Download Steno`s Laws or Relative Dating Principles

Steno’s Laws or Relative Dating Principles
The following 3 Laws or Principles help to figure out the relative ages of fossils and sedimentary
layers (relative = which fossil is older or younger than another). Sometimes these 3 laws are
called Steno’s Laws.
Law of Superposition = in an undeformed series of rock layers, the younger rocks lie on top of
the older rocks. Think of the layers of paint on a wall – the oldest layer was put on first, and is at
the bottom, while the newest (and youngest) layer is at the top.
It was recognized in the 1600’s that in a sedimentary sequence, the older beds are on the bottom,
and the younger beds are on the top. This has come to be called the Principle of Superposition.
You can visualize how this occurs if you imagine a stack of newspapers in the corner of a room.
Every day you put another newspaper on the pile. After several weeks have passed, you have a
considerable stack of newspapers, and the oldest ones will be on the bottom of the pile and the
most recent ones will be on the top. This fairly obvious, but very important fact about layering
was first noted by Nicholaus Steno, and is the first of three principles which have come to be
known as Steno’s Laws.
Law of Original Horizontality – sedimentary rocks are laid down in a horizontal manner. So, if
you see a tilted layer of sedimentary rocks, you know that it was originally horizontal, and was
tilted after it was laid down.
Steno’s second law is the Principle of Original Horizontality, which states that sediments are
deposited in flat, horizontal layers. We can recognize this easily if we consider a sedimentary
environment such as the sea floor or the bottom of a lake. Any storm or flood bringing sediment
to these environments will deposit it in a flat layer on the bottom because of the sedimentary
particles settling under the influence of gravity. As a result, a flat, horizontal layer of sediment
will be deposited.
Law of Uniformitarianism – the geologic processes in the present day are the same processes of
the past. So, if you see what looks like water ripple marks in a sandstone, and you know that
water forms ripple marks like that today, then you can know that water formed ripple marks in
the same way in the past. This law helps to figure out the environment in which the fossil
organism lived.
Steno’s third law is the Principle of Uniformitarianism. Uniformitarianism is the assumption
that the physical laws of nature as we now know them have always been in operation. Through
this assumption, scientists are able to study patterns and rates of change in the earth through
geologic time. The earth’s history is exceedingly long and complicated. By keeping
uniformitarianism in mind, we can make better sense of the past.
Earth’s atmosphere and surface water act together to decompose and wear away the rocks on the
surface of the planet. The sediments formed by this erosional process are deposited in horizontal
layers. This process is called the principle of horizontality. Over time, these sediments harden
and bond together and become the rock layers of today. Because the first, or oldest, sediments
are found on the bottom layer, the age of sediments and rocks becomes progressively younger
from bottom to top. This is called the principle of superposition. Sedimentary rocks tell the
story of destruction and rebuilding. Geologists study sedimentary rocks to understand the past.
They use a relative time scale which places rocks and events in the chronological order in which
they formed. Using these principles, scientists can use fossils to establish the age of the rock.
C
B
A
Vertical geologic section.
Layer A is the oldest, layer C is the youngest.
In summary, the three principles which we call Steno’s Laws are:
The Principle of Superposition
The Principle of Original Horizontality
The Principle of Uniformitarinanism
Of the three, the Principle of Superposition is most directly applicable to relative dating. We can
examine any sequence of sedimentary strata and determine in a relative sense which beds are
older and which beds are younger. All that we need to know is whether the beds are right-sideup or not. This complication comes because tectonic forces can cause sedimentary sequence to
be tilted, folded, faulted, and overturned. Although sediments are originally deposited in
horizontal layers, they do not always remain horizontal. A trip to the mountains or a quick look
through your textbook is probably all that is needed to convince you that any sedimentary
sequences consist of beds which dip at some angle to the horizontal. And in some cases, the beds
are vertical or overturned.