[SOLVED] GEOSCIENCE 001 SPRING 2022 LAB 5 GEOLOGIC TIME

GEOSCIENCE 001 SPRING 2022

LAB 5:  GEOLOGIC TIME

This lab is modified from “Geology from Experience” byE. kirsten peters and Larry E. Davis,from “Investigating Earth” by C. Gil    Wiswall and Charles H. Fletcher, and by the geologic history lab developed by Michael Harris of the Department of Geology at James Madison University.

Introduction

Large sequences of layered sedimentary rocks can represent millions of years of elapsed time.  Each distinct layer or bedin a sequence is called STRATUM, and multiple layers are collectively called STRATA.  The study of sequences of strata is called STRATIGRAPHY. A sequence of strata may also be referred to as a STRATIGRAPHIC SECTION.

Extrusive igneous rocks such as lava flows and ash falls may also form as beds and can occur as discrete layers in a sequence of sedimentary rocks or they may cut across pre-existing layers of rock. All sequences of rock world-wide are sometimes collectively referred to as the ROCK RECORD. Each rock type and geologic structure in a sequence represents a different geologic event. As geologists we want to understand the history of the Earth and the rate of geologic processes. To do this, we must be able to date stratigraphic sections and the rock record. This lab provides exercises to investigate the two types of dating that geologists use, RELATIVE and NUMERICAL or ABSOLUTE DATING.

Relative Dating

RELATIVE DATING refers to the placing of events in the order in which they occurred without any understanding of the actual time or absolute time during which any one event occurred.  In other words, we can discuss that a certain event happened first, or previous to the next event, or another event

couldn,t have occurred until other events had. A set of very simple principles in conjunction with careful observation allows the determination of the relative order of geologic events represented by the rocks in   an exposed stratigraphic section.

Principles of Geology

An understanding of stratigraphy begins by recognizing certain principles. Natural processes such as erosion, deposition, and plate tectonics, and the natural laws of gravity and isostasy that have produced  the current features of the Earth, are thought to have operated in the same way in the distant past as they do now. This idea is known as the PRINCIPLE OF UNIFORMITARIANISM and was first stated by

James Hutton in the 18th  century. Given an understanding of uniformitarianism, the relative timing of geologic events can be determined by applying other simple ideas, collectively known as the PRINCIPLES OF GEOLOGY which include:

1)  THE PRINCIPLE OF ORIGINAL HORIZONTALITY — sediments that are deposited or precipitated on the Earth,s surface are done so in mostly horizontal layers.  Thus, if the rocks are noted to be tilted,  folded,  or metamorphosed,  then  these  events must have  followed deposition and lithification;

2)  THE PRINCIPLE OF SUPERPOSITION — in a series of layered rocks that have accumulated on the Earth,S Surface, the oldeSt rockS are at the bottom of the Sequence and the youngeSt are at the top;

3)  THE PRINCIPLE OF CROSS-CUTTING RELATIONSHIPS — any geologic feature that is crosscut or modified by another feature must be the older unit, i.e., the crosscutting feature is the younger feature, it needed something to already be there for it to cut across.

THINGS TO REMEMBER — each of the rock types, sedimentary, igneous, or metamorphic, represent a unique geologic event, which is referred to using the appropriate terminology that reflects the process(es) that formed it, i.e.;

o Sedimentary rocks are deposited, so we refer to the DEPOSITION of a sedimentary rock (e.g., deposition of shale or sandstone)

o Plutonic rocks are intruded, so we refer to the INTRUSION of a plutonic rock (e.g., intrusion of granite or mafic dike)

o Volcanic rockS are erupted onto the Earth,S Surface, So we refer to the ERUPTION of a volcanic rock (e.g., eruption of basalt or rhyolite)

o Metamorphic rocks form through the METAMORPHISM of a protolith. The protolith, not the metamorphic rock, was deposited, intruded, or extruded. We report the event that formed the protolith, AND THEN the metamorphic event.   The metamorphism itself is considered a separate event. Deformation and erosion of rocks are also geologic events. Some but not all of the rock units may be folded or faulted, and the discussion of the relative ages of these deformation events follows the same as if talking about the rock units.  Erosion events are also discussed as events relative to other events.

Unconformities

Unconformities are (usually) irregular contacts between strata in a stratigraphic sequence produced during periods of erosion or non-deposition. These contacts, thin boundaries between layers of rock, represent episodes of missing information, where there are no recorded rock units.  Unconformities are labeled according to the nature of the strata above and below the unconformity.  There are three types:

1)  DISCONFORMITY — a boundary between parallel, undeformed layers of rock, usually formed due to erosion or non-deposition;

2)  NONCONFORMITY — a boundary between  layers of sedimentary rock overlying igneous or metamorphic rocks. This relationship usually indicates that the underlying igneous or metamorphic rocks were exposed before being buried and deposited upon;

3)  ANGULAR UNCONFORMITY — a boundary between layers of sedimentary rock overlying

tilted or folded beds.  This relationship suggests that the older rocks were deformed, exposed, and then deposited upon.