocks in mountain belts
The central core of many mountain belts is made of metamorphic rocks. Here in the Austrian Alps, the mountains in this view are built of granites, gneisses and schists formed in the Carboniferous and Permian periods, then metamorphosed again in the Tertiary period when the Alps were formed, and finally uplifted to the surface over the past 20 to 30 million years.

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Metamorphic rocks in mountain belts
Rocks metamorphosed at the deepest levels of the Earth's crust are brought up to the surface when mountain ranges are uplifted and eroded. The banded rocks in the cliffs beyond the mountain hut were originally lavas and sediments formed in the sea. They were metamorphosed at a depth of 70 km beneath the Alps, and now form Alpine peaks 3 km above sea level.

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Metamorphic rocks of different compositions
Metamorphic rocks can be formed from any type of older material. Here, the lighter brown rocks in the upper part of the cliff are calcareous schists formed from impure limestones, and the dark rocks below are mainly metamorphosed basalts. As Sifah, Oman.

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Schist
Huge thicknesses of sandstone, siltstone and mudstone, deposited in ocean basins, are folded, squeezed and metamorphosed into schists when continents collide. These steeply-dipping schists are now exposed in the Kuiseb Canyon, Namib Desert Park, Namibia.

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Schist
The outlines of tightly-folded strata can be seen in this outcrop of mica schist, from the Eastern Alps, Austria.

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Gneiss
Ancient gneisses, of Precambrian age, underlie the interior of many continents. They can look very complicated: this example is a mixture of white and pink granitic-looking material and streaky grey gneiss. Lewisian Gneiss, from Loch Laxford, NW Scotland.

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Gneiss, with tight folds
Layers of different composition show up boldly in these gneisses. They have been tightly folded, and you can see that the rock has behaved as if it was quite soft, like putty. In fact rock, even hot rock, is very stiff, and it probably would have taken many thousands of years to deform the rock into these shapes. Lower Orange River region, South Africa.

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Gneiss
This photograph shows the streaky banded texture typical of many gneisses. The light streaks are feldspar and quartz, the dark bands are mica and hornblende. Namaqualand, South Africa.

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Marble
Pure marble is often white, but the presence of other mineral impurities gives coloured varieties. These marbles have bands and veins coloured yellow-green and grey-black by serpentine, mica and iron oxide. In this quarry the marble was being cut into large slabs for ornamental use. Ledmore, Assynt, NW Scotland.

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Gneisses cut by granite
Bands of black basic gneiss and grey acid gneiss were formed from igneous rocks more than 2000 million years ago. In a later event, they have been cut across by intrusions of pink granite. This is one of the roadside stops on the signposted "Rock Route", near Loch Laxford, NW Scotland.

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Metamorphic minerals (detail)
Metamorphism causes new minerals to grow in a rock that was originally formed as a sediment. Here you can see rounded red garnet and a bladed grey mineral (kyanite) set in dark mica. Namaqualand, South Africa.

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Metamorphic minerals: garnet
This is a metamorphosed basic igneous rock in which you can see large red garnet crystals, outlined by a rim of pale feldspar, set in dark pyroxene. Scourie, Sutherland, NW Scotland.

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Metamorphic minerals: hornblende
A metamorphosed volcanic rock. The layering (perhaps originally layers of volcanic ash of different composition) has been folded, and the dark bands are now made of blocky black crystals of hornblende, set in pale feldspar. Lower Orange River region, South Africa.

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Migmatite: partly melted rock
Migmatites are mixed rocks, part igneous, part metamorphic. They form when rocks are heated to the point where they begin to melt. The molten material partly separates out, but crystallises before it can move right away. In this outcrop, the lighter parts are granite formed from crystallised melt, the darker rocks are unmelted parts of the original metamorphosed sediment. Cape Town, South Africa.


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D.J. Waters, Department of Earth Sciences, June 2004