OREGON GEOLOGY published by the Oregon Deportment of Geology and Mineral Industries OREGON GEOLOGY Barnett appointed to OOGAMI Governing Board --VOlUME 60, i'lUM6EIt , JAN /fEB Me... N. Barnett at Pa1Iond ho.I bMn oppoinl8d b)' c:.o... .... 10M 16tz1lo ... ."d ....1.rnMI by !he Or."", ...... ... .. -~....... " "" _..-.-- _-_._--_ - .. .. '--. S,n." lor • fOUl-year Ie!m be""'" OKombel " _"'.eI ,"7, 0$ c.c:-mIn, 1\oaI,j ".,..,w.., of ,110 Ole,,,,, -~ o.pll"",nt aI c.doC one! Mlnaol Induwles --- , ~. - (OOCoAMl) Bornoort ."a: .." John W. S\olJl~ '" _.- -_-- ..... Pa1Iond • .me....-..d two f""' - ~ _.., Ihe Go.<- --. _u_- --~- """"...... d ----______- - .. ~ ... _. ... _.>m. ......... ,... ... ........ - ---- _ 1oIoM.,.,- ... _ ,, -""'"'+ .......,0>, ... '"___ cc,.", .. _ ...' , """.~ ""1>'11 .n-, - ~- ,...,...-..... .. ,_.c:.- .......... ---__ (0'1" ''''''''_lM'I . ,. ,, ~ -,--~. " ~ -., W»o-~ _IM',_->cuo' -- .... I"",. ..... _ "'" ....... ....._ _-..., r-__.. , , ':--,--.. ,1>._ .- ..._ _.. ........ _ -...m.I>W __ .n-".., .... _"'Jr--. -",--....,-- ..._., , AriHoI It. "tftOft ,...I"-_.. ,_..,. ___ u , _ , -, ... ..... .. "' .... .............. .......... BlIlIOn Is tho M.".,. 01 1"- Humon ~ - ""-__",-_._- . ,,,._.nn_ .. _. _ __ ot _- •.. ...... OpeI.lioN 00tp0." ......1 01 PortIa..a General E~1c Company (PGEJ. $110 ".. bftn wo<ldn, .mt. P'GE oIr>e. '918. mostly ... ........,..,. /v..ctloro. ",,j _.'_'_._-""'_--~"'----"--.""-"--"'"_._- ___ ".... _ _ ' F........ p''"''''''"''"''tIy ... II>e .." af ........., RQoourcn. Silo _ .... _.. __ .. _R__ .. _• ott.odod "-PP-.... ~ one! " .....Iod hom AbiIono Chnstian 1..Wvoni1y 11\ .... Shoo 10 rrwrI.cI 0IId : -_ .. __ .. ..... two -.,..I ~... She Is .., Itw M,->, -----""'---........ .. .-_ -- CcundI 01 INS. alia, Asmy Gt-v..... .. and""- --,.-_............ -_--- 11\ IN """" ".ObtIy 01 r. clwW ~_ ...... " ... _ ...... '+ I'" .-'_ ,?to ,.,_._JIIo .. .... _ ,.... .. ___ __ .. _ III I H IIo"" to """" krnoott. IN tl..... momber bcwd ......... Joc:~ G. .."...,. at Enlorprloo ond .. ................ ' ....,., .. ...--. , OQI,oId W. C.. ImI1 .... 01 loy. 0 __.. ..... ." I .... _- DfI>OIt _"_>0'Or_" ,•__ ...... _ ",_,.... _- __. --_.. .. _.. ..... __ ..... _- Correction .. c-.." , ...__ __(..dO ... Tho",...,. 1,,1uI year., -oIu"" of 01."", GoooIw'. .--~-.... _no .. pullllohM .., po&e , 'j() oj the last ~..".. -- '( , -" bor to.Ins IOITII ........ lot wHdo _ .. ...., .... ... ,,,n......... _10<1 ........_-- ___.. _ ____--- -- ......... --.. .. _--- II ,..... """" 10 ...... .., ,....... copy of IN ",... pio_ -_ .. _ .. ,.., _ DO m".. , a -- n.. tho /doom,: Undw "Fitid trip JUIdu,. the """* r.f......... fur eow_photo Ihe two tIItrlel shodd be r_rMd. _ ...... -.., .. s.. ... ,..,1 _ ,.,. c-o..I IJn.cI« • Mined land rtdarnation.· lho ..",. applies _ flow. __ ............ lot!, ..., ,"".... 004 ... lho ft<I l ond tlurd tfltry. _,",500"" -. 0.-.• ..,. ___ Iho_ to __ ...,.... ... t ." ......_-'" c.- UI\dof ·"&1Jnomil.· Ihe P*I" 'of......... lor ·Cenl.. fur lMooml ImIn<!oIiorl MoppIn, • ond · Huard _.-_ .... _...... .. _10~...... ___.-'" --- -... _.. ..., adopt"" . , .' ..... shodd be .-..!. 