نجوم المرمر

نجوم المرمر


 
الرئيسيةمكتبة الصورس .و .جالأعضاءالمجموعاتالتسجيلدخول

شاطر | 
 

 some geologic principles and stratigrafy

استعرض الموضوع السابق استعرض الموضوع التالي اذهب الى الأسفل 
كاتب الموضوعرسالة
Geo Ahmad
رئيس المنتدى
ورئيس المجلس الاعلى للمنتدى
رئيس المنتدى  ورئيس المجلس الاعلى للمنتدى
avatar

ذكر عدد الرسائل : 542
العمر : 28
الموقع : wwww.marmarstars.ahlamontada.net
العمل/الترفيه : واحد ماشي في كلية العلوم
المزاج : عالي اوي وهاي
السٌّمعَة : 2
نقاط : 22
تاريخ التسجيل : 14/08/2008

بطاقة الشخصية
الأنتشار في المكان:
100/100  (100/100)

مُساهمةموضوع: some geologic principles and stratigrafy   الأربعاء فبراير 25, 2009 9:31 pm



An Early Scientist Discovers Some Basic Rules

Nicholaus Steno (a.k.a. Niels Stensen) was a Danish physician living in Florence, Italy in the 1600s. He observed sedimentary rocks and formulated three basic principles of historical geology, which are commonly referred to as "Steno's Laws."


  1. Principle of Superposition

    1. Oldest rocks on the bottom
    2. Younger rocks on top


  2. Principle of Original Horizontality
    Sediments are deposited in flat, horizontal layers.
    (Steeply dipping or folded rock layers indicate tectonic disturbance after deposition.)

  3. Principle of Original Lateral Continuity
    Sediments are deposited over a large area in a continuous sheet. Rock layers extend continuously in all directions, until they thin out at the edge of the depositional basin, or grade into a different type of sediment.
These three principles are the basis of stratigraphy, the study of layered rocks. Stratigraphy allows geologists to place rock units into a sequence, helpng them interpret Earth history.


The Principle of Cross-Cutting Relationships

Discussed by English geologist Sir Charles Lyell, 1830s.
Applies to geologic features like faults, or intrusions of igneous rock or veins of minerals, or ancient erosion surfaces called unconformities that cut across pre-existing rocks.
Examples:


  1. The fault is younger than the rocks it cuts.
  2. The intrusion is younger than the rocks it cuts.

  3. Note the irregular erosional surface. This is an unconformity.
    The unconformity is younger than the rocks that have been eroded.
The Principle of Inclusions

Note that there are fragments of eroded rock overlying the unconformity in the diagram above. These are clasts or inclusions.
The inclusions are older than the rocks which contain them.
The clasts (in the bed above the unconformity) are derived from the underlying (older) bed. The gravel clasts are older than the layer which contains them. The layer containing the gravel must be younger than the layer from which the clasts originate.
There are two fundamentally different types of inclusions.


  1. Inclusions in sedimentary rocks - the gravel clasts in the layer above an unconformity.

    Gravel clasts in a sedimentary rock above an unconformity. The gravel clasts are older than the bed of sedimentary rock which contains them.
  2. Inclusions in igneous rocks.
    A xenolith is a fragment of the surrounding rock which has broken off during an intrusion and fallen into the magma.
    The xenolith is older than the igneous rock which contains it.

    A xenolith in a granite intrusion at Stone Mountain, Georgia. The xenolith is older than the granite.
The diagram below compares inclusions in a sedimentary rock (A) with inclusions in an igneous rock (B).

In the diagrams above, which are the gravel clasts and which are the xenoliths?
In each diagram, determine the origin of the inclusions and whether they are older or younger than the rocks that contain them.


Interpreting the Sequence of Events

The sequence of geologic events in an area can be solved by applying the fundamental principles of geology. In this diagram, which shows a hypothetical exposure of rock, such as might be seen in a roadcut or in the walls of the Grand Canyon, it is possible to determine the order in which the various geologic events occurred.


The sequence of events is as follows:


  1. Deposition of sedimentary rocks D.

  2. Fault B.

  3. Intrusion of igneous rock C.

  4. Erosion, forming the unconformity.

  5. Deposition of sedimentary rocks E.


_________________
هعيش مصري و جيو و هفضل مصري و جيو






الرجوع الى أعلى الصفحة اذهب الى الأسفل
معاينة صفحة البيانات الشخصي للعضو
 
some geologic principles and stratigrafy
استعرض الموضوع السابق استعرض الموضوع التالي الرجوع الى أعلى الصفحة 
صفحة 1 من اصل 1

صلاحيات هذا المنتدى:لاتستطيع الرد على المواضيع في هذا المنتدى
نجوم المرمر :: مدينة العلوم :: الحي العلمي الجيولوجي-
انتقل الى: