Voorbereidingsopdracht Matchingsdag Aardwetenschappen

Deze tekst komt uit het boek Earth System History van Steve M. Stanley. Dit boek wordt gebruikt tijdens het
eerstejaars vak: Geschiedenis van de aarde. De verschillende groepen gesteente worden besproken en de daarbij
behorende gesteentecyclus. Mocht je moeite hebben met het begrijpen van de tekst, dan raden we je aan om toch
verder te lezen. De tekst wordt dan vaak vanzelf duidelijk.

Earth as a system
Few people recognize, as they travel down a highway
or hike along a mountain trail, that the rocks they see
around them have rich and varied histories. Unless
they are geologists, they probably have not been
trained to identify a particular cliff as rock formed on a
tidal flat that once fringed a primordial sea, to read in a
hillside’s ancient rocks the history of a primitive forest
buried by a fiery volcanic eruption, or to decipher
clues in lowland rocks telling of a lofty mountain chain
that once stood where the land is now flat. Geologists Figure 1: Ripples in sediments and sedimentary rocks.
can do these things because they have at their service a Left figure shows sand along a beach, exposed to the air at low
wide variety of information gathered during the two tide. The right figure shows a large block of sandstone with
centuries when the modern science of geology has similar ripples.
existed.
Similarly, when we encounter ancient rocks that
The principle of uniformitarianism closely resemble those forming today from volcanic
Fundamental to the modern science of geology is the eruption of molten rock in Hawaii, we assume that the
principle of uniformitarianism – the understanding ancient rocks are also of volcanic origin. We cannot
that there are inviolable laws of nature that have not observe rocks twisting into contorted configurations
changed in the course of time. Of course, uniform- like those seen I mountains, but we can witness the
itarianism applies not only to geology but to all breaking, bending, and uplift of rocks during
scientific disciplines – physicists, for example, invoke earthquakes, and we can easily imagine that the
the principle of uniformitarianism when they assume immense forces that produce these effect can strongly
that the results of an experiment conducted on a given contort rocks deep within Earth and elevate them into
day will be applicable to events that take place a day, a mountains.
year, or a century later.
The principle of uniformitarianism informs geologist’s The nature and origin of Rocks
interpretations of even the most ancient rocks on Rocks consist of interlocking or bonded grains of
Earth. It is in the present, however, that many geologic matter, which are typically composed of single
processes are discovered and analyzed. The applica- minerals. A mineral is a naturally occurring inorganic
tion of these analyses to the study of ancient rocks, in solid element or compound with a particular chemical
accordance with the principles of uniformitarianism, is composition or range of compositions and a
called actualism. Actualism is commonly expressed by characteristic internal structure. Quartz, which forms
the phrase ‘The present is the key to the past’. When we most grains of sand, is probably the most familiar and
see ripples on the surface of an ancient rock composed widely recognized mineral; the materials we call
of hardened sand (sandstone), we assume that they limestone, clay and asbestos consist of other minerals.
formed in the same way that similar ripples develop Most rocks are formed of two or more minerals.
today under the influence of certain kinds of water
movement or wind (Figure 1).

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Figure 2: Interlocking grains in granite. The pink and Figure 3: Intrusive igneous rock. The dark bodies are pieces
white grains are two kinds of feldspar, the gray grains are of surrounding rock that magma incorporated before it
quartz, and the black grains are mafic minerals. solidified into igneous rock. The light-colored diagonal bands
on the left are veins that formed when a second body of
magma intruded the main body of igneous rock.
Igneous, sedimentary and metamorphic rocks
can form from one another
On the basis of modes of origin, many of which can be Even intrusive rocks that form deep within Earth can
seen operating today, early uniformitarian geologists eventually be exposed at the surface if they are up-
came to recognize three basic types of rocks: igneous, lifted by earth movements and overlying rocks are
sedimentary, and metamorphic. stripped away. Weathering is a collective term for the
Igneous rocks, which form by the cooling of molten chemical and physical processes that break down
material to the point at which it hardens, or freezes rocks of any kind at Earth’s surface (Figure 4). Water
(much as ice forms when water freezes), are composed carries some product of weathering away in solution.
of bonded grains, each consisting of a particular Solid products are removed by erosion, the process
mineral (Figure 2). The igneous rock most familiar to that loosens pieces of rock and moves them downhill.
non-geologist is granite. Molten material, or magma, After erosion sets these pieces of rock in motion,
that turns into igneous rock comes from great depths moving water, ice, or wind transports them to a site
within Earth, where temperatures are very high. This where they accumulate as sediment.
material may reach Earth’s surface through cracks and Sediment is material deposited on Earth’s surface by
fissures in the curst and then cool to form extrusive, water, ice or air. Grains of sediment accumulate in a
or volcanic, igneous rock, or it may cool and harden variety of settings, ranging from surfaces of desert
within Earth to form intrusive igneous rock(Figure 3). dunes to river channels, lake bottoms, sandy beaches,
and the floor of the deep sea. After grains have
accumulated as loose sediment, they can become
bonded together to from solid sedimentary rock by
either of two processes: the grains may become
mutually attached by compression of the sediment
after burial, or they may be glued together by
precipitation of mineral cement from watery solutions
that flow through the sediment. The processes that
turn loose sediment into solid rock are collectively
termed lithification.
The most common grains produced by lithification are
particles of sand and clay. Sand particles are grains
that weathering releases from preexisting rocks,
generally without chemical alteration. Sand grains are
Figure 4: Granite boulders accumulated from weathering globular, and they do not stick together well when
and erosion compacted.

