WINSTIAKA$FIGURE 1: Cross-section of the earth's surface showing a sequence of layered rocks.Is the lava flow older or younger than the rocks that are......immediately above it:Examine Figure 1, and using the descriptions of the mode of formation of lava flows and sills, answer thefollowing questions...beneath it:Is the sill older or younger than the rocks that are immediately......above it:...beneath it:SILL93LAVAFLOW Many aspects of rock origins are best understood if a rock is considered in terms of its relationships with itsneighbors. Such relationships are best seen 'in the field', where the rock is in its natural setting. As an example ofthis approach, we will consider several field situations to see what light they shed on the origin of rocks.ProcedureThree duplicate sets of samples are placed about the room. Work with one set. Each set has 5 boxes, one for each ofthe 5 exercises that comprise this laboratory. The exercises may be done in anyorder.Sample Set #1Age Relationships of Layered RocksGeologists quantify the immense time spans involved in earth history by establishing a reference point intime, and then counting backwards from that reference point using convenient time units. Geologists usethe modem era as a reference point and reckon time backwards to the time of occurrence of a geologicevent, as for example, the formation of a rock. Thus, to say that a sandstone is 100 million years old meansthat 100 million years ago, that sandstone was in the process of formation, perhaps as a sandy beach alongan ancient coastline. This is a familiar approach because it is essentially the same as we take in dealing witheveryday life. The major difference is that we use different time units (weeks, days, minutes) to measurethe short time scales characterizing our day-to-day affairsGeologists can determine the relative ages of the layers in a sequence of layered rocks from the positions ofthe layers in the sequence. Generally, a given layer is younger than (i.e. formed more recently) the layerbelow it and older than (i.e. formed before) the layer above it. This idea is referred to as the Principle ofSuperposition. Figure 1, on the next page, is a cross section showing a horizontal sequence of interbeddedsedimentary and igneous rocks, and illustrates how this principle can be applied to interpreting the relativeages of rocks. Figure 1 illustrates also that sometimes the age of specific rock layers may not adhere to the'younging upward' tenet expressed in the idea of superposition. These exceptions are, however, relativelyrare.There are two ways in which the igneous layers diagrammed in Figure 1, may have become part of thissequence:Lava Flow: An igneous layer may be a lava flow which was erupted onto the earth's surface froma volcanic vent. The flowing lava covered the sedimentary rock layers beneath it. At a later time,after the lava had cooled and turned to rock, it was buried by a new influx of sediment, thusbecoming part of the layered sequence.Sill: Molten rock, working its way upward through the layered sequence, did not reach the surface ofthe earth. Instead, while still below ground level, it wedged, or intruded, itself laterally betweentwo of the sedimentary layers in the sequence, where it eventually cooled and became a rock. Thisintrusive body, called a sill, thus became part of the layered sequence.92TARI

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Geologists study the history of earth by looking at the different layers of rocks. Relative ages of…

Is the lava flow older or younger than the rocks that are... ...immediately above it: ...beneath it: Is the sill older or younger than the rocks that are immediately... ...above it: ...beneath it: