ROCKS AND THE ORIGIN OF THE EARTH

No matter where we travel on the face of the land we will see rocks. Solid rock ramparts form our seacoasts for hundreds of miles. Over a vast area of northern Canada rocky ledges and hills alternate with gravel or sand-filled valleys. Even in places where soils cover the surface it only takes a few feet of digging to get down to the solid rock. The soils themselves are often full of boulders and pieces of rock which have come from solid ledges nearby. To see why this is, it is necessary that we review a little of the history of the earth from its earliest beginnings.

It seems that a little more than five billion years ago the earth was a blob of hot gases, which was part of a series of such masses in space. As heat was gradually lost the gases condensed to liquid and the heavier parts of the liquid settled towards the centre of the mass. Still further cooling of the earth's mass led to solidification of the outer part to form a solid crust. Finally the outside part of the earth cooled enough for rain to condense from the atmosphere and fall on the newly formed surface. Soon rivers began to form and the great ocean basins to fill with water. From that time to this the face of the land has been undergoing slow but constant changes.

Earth's Interior

We can walk over and examine the outside of the earth to see what it is like but our knowledge of the interior is not so easily acquired. From the highest mountain to the greatest depth of the sea is only about 14 miles. This not much of a scratch on the ball of the earth which is some 8,000 miles in diameter. The deepest mines are only about two miles deep and drills have penetrated only about four miles below the surface. Erosion has stripped off the outer cover in some parts of the world so that we can see rocks which at one time were perhaps as much as ten miles below the surface. All of this means that our knowledge of the interior of the earth has to come by indirect means.

Most of the information so far obtained comes from earthquake waves which penetrate right through the fabric of the earth. From a study of these and how they are affected at different depths we have come to know that the earth is made up of a number of concentric shells. A great mass of heavy material some 4,000 miles in diameter forms the central core of the earth. We cannot tell for sure whether this is actually solid or whether it is a liquid which behaves like a solid because it is under such enormous pressure due to the weight of all the outer part of the earth. This innermost core is covered by another shell of an intermediate type of rock material some 1,800 miles thick. The outermost shell or crust of the earth is about 25 miles thick, and seems to consist of two types of rock material, the one lighter by weight and lighter by colour than the other. The continents seem to be made up largely of the lighter material with a layer of the darker and heavier underneath. The great ocean basins on the other hand seem to have little or none of the lighter rock material on their floors.

Now, we have seen something of the rocky framework of the earth, and how at first the surface of the earth was made of nothing but rocks which had crystallized from the molten mass from which the earth began. We have seen, too, how erosion of these began with the first rains.

White igneous rock has invaded dark metamorphosed gray gneisses in this roadcut along the Trans-Canada Highway between Glacier and Mount Revelstoke National Parks. In some places the gneisses have been shredded by the invading rock which tends in a general way to follow the well-marked banding in the gneiss. © D. M. Baird, 1963

Igneous Rocks

At many places in the world, volcanoes are spewing out masses of melted or molten rock which spread over the country as lava flows. As they cool they gradually solidify to make solid rock. Rocks which were thus at one time molten are called igneous rocks. People can actually see the molten material cool and solidify in the case of lava flows, but other rocks, which show evidence of having once been molten, have apparently solidified at great depths below the surface of the earth and are only visible to us after long erosion has stripped off the surface rocks and uncovered them. Lava flows are called extrusive igneous rocks because they are squeezed out or extruded onto the surface of the earth where they cool. The other types are called intrusive rocks because they cool and solidify deep below the surface where they cut into or intrude other rocks. Perhaps the biggest differences in appearance between the two types result from the different ways in which they cool.

Extrusive igneous melts cool very quickly when they reach the surface and are exposed to the air and cool surface rocks. As the temperature drops rapidly many crystals are formed but few of them have a chance to grow to any size before the rock has solidified. Thus the average grain size in lava flows is about the same as that of granulated sugar and the rocks are said to be fine grained. Extremely rapid cooling sometimes produces a rock with no crystals at all. It looks like black or dark brown glass and is called obsidian. When some igneous melts, which are charged with gases in solution, come to the surface, the gases bubble out of solution and the rock melt puffs up into a spongy mass. If this should solidify in the puffed up state, a rock froth or pumice is formed. After violent explosions of volcanoes in some parts of the world the surrounding oceans have been found to be covered with floating pieces of pumice formed in this way.

Ordinary lava flows have a wide range of chemical and mineralogical composition because the original melts they come from are of various compositions. Out of a great variety of names we may find it useful to remember rhyolite for light-coloured, pink or red, lava flows and basalt for dark lavas.