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Figure 5: Horizontal bedding in the Grand Canyon. Figure 6: Metamorphic rock.

Loose sand therefore becomes solid sandstone only solid rock, this new rock is by definition igneous rather
when cement precipitates between adjacent grains, than metamorphic. Metamorphism produces minerals
locking them together. Tiny clay particles form by the and textures that differ from those of the original rock
chemical breakdown of certain minerals: they are and that are characteristically arrayed in parallel wavy
chemical products of weathering. Clay is a flakey layers (Figure 6). The two groups of rocks that form at
material that compacts to form the soft rock known as high temperatures – igneous and metamorphic rocks –
shale. are commonly referred to as crystalline rocks.
Other sedimentary rocks consist of fragments of Figure 7 summarizes the various possible relation-
skeletons of once-living organisms. Many limestones ships among igneous rocks, metamorphic rocks, and
are formed of such material, including bits of broken sedimentary rocks that are composed of debris from
seashells. Cementation turns accumulations of this other rocks. Any kind of rock can be metamorphosed,
limey debris into solid rock. or it can be melted to produce magma or weathered to
Sediments usually accumulate in discrete episodes, produce sediment.
each of which forms a tabular layer known as a
stratum, or bed. A breaking wave can create a
stratum, for example, and so can the spreading waters
of a flooding river. Even after lithification, a stratum
tents to remain distinct from the one above it and the
one below it because the grains of adjacent strata
usually differ in size or composition. Because of such
differences the contacting surfaces of the strata usually
adhere to each other only weakly, and sedimentary
rocks often break along these surfaces. As a result,
sedimentary rocks exposed at Earths’ surface often can
be seen to have step like configuration when viewed
form the side (Figure 5). Stratification and bedding
are the synonymous words used to describe the
arrangement of sedimentary rocks in discrete layers.

Metamorphic rocks form by the alteration, or

metamorphism, of rocks within Earth under conditions
of high temperature and pressure. By definition, Figure 7: Relationships in the rock cycle. Any of the three
basic kinds of rock – igneous, sedimentary, or metamorphic –
metamorphism alters rocks without turning them to
can be transformed into another rock of the same kind or
liquid. If the temperature becomes high enough to melt either of the other two kinds.
a rock and the molten rocks later cools to form a new

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The rock cycle relates all kinds of rocks to one The rock cycle is actually a complex of many kinds of
another cycles in which components of any body of rock –
whether igneous sedimentary, or metamorphic – can
After rocks form, they are subject to many kinds of become part of another body of rock of the same kind
change. Central to the uniformitarian view of Earth is or either of the other two kinds. In other words, as
the rock cycle – the endless pathway along which partly illustrated by figure 7, any rock may be (1)
rocks of various kinds change into rocks of other kinds. melted to form magma that later cools to form igneous
Two simple principles are useful for recognizing steps rock, (2) incorporated in magma without melting, (3)
of the rock cycle. weathered to form debris that becomes part of
1. The principle of intrusive relationships states sedimentary rock, or (4) turned into metamorphic
that intrusive igneous rock is always younger than rock by exposure to high temperatures and pressures.
the rock that it invades. Figure 9 illustrates the rock cycle with a hypothetical
2. The principle of components states that when example that includes igneous, sedimentary, and
fragments of one body of rock are found within a metamorphic rocks.
second body of rock, the second body is always Movements of Earth play a key role in the rock cycle.
younger than the first. The second body may be a When mountains rise up, for example, weathering and
body of sedimentary rock in which the fragments erosion wear them down to expose rocks that formed
have come from another body of rock, or it may be deep within the planet. Over vast stretches of time,
a body of igneous rock that contains distinctive these destructive processes level mountains, and
pieces of older rock that magma engulfed before it streams and rivers carry the resulting sediments of
cooled. faraway depositional settings.

Figure 8: The rock cycle. On the left is an igneous intrusion

formed by magma that melted sedimentary rock and
incorporated it. Some of the magma containing the melted
sedimentary rock was extruded from volcanoes. Those volcanoes
are now inactive and are eroding, along with exposures of the
intrusive igneous rock and the metamorphic rock formed during
the intrusive activity. The resulting sediment is accumulating in
water nearby. Thus sediments have become igneous and
metamorphic rocks, and those rocks have yielded younger
sediment, completing the cycle. The volcano on the right is still
active.