0 , Geology along U.S. Highways 197 and 97 between The Dalles and Sunriver, Oregon by Gary Smith, Department ofEarth and Planetary Sciences, University ofNew Mexico, Albuquerque, New Mexico 87131 This highway-geology gUide was prepared for an as-yet unpublished travel brochure for the general public. We thank the author for his permission to print it here and for his help in adapting it to a different format. -Ed Introduction A LAND BUILT BY VOLCANIC ERUPTIONS viscous ("sticky") than andesitic or rhyolitic lava. The west coast of North America is referred to as an Basaltic lava, therefore, is more fluid and moves as a active continental margin. Large-scale geological pro­ thinner and more rapidly flowing lava flow. Andesitic cesses involving horizontal motions of the outermost and rhyolitic lavas show greater resistance to flow and layers of the earth cause earthquakes, volcanic erup­ generally do not travel more than a"few miles from their tions, and the uplift of mountains. Volcanoes are re­ eruptive vents. In some cases, silicic lava does not flow sponsible for the formation of most of the rocks formed away at all, but accumulates as a high, steep-sided, in Oregon over the past 45 million years. The rocks and bulbous mass of lava on top of the vent; these features landscapes that one sees while driving between The are called lava domes. Dalles and Sunriver are nearly all related, in one fashion Volcanic eruptions are driven by gases that are dis­ or another, to volcanism. solved in the magma where it is subjected to the great pressures of overlying rock within the earth. As the TYPES OF VOLCANOES AND VOLCANIC ROCKS magma rises near to the earth's surface, this pressure (significant terms printed in boldface) diminishes until it is insufficient to keep the gas from Volcanic rocks are solidified magma, molten material forming bubbles in the melt. This process is analogous produced at depths of 75-150 km beneath the earth's to the formation of bubbles in a carbonated beverage surface. The chemical composition and physical proper­ when the container is opened. Gas bubbles rise in the ties of magma are variable, depending on the depth and magma, pushing some of the magma along, and pro­ temperature at which the rocks melt, the type of rocks duce explosions when they burst through to the sur­ that are melted, and the degree of solidification that face. Basaltic magmas contain less dissolved gas than occurs as the magma cools on the way to the surface. more silica-rich magma varieties; consequently, erup­ This variation produces a wide range of volcanic rocks tions of basaltic volcanoes are less violent than those and an equally varied array of landscape features result­ occurring at andesitic or rhyolitic volcanoes. ing from volcanic eruptions. Silica, a compound of the During explosive eruptions, some of the bubble-rich elements silicon and oxygen, is the most important lava is blown out as a froth, which is qUickly quenched constituent of magma, comprising about 45-75 percent to volcanic glass upon contact with the cooler atmo­ of the melt by weight. sphere and broken into fragments of Widely ranging Volcanic rocks represent solidified lava, as magma is sizes as the bubbles continue to expand. These frag­ known when it reaches the earth's surface, and are ments, called pyroclasts (from Greek words meaning named on the basis of silica content and the silicate "fire broken"), include volcanic ash, pumice, and cin­ minerals that they contain. Rocks formed by crystalliza­ der. A rock