Page 161 - Laboratory Manual in Physical Geology

ACTIVITY that cut across layers of bedrock). The dikes can occur as
sheet dikes (nearly planar dikes that often occur in parallel
5.9 Geologic History of pairs or groups), ring dikes (curved dikes that form circu-
lar patterns when viewed from above; they typically form
Southeastern Pennsylvania
under volcanoes), or radial dikes (dikes that develop from
THINK | How can the shapes of bodies of the pipe feeding a volcano; when viewed from above, they
radiate away from the pipe).
About It igneous rock be used to classify them When magma is extruded onto Earth’s surface it is called
and infer their origin? lava . The lava may erupt gradually and cause a blister-like lava
dome to form in the neck of a volcano or a lava flow to run
OBJECTIVE Analyze bodies of igneous rock in from a volcano. The lava may also erupt explosively to form

southeastern Pennsylvania, using a geologic map, and pyroclastic deposits (accumulations of rocky materials that have
infer their origin. been fragmented and ejected by explosive volcanic eruptions).
All of these extrusive (volcanic) igneous processes present geo-
PROCEDURES
logic hazards that place humans at risk.
1. Before you begin , read about Intrusion, Eruption,
When you examine an unopened pressurized bottle
and Volcanic Landforms. Also, this is what you
of soft drink, no bubbles are present. But when you open
will need :
the bottle (and hear a “swish” sound), you are releasing
___ Activity 5.9 Worksheet (p. 152 ) and pencil the pressure that was containing the drink and allowing
2. Then follow your instructor’s directions for bubbles of carbon dioxide gas to escape from the liquid.
completing the worksheet. Recall that magma behaves similarly. When its pressure is
released near Earth’s surface, it’s dissolved gases expand and
make bubbly lava that may erupt from a volcano. In fact,
early stages of volcanic eruptions are eruptions of steam
Intrusions have different sizes and shapes. Batholiths and other gases separated from magma just beneath Earth’s
( FIGURE 5.15 ) are massive intrusions (often covering regions surface. If the hot, bubbly lava cannot escape normally

2
of 100 km or more in map view) that have no visible from the volcano, then the volcano may explode (like the
bottom. They form when small bodies of lava amalgamate top blowing off of a champagne bottle).
(mix together) into one
large body. To observe
one model of this amal-
gamation process, watch Lava flow
the blobs of “lava” in a Volcanic Volcanic cone
of pyroclasts
lighted lava lamp as they neck
rise and merge into one
large body (batholith) at Laccolith
Radial
the top of the lamp. Sill dike Radial
Sill dike
Smaller intrusions
(see FIGURE 5.15 ) include Sheet Ring Sheet

dike dike
sills (sheet-like intrusions dike
that force their way be- Sheet Sill Sheet
dike dike
tween layers of bedrock),
laccoliths (blister-like Pipe
sills), pipes (vertical tubes
or pipe-like intrusions
that feed volcanoes), and
dikes (sheet-like intrusions
Batholith

FIGURE 5.15 Intrusive and extrusive igneous rock bodies.

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