Masses of molten material which cool at a depth below the surface of the earth must cool more slowly than those which cool at the surface because of the insulating effect of the rocks overhead. Slow cooling under undisturbed conditions produces a coarser grain size. Fudge-makers know that slow cooling makes coarse, sugary fudge, whereas faster cooling with constant beating makes far finer grain and smoother texture. When talking of igneous rocks the word "texture" refers to the grain size and arrangement of grains in the rock. Thus intrusive igneous rocks have a coarse grain size or texture when crystals are the size of peas or larger.

Coarse-grained, intrusive rocks sometimes occur in enormous masses. One of these masses forms the entire Coast Range of British Columbia, a belt of mountains more than a thousand miles long and a couple of hundred miles wide. Such enormous masses are usually made of a light pink or grey rock called granite, which is composed of the three minerals, quartz, feldspar and mica. Coarse-grained rocks are sometimes very dark grey or black and are then usually called gabbro. If we were to compare chemical analyses of a granite with a rhyolite or a basalt with a gabbro, we would see that they are really the same compositions in each case, so that the differences between them are only differences in grain size and, thus, differences in their manner of cooling.

A peculiar combination of two histories may result in a combination of characteristics. Some of the most unusual rocks result from masses of molten material which begin to cool slowly at depth with the formation of some large crystals. If, at this stage of partial cooling and solidification, the mixture of some large crystals swimming in a molten mass of melted rock should be caught up and spewed out of a volcano, the remaining molten part may cool very rapidly. Thus is produced a rock which consists of a few large crystals scattered through a fine-grained background. This is called a porphyry and is usually named from the large crystals so that we might have quartz porphyry or feldspar porphyry and so on.

One last note about igneous rocks—when volcanoes erupt violently great clouds of dust or ash, fragments of broken rock and bits of liquid rock, which cool and solidify while in flight through the air, are blown over the country. This debris may collect in a blanket tens of feet thick or be mixed with muds and silts in rivers or the bottom of the sea. Dust and ash from one gigantic eruption in 1883 at Krakatoa, between Sumatra and Java, was blown into the upper reaches of the air where currents sifted it all over the earth causing brilliant sunsets around the world for many years.

Many of our finest building stones are igneous rocks. Public buildings and commercial establishments often have polished igneous rocks as panels, pillars or facings along the sidewalk or between windows. Some of the best places to study igneous rocks are in cemeteries where polished and rough-hewn slabs are brought from all over the world.

Weathering and Erosion

A nail left on the ground very soon rusts away in the air and rain. A sandstone monument soon becomes weatherbeaten and begins to crumble. Buildings built of brick soon lose their newness and have to be repaired. Rocks which are exposed to the weather similarly begin to go to pieces almost immediately. The processes of breakdown and decay of rocks on the surface of the earth are collectively called weathering.

When rain falls on the surface of a rock a little may penetrate into pores and tiny cracks in the rocks. If the water freezes, its expansion may force apart the individual grains in the rock and loosen them. Some minerals which alter chemically on the surface may swell and force apart adjacent mineral grains. Plants force their roots into tiny fractures and split rocks. These processes are called disintegration because the rock goes to pieces by physical breakdown.

The Grand Cycle of Erosion © Parks Canada, 1963

Chemical decay of minerals, mentioned above as contributing to the physical breakdown of rocks, is going on all around us. Certain kinds of rock are soluble in rain water and, as the years pass, the surface of the rock is gradually dissolved away. Water, which passes down cracks and fissures, may dissolve the rock and make the openings larger thus allowing even more water to pass through. Rotting wood and vegetable debris produce complicated chemical compounds which are added to the water and assist in the breakdown of the rocks which it passes through. Oxygen, carbon dioxide and water vapour in the air react with some rocks to produce new mineral substances. These chemical changes in rocks at the surface are together called decomposition. Thus weathering of rocks is the sum total of all processes of physical and chemical changes in rocks at the surface.

Tall remnants left by erosion in loosely consolidated, clayey gravels overlook islands of sand and gravel in the Bow River, Banff National Park. The mountains in the background are slowly wasting away to make these river sediments which are now in transit to the sea. © National Film Board of Canada, 1963

Erosion on the other hand is the transportation of materials which come from weathering of rocks. Rivers carry muds and silts to the sea. Glaciers push and scrape piles of rock waste down their valleys. Wind carries fine dust and sand from some areas and spreads them over others. Waves along the shoreline cut into cliffs and carry the resulting sands and gravels along the shore or out into deep water. In later chapters we will examine each of these agents of erosion in detail to understand what kind of scenery they produce, but at this point, we wish only to see enough of them to understand that the sedimentary materials which they carry are eventually dumped into the sea, or into lakes or remote corners of the desert. Here they may become solidified into sedimentary rocks.

Sedimentary Rocks

Sedimentary rocks are rocks which at one time were sediments like sands, gravels, muds and silts. The original materials, which came from the physical and chemical breakdown of rocks, were transported by rivers, glaciers, wind, or waves and eventually were deposited in deep, quiet water offshore in the ocean, in river deltas, or in certain parts of deserts. After these sediments were buried by other sediments, sometimes to depths of hundreds or even thousands of feet, they hardened or were cemented together to form solid rocks. It is by these processes that sands have become sandstone, gravels have become conglomerate, and muds have become shale.

In certain, very special parts of the sea, marine muds are made mostly of calcium carbonate which has been precipitated from sea water by organisms or by chemical means. In some places millions of tiny creatures, which have skeletons of calcium carbonate, accumulate on the sea bottom to form very fine-grained muds or oozes. When these deposits of calcium carbonate become solidified they form limestone.

The remains of living things are sometimes entombed in sediments as they accumulate on the bottom of the sea. If the remains, or their imprints, survive the changes which make rocks out of soft muds and sands they may be preserved indefinitely and are called fossils. Here we see imprints of trilobites, creatures which have been entirely extinct for 200 million years, in shales from a famous fossil locality in Yoho National Park. The two large trilobites catch the eye at first but you may see several smaller ones of different species between them and in the upper left corner. © Geological Survey of Canada, 1963

Fossils

As some of these sedimentary materials were accumulating, the remains of living things, plants and animals of the day, were buried in them. As the enclosing sediments gradually turned to solid rocks, the plant and animal remains were preserved. The remains of these ancient creatures in the rocks are called fossils. In some rocks, fossils are so abundant that the main part of the rock is made up of shells and bones. In others no fossils are found at all. This is just what you would expect because some parts of the sea bottom are even now covered with pieces of shells and bones and others are clean sands and muds.

Preservation of ancient living things is sometimes very nearly perfect. Thus in Western Canada we find dinosaurs, that roamed the country which is now Alberta, in such perfect preservation that the contents of their stomachs tell us what they ate for their last meal about 100 million years ago. Fossil trees and leaves tell us accurately of the forests of 250 million years ago in the Cape Breton area in Nova Scotia. In shales and limestones, in Yoho National Park, imprints of animals, which have long been extinct, tell us what lived on the bottom of the sea 480 million years ago when the landscape of North America was very different from that of today.

Almost all fossils are found in sedimentary rocks for this is about the only place where they could be preserved.

Metamorphic Rocks

Almost everyone is familiar with the manufacture of bricks. Wet clay or special muds are pressed into moulds and made into brick shapes. These are then allowed to dry a little and placed in kilns or furnaces where they are heated until they are red hot. When they cool, a strong rock-like material makes up the brick. The same kind of thing sometimes happens in nature, Where lava flows or intrusive igneous masses come into contact with muds or shales the latter are baked and altered into entirely new kinds of rocks. New kinds of rocks which result from the alteration or changing of other rocks are called metamorphic rocks.

Ancient rocks on Beausoleil Island in Georgian Bay Islands National Park have been profoundly altered by elevated temperature, and enormous pressures to form the metamorphic rocks shown here. The light diagonal streak is a vein of quarts which invaded the older rocks as a fluid mass at very high temperature and then froze. © D. M. Baird, 1963

Now, to change rocks the conditions do not have to be as severe as they are in a brick kiln. Small changes may take place if rocks are heated only a little. If they are subjected to high pressures, such as would happen if a sedimentary rock were to be buried deeply by other sedimentary materials, then certain changes may take place. Thus the sedimentary rock, shale, becomes slate under high pressure and elevated temperature. Limestone becomes marble because of a coarsening of grain under high pressure and temperature. Sandstone may be recrystallized to form quartzite which is a strong rock made of interlocking grains of quartz instead of simply cemented sand grains.

Under extreme conditions the rocks which result from the change of sedimentary or even igneous rocks bear little or no resemblance to the original. New minerals appear by reaction of the old minerals with one another and a new appearance altogether may result from the development of flat mineral grains, or needle-like minerals. In some cases the minerals tend to clump together in bands and new families of rocks, the schists (finely banded) and gneisses (more coarsely banded, say 1/4 inch or more thick) appear.

One would expect that metamorphic rocks would be more commonly found in some places than in others. One would expect that new rocks would be formed by heat and the addition of hot gases and liquids where large masses of igneous rocks butt up against other rocks. Great pressures and sometimes heat are generated during the building of mountains so metamorphic rocks would be expected in mountain belts.

Many millions of years ago a mass of limestone boulders became embedded in a matrix of silt on the bottom of the sea. Thin layers of mud and limy mud were laid down on top of this when conditions became more calm and stable. Now we see these ancient sediments as solid rocks. The boulder-filled mass is now conglomerate and the mud and limy mud have become shale and impure limestone. Note how very even the bedding or layering is in the upper zone. © D. M. Baird, 1963