f9rmed of consolidated ash is called tuff. tion of relatively low-silica lavas are called basalt, those Most pyroclastic fragments are blown skyward by derived from high-silica lava are known as rhyolite, and volcanic explosions and fall back to the surface. The the most common intermediate composition is named bulk of this fallout material accumulates adjacent to the andesite. Low-silica lavas contain more iron and mag­ vent from which it was erupted, but if the fragments are nesium than higher silica varieties, which gives them ejected to great height, a considerable volume may be darker colors. Basalt is generally dark gray or black, carried away by the wind and fall back to the earth at andesite is light gray or greenish gray, rhyolite is most great distances. The resulting deposits become thinner commonly white, pink, or tan. and composed of successively smaller particles at The composition of the magma not only determines greater distances from the volcano. Because eruptions what minerals will crystallize from it, but it also controls of gaseous rhyolite are the most violent, rhyolitic tuff is the nature of the volcanic eruptions that occur when more voluminous and widespread than that of basaltic the magma arrives at the surface. Basaltic lava is less or andesitic composition. In some cases, rhyolitic and, OREGON GEOLOGY, VOLUME 60, NUMBER I, JANUARY/FEBRUARY 1998 3 .. o ::l1li m G\ o • Z c,• ~ .... G\ ~ .I~I ' m X::::!o o r-<X tIlCD r­ 5 <:: .., r- til til til <:: ~ <:: til C3 Q til OJ E ~ ~o ~ 3: "J1 CB Qi m o .., ~ Q ~ CB Z ~ C 3: l1li m ::l1li :~~~~~, :- I·"..,/'-", .. ·.. ·' liL!L bill D O ...... "Uroo C""O +=>< i 0 6"§:~~~ p:l}'sog: Q. 61li?~~g> g: Cil 3Q. ~ :::r.o ",9:n III en =0 3 ::J 0t1131ll 315'6'£9: -CD = III ~~~3 a en::J :!o~ -::::::S0::1 ":it== = li?~3 ~ go _olll'<O ::J Q)".:-!":J o· m CD 01 '<::J =_ ~~~~ Q:JCD...,..­ < 3'<., l1li (J) "TI0 =roo en C.1ll 0 III ::l1li 5'~ § !»oreg [ III ::J'<; C. glll';:;~ o oCil~ o (J)~< ~ '<IllIllCD Q.S'! ~U1@ ...... 35.~a CD ;::::;':J .., ~ § "< ..... - . o III Ill o o ~. -,@ ~. 0 c. g.·q.Nc. == o (J).o c:: ' ::J -CD +=> !'l (J) + Generalized geologic map of road guide area: Left, The Dalles to Madras; right, Madras to Sunriver. Numbers refer to numbered sites in text. less often, andesitic pyroclasts are not simply ejected cade volcanoes. Eruptions from small rhyolite volcanoes upward, but may flow downslope as rapidly moving, in central Oregon also produced widespread ash-flow hot avalanches of ash and pumice, called ash flows. tuffs. Rhyolite lavas formed steep domes south of Ash flows may devastate hundreds of square miles Madras. John Day Formation tuff and Clarno Formation adjacent to volcanoes. The resulting rocks, composed mudflow deposits and associated sediment, are famous principally of pumice set in a matrix of finer ash and for the abundant and well-preserved plant and verte-. minerals crystallized from the magma are called ash­ brate fossils they contain. The story that these fossils tell flow tuffs. about the climate and landscape of long-ago Oregon Volcanic landforms vary as a result of the difference can be learned from visits to any of the various units of in eruption character exhibited by magmas of different John Day Fossil Beds National Monument located in composition. Basaltic magma erupts without prodUcing eastern Oregon.